A general mechanism of KCNE1 modulation of KCNQ1 channels involving non-canonical VSD-PD coupling

Evaluating sequential and allosteric activation models in IKs channels with mutated voltage sensors

The ion-conducting IKs channel complex, important in cardiac repolarization and arrhythmias, comprises tetramers of KCNQ1 α-subunits along with 1-4 KCNE1 accessory subunits and calmodulin regulatory molecules. The E160R mutation in individual KCNQ1 subunits was used to prevent activation of voltage sensors and allow direct determination of transition rate data from complexes opening with a fixed number of 1, 2, or 4 activatable voltage sensors. Markov models were used to test the suitability of sequential versus allosteric models of IKs activation by comparing simulations with experimental steady-state and transient activation kinetics, voltage-sensor fluorescence from channels with two or four activatable domains, and limiting slope currents at negative potentials. Sequential Hodgkin-Huxley-type models approximately describe IKs currents but cannot explain an activation delay in channels with only one activatable subunit or the hyperpolarizing shift in the conductance-voltage relationship with more activatable voltage sensors. Incorporating two voltage sensor activation steps in sequential models and a concerted step in opening via rates derived from fluorescence measurements improves models but does not resolve fundamental differences with experimental data. Limiting slope current data that show the opening of channels at negative potentials and very low open probability are better simulated using allosteric models of activation with one transition per voltage sensor, which implies that movement of all four sensors is not required for IKs conductance. Tiered allosteric models with two activating transitions per voltage sensor can fully account for IKs current and fluorescence activation kinetics in constructs with different numbers of activatable voltage sensors.

Whole-exome sequencing of pathogenic genes in a family with congenital heart disease: A case report

RATIONALE

Congenital heart disease (CHD) is the most common birth defect and an important cause of noninfectious deaths in infants and children. It has high prevalence globally, placing an enormous burden on society and families. Studies of individuals with hereditary or sporadic CHD have provided strong evidence for its genetic basis. The aim of this study was to identify causative gene variants in a Chinese family with congenital heart disease.

PATIENT CONCERNS AND DIAGNOSES

Three generations of a CHD family were recruited. Proband III.9 was diagnosed with congenital heart disease at age 11 months, and the echocardiogram showed arterial ductus arteriosus, with a left-to-right shunt at the level of the arteries. Precedent III.10 was a twin of Proband III.9 who was diagnosed with congenital heart disease at age 11 months, in whom the echocardiogram revealed an arterial ductus arteriosus, an unenclosed patent ductus arteriosus, and a left to right shunt at the level of the arteries (second figure). III.8 was diagnosed with congenital heart disease at age 15, but echocardiography in this study showed no abnormalities. No cardiac abnormalities were detected in any of his parents, grandparents, or maternal grandparents. We performed whole-exome sequencing on CHD sufferers and their unexpressing family members to investigate the genetic causes of CHD in this family line. Exome sequencing identified 4 mutation sites in this family line. The variant c.3245A>G (p.His1082Arg) of the AMER1 gene was consistent with concomitant X-chromosome recessive inheritance, the variant c.238G>C (p.Val80Leu) of the KCNE1 gene was consistent with autosomal accessory inheritance, and the other 2 variants did not conform to the law of the mode of inheritance of the disease.

OUTCOMES

The first identified variant, c.3245A>G (p.His1082Arg) of the AMER1 gene, with X-chromosome recessive inheritance, and the variant c.238G>C (p.Val80Leu) of the KCNE1 gene, which has been reported as autosomal dominant, may be the causative agent of CHD in this family line. These findings broaden the genetic scope of congenital heart disease and could help in the development of targeted drugs for the treatment of congenital heart disease.

Mechanistic understanding of KCNQ1 activating polyunsaturated fatty acid analogs

The KCNQ1 channel is important for the repolarization phase of the cardiac action potential. Loss of function mutations in KCNQ1 can cause long QT syndrome (LQTS), which can lead to cardiac arrythmia and even sudden cardiac death. We have previously shown that polyunsaturated fatty acids (PUFAs) and PUFA analogs can activate the cardiac KCNQ1 channel, making them potential therapeutics for the treatment of LQTS. PUFAs bind to KCNQ1 at two different binding sites: one at the voltage sensor (Site I) and one at the pore (Site II). PUFA interaction at Site I shifts the voltage dependence of the channel to the left, while interaction at Site II increases maximal conductance. The PUFA analogs, linoleic-glycine and linoleic-tyrosine, are more effective than linoleic acid at Site I, but less effective at Site II. Using both simulations and experiments, we find that the larger head groups of linoleic-glycine and linoleic-tyrosine interact with more residues than the smaller linoleic acid at Site I. We propose that this will stabilize the negatively charged PUFA head group in a position to better interact electrostatically with the positively charges in the voltage sensor. In contrast, the larger head groups of linoleic-glycine and linoleic-tyrosine compared with linoleic acid prevent a close fit of these PUFA analogs in Site II, which is more confined. In addition, we identify several KCNQ1 residues as critical PUFA-analog binding residues, thereby providing molecular models of specific interactions between PUFA analogs and KCNQ1. These interactions will aid in future drug development based on PUFA-KCNQ1 channel interactions.

Electro-mechanical coupling of KCNQ channels is a target of epilepsy-associated mutations and retigabine

KCNQ2 and KCNQ3 form the M-channels that are important in regulating neuronal excitability. Inherited mutations that alter voltage-dependent gating of M-channels are associated with neonatal epilepsy. In the homolog KCNQ1 channel, two steps of voltage sensor activation lead to two functionally distinct open states, the intermediate-open (IO) and activated-open (AO), which define the gating, physiological, and pharmacological properties of KCNQ1. However, whether the M-channel shares the same mechanism is unclear. Here, we show that KCNQ2 and KCNQ3 feature only a single conductive AO state but with a conserved mechanism for the electro-mechanical (E-M) coupling between voltage sensor activation and pore opening. We identified some epilepsy-linked mutations in KCNQ2 and KCNQ3 that disrupt E-M coupling. The antiepileptic drug retigabine rescued KCNQ3 currents that were abolished by a mutation disrupting E-M coupling, suggesting that modulating the E-M coupling in KCNQ channels presents a potential strategy for antiepileptic therapy.