KCNQ potassium channel mutations cause cardiac arrhythmias in Drosophila that mimic the effects of aging

Abstract

Population profiles of industrialized countries show dramatic increases in cardiovascular disease with age, but the molecular and genetic basis of disease progression has been difficult to study because of the lack of suitable model systems. Our studies of Drosophila show a markedly elevated incidence of cardiac dysfunction and arrhythmias in aging fruit fly hearts and a concomitant decrease in the expression of the Drosophila homolog of human KCNQ1-encoded K(+) channel alpha subunits. In humans, this channel is involved in myocardial repolarization, and alterations in the function of this channel are associated with an increased risk for Torsades des Pointes arrhythmias and sudden death. Hearts from young KCNQ1 mutant fruit flies exhibit prolonged contractions and fibrillations reminiscent of Torsades des Pointes arrhythmias, and they exhibit severely increased susceptibility to pacing-induced cardiac dysfunction at young ages, characteristics that are observed only at advanced ages in WT flies. The fibrillations observed in mutant flies correlate with delayed relaxation of the myocardium, as revealed by increases in the duration of phasic contractions, extracellular field potentials, and in the baseline diastolic tension. These results suggest that K(+) currents, mediated by a KCNQ channel, contribute to the repolarization reserve of fly hearts, ensuring normal excitation-contraction coupling and rhythmical contraction. That arrhythmias in both WT and KCNQ1 mutants become worse as flies age suggests that additional factors are also involved.

Fig. 1.

Cardiac KCNQ levels are reduced with age, and KCNQ mutants are susceptible to pacing-induced failure. (A) KCNQ RNA levels isolated from hearts dissected at 5 weeks are decreased to 33% relative to the 1-week levels (mean ± SD of three independent experiments; see also SI Fig. 5E). A control gene SH3β was only reduced to 72% of 1-week levels. (B) (Left) One-week-old KCNQ mutant flies (KCNQ¹⁸⁶ and KCNQ³⁷⁰) exhibit significantly elevated heart-failure rates (∗, P < 0.01, χ² analysis) in response to electrical pacing compared with WT controls (KCNQ⁹⁷). (Right) Mesodermal overexpression of the WT KCNQ cDNA (light bars) reduces the failure rate to control levels (dark bars).

Fig. 2.

Cardiac arrhythmias in old and KCNQ mutant flies. (A) M-mode traces (10 s) prepared from high-speed movies of intact flies. (B) Representative M-mode traces from semiintact Drosophila preparations. Arrhythmic heartbeats are evident in KCNQ mutants as early as 1 week of age. (C) Visual quantification of aberrant heartbeats in random, 10-s M-mode traces. KCNQ mutants have an elevated incidence of arrhythmia at young ages compared with WT flies. (D) Automated analysis of heart period obtained from 10-s high-speed video images. Mean (± SEM) heart period increased significantly with age in most KCNQ mutant flies compared with age-matched controls (∗, P < 0.05). Genotype-by-age analysis of the mutant versus WT curves also shows significant differences (∗, P < 0.05). (E) SD of the heart periodicity was used as an overall measure of arrhythmicity (arrhythmia index ± SEM). All time points (except 1 week) and overall curves for both KCNQ mutants were significantly different from the WT controls (∗, P < 0.01, analysis of covariance regression analysis). Age-by-genotype analysis also shows a significant difference between WT and mutants (∗, P < 0.01). For C–E, ◇, WT; □, KCNQ186; ▵, KCNQ³⁷⁰; n = 20–30 flies per data point.

Fig. 3.

KCNQ mutants exhibit early onset of prolonged, fibrillatory contractions. (A) Systolic interval lengths for all heartbeats in each record were computed and averaged for each fly (∗, P < 0.05, analysis of covariance regression analysis and independent samples t test). (B) Incidence of fibrillation, plotted as a percentage of total flies (∗, P < 0.05, Pearson's χ² test). For A and B, ◇, WT; □, KCNQ¹⁸⁶; ▵, KCNQ³⁷⁰; n = 20–30 flies per data point. (C) Arrhythmicity index from semiintact fly heart preparations in 3-week-old flies. Overexpression of the WT UAS-KCNQ in KCNQ mutant flies (dark blue/red bars) significantly reduced the arrhythmicity index compared with control flies with the UAS-KCNQ construct or the 24B driver alone (light blue/red bars; ∗, P < 0.05). (D) Combined data, showing the distribution of systolic intervals for control and KCNQ mutant flies. Systolic intervals lasting >1.5 s were grouped together and are shown in the last bar of each histogram. The bottom row of the histograms shows rescue of the 3-week-old KCNQ¹⁸⁶ mutant phenotype by overexpression of KCNQ transgene in all mesoderm (on the left) and improvement in the cardiac status of 7-week-old WT flies by overexpression of KCNQ transgene specifically in all cardiomyocytes (right) using the GMH5 driver (17).

Fig. 4.

Field potential and tension recordings from KCNQ mutant hearts. (A) Spontaneous field potential recordings from semiintact fly heart preparations. WT heart (3 weeks) exhibits regular upward depolarizing potentials and downward repolarizing potentials. (B) Depolarizing potentials from KCNQ null mutant heart (3 weeks) are similar to WT in appearance, but the repolarizing potentials (arrows) are relatively smaller and/or delayed compared with WT. (C) M-mode trace from a WT fly with the same 2-s time base as for the field potential recordings. (D) Two-second M-mode from a KCNQ¹⁸⁶ mutant fly. (E) (Upper) Baseline tension recordings across an individual WT heart tube show rhythmic heart contractions. (Lower) Tension recordings before, during, and after a 5-Hz electrical stimulation signal (solid bar). (F) (Upper) Baseline tension development in KCNQ mutant heart showed more prolonged contractions and slower heart rate than WT. (Lower) Tension recordings from a KCNQ mutant heart before, during, and after 5-Hz electrical stimulation (solid bar).