Prolonged repolarization and triggered activity induced by adenoviral expression of HERG N629D in cardiomyocytes derived from stem cells.
OBJECTIVE
The long QT syndrome, N629D HERG mutation, alters the pore selectivity signature sequence, GFGN to GFGD. Heterologous co-expression of N629D and the wildtype HERG resulted in a relative loss of the selectivity of K+ over Na+, but its physiologic relevance has not been assessed in cardiac myocytes.


RESULTS
Accordingly, N629D was overexpressed, via adenoviral gene transfer, in cardiomyocytes derived from mouse stem cells. Three IKr phenotypes were observed: (1) the wildtype-like IKr showed inward rectification and a positive tail current; (2) the N629D-like IKr showed outward rectification and an inward tail current; and (3) intermediate IKr showed a small outward tail current. Action potentials (AP) were paired with the IKr measurements in each cell. Resting membrane potential (RMP) was critically dependent on the IKr phenotype. The resting membrane potential of the cells was -61 +/- 5 mV (n=40) in wildtype, -63 +/- 3 mV (n=18) in wildtype-like IKr phenotype, -30 +/- 2 mV (n=12) in N629D-like and -47 +/- 2 mV (n=24) in intermediate phenotype (p<0.00001). Triggered action potential durations (APD) were: 62 +/- 12 ms (n=6) in wildtype, 65 +/- 11 ms (n=6) in wildtype-like IKr phenotypes and 106 +/- 10 ms (n=6) (p<0.01) in intermediate IKr phenotypes. Lowering [K+]o hyperpolarized wildtype cells and cells with a wildtype-like IKr phenotype, but depolarized those with intermediate phenotype (from -45 +/- 1 to -35 +/- 0.5 mV (n=12), p<0.01). In 6 of 12 cells, with intermediate phenotype, the hypokalemia-induced depolarization resulted in triggered activity. TTX suppressed this triggered activity.


CONCLUSIONS
Overexpression of N629D in cardiomyocytes derived from stem cells results in phenotypic variability in IKr, which was the critical determinant of the resting membrane potential, action potential duration and arrhythmogenic response to low [K+]o.
Teng
GuoQi
G
Department of Medicine, University of Calgary, Calgary, Alberta, Canada.
Zhao
Xiang
X
Cross
James C
JC
Li
Pin
P
Lees-Miller
James P
JP
Guo
Jiqing
J
Dyck
Jason R B
JR
Duff
Henry J
HJ
D000200:Action Potentials; D000256:Adenoviridae / Q000235:genetics; D000818:Animals; D027682:Cation Transport Proteins / Q000235:genetics*; D002454:Cell Differentiation; D002478:Cells, Cultured; D051638:Ether-A-Go-Go Potassium Channels; D005822:Genetic Vectors / Q000494:pharmacology; D049452:Green Fluorescent Proteins; D008133:Long QT Syndrome / Q000378:metabolism*; D008164:Luminescent Proteins / Q000235:genetics; D008564:Membrane Potentials; D051379:Mice; D032383:Myocytes, Cardiac / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D017354:Point Mutation; D015221:Potassium Channels / Q000235:genetics* / Q000378:metabolism; D024642:Potassium Channels, Voltage-Gated; D013234:Stem Cells / Q000166:cytology; D014161:Transduction, Genetic / Q000379:methods
D027682:Cation Transport Proteins; D051638:Ether-A-Go-Go Potassium Channels; C092509:KCNH6 protein, human; D008164:Luminescent Proteins; D015221:Potassium Channels; D024642:Potassium Channels, Voltage-Gated; D049452:Green Fluorescent Proteins