Delayed Ventricular Repolarization and Sodium Channel Current Modification in a Mouse Model of Rett Syndrome

Abstract

Rett syndrome (RTT) is a severe developmental disorder that is strongly linked to mutations in the MECP2 gene. RTT has been associated with sudden unexplained death and ECG QT interval prolongation. There are mixed reports regarding QT prolongation in mouse models of RTT, with some evidence that loss of Mecp2 function enhances cardiac late Na current, INa,Late. The present study was undertaken in order to investigate both ECG and ventricular AP characteristics in the Mecp2Null/Y male murine RTT model and to interrogate both fast INa and INa,Late in myocytes from the model. ECG recordings from 8–10-week-old Mecp2Null/Y male mice revealed prolongation of the QT and rate corrected QT (QTc) intervals and QRS widening compared to wild-type (WT) controls. Action potentials (APs) from Mecp2Null/Y myocytes exhibited longer APD75 and APD90 values, increased triangulation and instability. INa,Late was also significantly larger in Mecp2Null/Y than WT myocytes and was insensitive to the Nav1.8 inhibitor A-803467. Selective recordings of fast INa revealed a decrease in peak current amplitude without significant voltage shifts in activation or inactivation V0.5. Fast INa ‘window current’ was reduced in RTT myocytes; small but significant alterations of inactivation and reactivation time-courses were detected. Effects of two INa,Late inhibitors, ranolazine and GS-6615 (eleclazine), were investigated. Treatment with 30 µM ranolazine produced similar levels of inhibition of INa,Late in WT and Mecp2Null/Y myocytes, but produced ventricular AP prolongation not abbreviation. In contrast, 10 µM GS-6615 both inhibited INa,Late and shortened ventricular AP duration. The observed changes in INa and INa,Late can account for the corresponding ECG changes in this RTT model. GS-6615 merits further investigation as a potential treatment for QT prolongation in RTT.

QTc interval and ventricular action potential prolongation in the Mecp2Null/+ murine model of Rett syndrome

“…The results of this study are consistent with earlier data showing QT c interval prolongation in this model of RTT (Cheng et al, 2022;Herrera et al, 2015;Herrera et al, 2016;McCauley et al, 2011). The direct measurement of APs from Mecp2 Null/+ ventricular myocytes has also demonstrated delayed repolarization and showed results that are similar, albeit not identical, to those from younger Mecp2 Null/Y male myocytes (Cheng et al, 2022). The beneficial effects of GS-6615 in reducing APD 90 and AP triangulation are similar to those reported recently in myocytes from male RTT mice and support the further preclinical investigation of this as a potential antiarrhythmic strategy in RTT.…”

“…The same recording conditions and stimulation rate (1 Hz) were used here for female Mecp2 Null/+ myocytes as in our recent report that included AP data from myocytes from Mecp2 Null/Y male animals from the same RTT model. It is instructive, therefore, to compare the differences in AP parameters between WT females observed in this study with those seen in males under identical experimental conditions (Cheng et al, 2022). In ventricular myocytes from 8-10 week Mecp2 Null/Y animals, AP durations at 75% and 90% of complete repolarization were increased in Mecp2 Null/Y compared to WT myocytes (with APD 90 prolonged by ~50 ms); AP triangulation (APD 90 -APD 25 ) was also increased by ~50 ms in Mecp2 Null/Y myocytes (Cheng et al, 2022).…”

“…The murine model of RTT employed for this investigation (the “Bird” strain (Guy et al, 2001 )), had deletions of the third and fourth exons of Mecp2 . Mice were genotyped as described previously (Cheng et al, 2022 ). McCauley et al reported no significant difference between QTc intervals in WT and Mecp2 Null/+ mice at 4–5 months of age, with differences becoming evident at older ages (10 months) (McCauley et al, 2011 ).…”

Characterization in Effective Stimulation on the Magnitude, Gating, Frequency Dependence, and Hysteresis of INa Exerted by Picaridin (or Icaridin), a Known Insect Repellent

“…In continued exposure to picaridin, further addition of ranolazine markedly resulted in an attenuation of picaridin-mediated increase of Hys (V) strength seen in GH 3 cells. Ranolazine has been previously demonstrated to be an inhibitor of I Na(L) [ 28 , 29 , 33 , 34 ]. Findings from the current observations, therefore, unveiled that the double V ramp -induced I Na(P) resulted in striking Hys (V) changes in voltage dependence and that such strength can be evidently enhanced by adding picaridin.…”

“…Of additional note, some inhibitors of Na V channels (e.g., KMUP-1 and ranolazine) were demonstrated to increase the inactivation rate of I Na [ 28 , 29 , 30 ], while some activations of Na V channels (e.g., tefluthrin) could preferentially retard the inactivation rate, as well as enhance the transient ( I Na(T) ) and late ( I Na(L) ) components of I Na [ 31 , 32 ]. KMUP-1 was viewed as a xanthine and piperazine derivative [ 30 ], and ranolazine as a piperazine derivative, which is structurally similar to trimetazidine and known to be a blocker of I Na(L) [ 28 , 29 , 33 , 34 ], while tefluthrin has been reported to be a synthetic type-I pyrethroid used to kill a wide range of insects [ 35 , 36 ]. However, whether and how picaridin can affect the magnitude, gating kinetics, and/or voltage-dependent hysteresis (Hys (V) ) of I Na remains mostly obscure, although it is an insect repellent which belongs to the piperidine chemical family.…”