Inducible over-expression of cardiac Nos1ap causes short QT syndrome in transgenic mice

Recent evidence demonstrated that alterations in the QT interval duration on the ECG are not only determined by mutations in genes for ion channels, but also by modulators of ion channels. Changes in the QT interval duration beyond certain thresholds are pathological and can lead to sudden cardiac death. We here focus on the ion channel modulator nitric oxide synthase 1 adaptor protein (Nos1ap). Whole-cell patch-clamp measurements of a conditional transgenic mouse model exhibiting cardiac-specific Nos1ap over-expression revealed a Nos1ap-dependent increase of L-type calcium channel nitrosylation, which led to increased susceptibility to ventricular tachycardias associated with a decrease in QT duration and shortening of APD₉₀ duration. Survival was significantly reduced (60% after 12 weeks vs. 100% in controls). Examination of the structural features of the hearts of transgenic mice revealed constant heart dimensions and wall thickness without abnormal fibrosis content or BNP production after 3 months of Nos1ap over-expression compared to controls. Nos1ap over-expression did not alter cGMP production or ROS concentration. Our study showed that myocardial over-expression of Nos1ap leads to the shortening of the QT interval and reduces the survival rate of transgenic animals, perhaps via the development of ventricular arrhythmias. We conclude that Nos1ap overexpression causes targeted subcellular localization of Nos1 to the CaV1.2 with a subsequent decrease of ADP₉₀ and the QT interval. This causes detrimental cardiac arrhythmias in transgenic mice.

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

Rett Syndrome (RTT) is a congenital, X-chromosome-linked developmental disorder characterized by developmental delay, dysautonomia, and breathing irregularities. RTT is also associated with sudden death and QT intervals are prolonged in some RTT patients. Most individuals with RTT have mutations in the MECP2 gene. Whilst there is some evidence for QT prolongation in mouse models of RTT, there is comparatively little information on how loss of Mecp2 function affects ventricular action potentials (APs) and, to-date, none on ventricular APs from female RTT mice. Accordingly, the present study was conducted to determine ECG and ventricular AP characteristics of Mecp2Null/+ female mice. ECG recordings from 12-13 month old female Mecp2Null/+ mice showed prolonged rate corrected QT (QTc) intervals compared to wild-type (WT) controls. Although Mecp2Null/+ animals exhibited longer periods of apnoea than did controls, no correlation between apnoea length and QTc interval was observed. Action potentials (APs) from Mecp2Null/+ myocytes had longer APD90 values than those from WT myocytes and showed augmented triangulation. Application of the investigational INa,Late inhibitor GS-6615 (eleclazine; 10 μM) reduced both APD90 and AP triangulation in Mecp2Null/+ and WT myocytes. These results constitute the first direct demonstration of delayed repolarization in Mecp2Null/+ myocytes and provide further evidence that GS-6615 may have potential as an intervention against QT prolongation in RTT.

Oxytocin does not directly alter cardiac repolarization in rabbit or human cardiac myocytes

Oxytocin, a nine amino acid peptide, is highly conserved in placental mammals, including humans. Oxytocin has a physiological role in parturition and parenteral administration of the synthetic peptide is used to induce labor and control postpartum hemorrhage. Endogenous levels of oxytocin before labor are ∼20 pg/mL, but pharmacological administration of the peptide can achieve levels of 110 pg/mL (0.1 nmol/L) following intravenous administration. Cardiac arrhythmia and premature ventricular contractions have been associated with oxytocin administration in addition to QTc interval prolongation. In the conscious rabbit model, intravenous oxytocin produced QT and QTc prolongation. The mechanism of oxytocin-induced QTc prolongation is uncertain but could be the result of indirect changes in autonomic nervous tone, or a direct effect on the duration of cardiomyocyte repolarization. The purpose of this study was to examine the ability of oxytocin to alter cardiac repolarization directly. Two conventional models were used: QTc interval evaluation in the isolated rabbit heart (IRH) and assessment of action potential duration (APD) in human ventricular myocytes (HVM). Oxytocin did not prolong QTc intervals in IRH or APD in HVM when tested at suprapharmacological concentrations, for example, up to 1 μmol/L. The results indicate that oxytocin has very low risk for eliciting QTc and APD prolongation directly, and infer that the QTc changes observed in vivo may be attributed to an indirect mechanism.

The variant hERG/R148W associated with LQTS is a mutation that reduces current density on co-expression with the WT

BACKGROUND

A variant of the ether-à-go-go related channel (hERG), p.Arg148Trp (R148W) was found at heterozygous state in two infants who died from sudden infant death syndrome (SIDS), one with documented prolonged QTc and Torsade de Pointes (TdP), and in an adult woman with QTc >500 ms, atrioventricular block and TdP. This variant was previously reported in cases of severe ventricular arrhythmia but very rarely in control subjects. Its classification as mutation or polymorphism awaited electrophysiological characterization.

METHODS

The properties of this N-terminal, proximal domain, hERG variant were explored in Xenopus oocytes injected with the same amount of RNA encoding for either hERG/WT or hERG/R148W or their equimolar mixture. The human ventricular cell (TNNP) model was used to test the effects of changes in hERG current.

RESULTS

R148W alone produced a current similar to the WT (369 ± 76 nA (mean ± SEM), n=13 versus 342 ± 55 nA in WT, n=13), while the co-expression of 1/2 WT+1/2 R148W lowered the current by 29% versus WT (243 ± 35 nA, n=13, p<0.05). The voltage dependencies of steady-state activation and inactivation were not changed in the variant alone or in co-expression with the WT. The time constants of fast recovery from inactivation and of fast and slow deactivation analyzed between -120 and +20 mV were not changed. The voltage-dependent distribution of the current amplitudes among fast-, slow- and non-deactivating fractions was unaltered. A 6.6% increase in APD90 from 323.5 ms to 345 ms was observed using the human cardiac ventricular myocyte model.

CONCLUSIONS

Such a decrease in hERG current as evidenced here when co-expressing the hERG/R148W variant with the WT may have predisposed to the observed long QT syndrome and associated TdP. Therefore, the heterozygous carriers of hERG/R148W may be at risk of cardiac sudden death.

Preservation of cardiomyocytes from the adult heart

Cardiomyocytes represent one of the most useful models to conduct cardiac research. A single adult heart yields millions of cardiomyocytes, but these cells do not survive for long after isolation. We aimed to determine whether inhibition of myosin II ATPase that is essential for muscle contraction may preserve fully differentiated adult cardiomyocytes. Using inhibitors of the myosin II ATPase, blebbistatin and N-benzyl-p-toluene sulphonamide (BTS), we preserved freshly isolated fully differentiated adult primary cardiomyocytes that were stored at a refrigerated temperature. Specifically, preserved cardiomyocytes stayed viable for a 2-week period with a stable expression of cardiac genes and retained the expression of key markers characteristic of cardiomyocytes. Furthermore, voltage-clamp, action potential, calcium transient and contractility studies confirmed that the preserved cardiomyocytes are comparable to freshly isolated cells. Long-term exposure of preserved cardiomyocytes to four tyrosine kinase inhibitors, sunitinib malate, dasatinib, sorafenib tosylate and imatinib mesylate, revealed their potential to induce cardiac toxicity that was manifested with a decrease in contractility and induction of cell death, but this toxicity was not observed in acute experiments conducted over the time course amenable to freshly prepared cardiomyocytes. This study introduces the concept that the inhibition of myosin II ATPase safeguards the structure and function of fully differentiated adult cardiomyocytes. The fact that these preserved cardiomyocytes can be used for numerous days after preparation makes them a robust and versatile tool in cardiac research and allows the investigation of long-term exposure to novel drugs on cardiomyocyte function.