A temperature-dependent in silico model of the human ether-à-go-go-related (hERG) gene channel

Abstract

Introduction: Current regulatory guidelines for assessing the risk of QT prolongation include in vitro assays assessing drug effects on the human ether-à-go-go-related (hERG; also known as Kv11.1) channel expressed in cell lines. These assays are typically conducted at room temperature to promote the ease and stability of recording hERG currents. However, the new Comprehensive in vitro Proarrhythmia Assay (CiPA) paradigm proposes to use an in silico model of the human ventricular myocyte to assess risk, requiring as input hERG channel pharmacology data obtained at physiological temperatures. To accommodate current industry safety pharmacology practices for measuring hERG channel activity, an in silico model of hERG channel that allows for the extrapolation of hERG assay data across different temperatures is desired. Because temperature may have an effect on both channel gating and drug binding rate, such models may need to have two components: a base model dealing with temperature-dependent gating changes without drug, and a pharmacodynamic component simulating temperature-dependent drug binding kinetics. As a first step, a base mode that can capture temperature effects on hERG channel gating without drug is needed.

Methods and results: To meet this need for a temperature-dependent base model, a Markov model of the hERG channel with state transition rates explicitly dependent on temperature was developed and calibrated using data from a variety of published experiments conducted over a range of temperatures. The model was able to reproduce observed temperature-dependent changes in key channel gating properties and also to predict the results obtained in independent sets of new experiments.

Discussion: This new temperature-sensitive model of hERG gating represents an attempt to improve the predictivity of safety pharmacology testing by enabling the translation of room temperature hERG assay data to more physiological conditions. With further development, this model can be incorporated into the CiPA paradigm and also be used as a tool for developing insights into the thermodynamics of hERG channel gating mechanisms and the temperature-dependence of hERG channel block by drugs.

Keywords: CiPA; Kv11.1; Markov model; Methods; Temperature; hERG.

Figure 1. Structure of the hERG model

C1 and C2 are closed states, O is open state, and the corresponding inactivated states are IC1, IC2, and IO. The transition between adjacent states is a first order reaction dependent on membrane voltage, temperature, with three free parameters A, B, and q (see Methods). Each state transition has a different set of free parameters, which are distinguished between each other by numeric suffixes.

Figure 2. Fitting to hERG biophysical data at 20°C

Experimental data, taken from Di Veroli et al (Di Veroli, et al., 2013), are shown as symbols, and simulated data are shown as solid lines. A, fraction of open channels over time during deactivation. B, fraction of open channels over time during activation. C, activation curve. D, inactivation curve. E, recovery and inactivation rate. The top panels show the voltage protocols used. Note that points in panel E were calculated using two separate protocols: recovery time constants (−100 to −40 mV) using the protocol 2D while inactivation time constants (−30 to +50 mV) using the protocol in 2E.

Figure 3. Fitting to hERG channel experiments at various temperatures

Experimental data, taken from Vandenberg et al (Vandenberg, et al., 2006), are shown as symbols and simulated data are shown as solid lines. A, activation curve. B, inactivation curve. C, activation time course. D, time constants of various processes at room or physiological temperatures; Measured are experimentally determined data from Vandenberg paper while Fitted are simulated using our model. The insets show the voltage command protocol used. Note that for A and B, the original Vandenberg study contains 14°C data, which are not used for model fitting here because the main purpose is to extrapolate between room temperature and physiological temperature.

Figure 4. Prediction of independent experiments at 35°C in HEK cells

The hERG model was used to predict the outcome of experiments from an independent study (Zhou, et al., 1998). Solid lines are model prediction while symbols are experimental data. A. Activation I–V relationship. B. Activation time course. C. Deactivation time course. D. Recovery from inactivation time constants. E. Inactivation time constants. The insets show the voltage command protocol used.

Figure 5. Prediction of independent experiments at room temperature in HEK cells

A new three-step voltage protocol was applied to HEK cells at room temperature (24°C) and the results (symbols) are compared to independent prediction of the model (lines). Error bars indicating standard deviation are also shown (N=3). The inset shows the voltage command protocol used. The three sections of symbols correspond to the pre-pulse (+20 mV), the first test pulse (−40 mV), and the second test pulse (−120 mV) respectively.