Bradycardic Effects of Mutations in the HCN4 Gene at Different Levels of Autonomic Tone in Humans

HCN4 channels are expressed in the human sinoatrial node (SAN) and conduct the hyperpolarization-activated 'funny current', If, also known as 'pacemaker current'. Several loss-of-function mutations in the HCN4 gene have been associated with human sinus bradycardia. Clinical observations suggest that bradycardic effects are present at all levels of autonomic tone. We assessed the effects of three different mutations in HCN4 on human SAN pacemaker activity at different levels of autonomic tone by incorporating experimentally identified mutation-induced changes in If into the recently developed Fabbri et al. model of a single human SAN cell. Different levels of autonomic tone were obtained through simulated administration of specific levels of acetylcholine (ACh) or isoprenaline (Iso). The G480R, A485V, and 695X mutations in HCN4 lowered the control beating rate from 74 to 62, 59, and 65 bpm, the ACh beating rate from 49 to 40, 37, and 45 bpm, and the Iso beating rate from 140 to 115, 100, and 109 bpm, respectively, all in accordance with the clinical observations. We conclude that experimentally observed changes in the expression and kinetics of If channels can explain the clinically observed bradycardic effects of loss-of-function mutations in HCN4 at different levels of autonomic tone

Computational modeling of human sinoatrial node: what simulations tell us about pacemaking

The Sinoatrial node (SAN) is the primary pacemaker in physiological conditions. SAN tissue is characterized by auto-ryhthmicity, i.e. it does not need external stimuli to initiate its electrical activity. The auto-rhythmic behavior is due to the spontaneous slow depolarization during the diastolic phase. Understanding the biophysical mechanisms at the base of diastolic depolarization is crucial to modulate the heart rate (HR). In turn, HR modulation is fundamental to treat cardiac arrhythmias, so that atria and ventricles can fill and pump the blood properly. The overall aim of the thesis is the investigation of the underlying mechanisms responsible for the pacemaking in human. To this end, a human computational model of the action potential (AP) of the SAN was developed. Pacemaking modulation at single cell level, effects of ion channel mutations on the beating rate and propagation of the electrical trigger from SAN to atrial tissue are the faced topics The human single cell SAN model was developed starting from the rabbit SAN by Severi et al.; the parent model was updated with experimental data and automatic optimization to match the AP features reported in literature. A sensitivity analysis was performed to identify the most influencing parameters. The investigation of pacemaking modulation was carried out through the simulation of current blockade and mimicking the stimulation of the autonomic nervous system. The model was validated comparing the simulated electrophysiological effects due to ion channel mutations on beating rate with clinical data of symptomatic subjects carriers of the mutation. More insights on pacemaking mechanisms were obtained thanks to the inclusion of calcium-activated potassium currents, which link changes in the intracellular calcium to the membrane. Finally, the propagation of the AP from the SAN to the atrial tissue and the source-sink interplay was investigated employing a mono-dimensional strand composed by SAN and atrial models