Dynamic action potential clamp as a powerful tool in the development of a gene-based bio-pacemaker.

The development of a genetically engineered 'biological pacemaker', or 'bio-pacemaker', is a rapidly emerging field of research. One of the approaches in this field is to turn intrinsically quiescent myocardial cells, i.e., atrial or ventricular cells, into pacemaker cells by making them express the cardiac hyperpolarization-activated 'pacemaker current' If (known in neurophysiology as Ih), which is encoded by the hyperpolarization-activated cyclic nucleotide-modulated (HCN) gene family. We carried out 'dynamic action potential clamp' (dAPC) experiments in which we record current from a HEK-293 cell transfected with HCN4, which is the dominant HCN isoform in the sinoatrial (SA) node. This HCN4-transfected HEK-293 cell is voltage-clamped by the action potential generated in a real-time simulation of a human atrial cell (Courtemanche-Ramirez-Nattel model). In a continuous feedback loop, this current is injected into the atrial cell, so that this cell effectively expresses an HCN4-based pacemaker current. With sufficiently high 'expression levels' of HCN4 current the atrial cell is turned into a pacemaker cell with an SA nodal like action potential. Lower expression levels are sufficient if the inward rectifier potassium current (IK1), which is largely responsible for the stable resting potential of atrial cells, is 'down-regulated' by 50%, thus mimicking the gene therapy strategy to create a bio-pacemaker by down-regulation of IK1 and (over-)expression of If. Our dAPC experiments provide direct insights into the effects of introducing HCN4 current into an atrial cell, illustrating that dynamic action potential clamp can be a powerful tool in the process of developing a gene-based bio-pacemaker.