Engineering a biological pacemaker: in vivo, in vitro and in silico models

Abstract

Several hundred thousand electronic pacemakers are implanted in the US each year to treat abnormally slow heart rates. Biological pacemaker research strives to replace this hardware, and the associated monitoring and maintenance, by using gene or cell therapy to create a permanent and autonomically responsive pacemaker. While there are numerous technological hurdles to overcome before this is a therapeutic reality, one critical issue is determining the optimal channel gene to employ in creating a biological pacemaker. This review discusses the pros and cons of various model systems for characterizing and evaluating the function of candidate channel genes. It is argued that a sequential approach that combines in silico, in vitro and in vivo models is required.

Figure 1

Role of pacemaker channels in regulating normal sinoatrial node automaticity. A) Representative sinoatrial node action potential (control: solid lines) and some of the contributing ion channels and exchangers. I_fis activated on hyperpolarization and provides current to initiate diastolic depolarization. Na/Ca exchange current and T-/L-type Ca currents contribute to late diastole and threshold. The potassium current I_krepolarizes the membrane. The effect of norepinephrine (NE) to increase diastolic slope and speed impulse initiation is represented by the dashed line. B) Depiction of an HCN pacemaker channel (2 of the 4 subunits composing a functional channel are shown), illustrating the 6 transmembrane spanning domains of each subunit. When the channel is open, Na influx depolarizes the cell. β1-adrenergic and M2-muscarinic receptors respond to NE and acetylcholine (ACh), respectively, to regulate adenylyl cyclase (AC) activity via G-protein coupling. AC, in turn, regulates intracellular cAMP level, which is sensed by a cAMP binding site in the carboxy terminus of the HCN subunit (adapted with permission from [16]).

Figure 2

Rationale for biological pacing. Top) Initiation of spontaneous rhythms by sinoatrial node cells. Action potentials (inset) are initiated via inward current flowing through transmembrane channels (see Figure 1). Current flowing via gap junctions to adjacent myocytes results in excitation and impulse propagation through the conducting system. Middle) Gene-based biological pacemaker implanted in atrium or ventricle. HCN channel genes are virally introduced into a group of myocytes (one shown), resulting in channel proteins incorporated in the myocyte membrane. When the membrane is hyperpolarized at the end of an action potential, the HCN channels open to induce inward current, which excites the myocyte to initiate an action potential that then propagates via gap junctions to neighboring cells. Bottom) Cell-based biological pacemaker shown implanted in ventricle. HCN channels are expressed in stem cells that are then implanted in myocardium, where they electrically couple to neighboring myocytes via gap junctions. When the stem cell membrane is hyperpolarized via current flow through the gap junctions from coupled myocytes, the HCN channels open to induce inward current, which travels through the gap junction to excite the coupled myocyte and initiate an action potential in the myocyte. In each depicted example, current spread requires the presence of gap junctions to electrically couple the cells (adapted with permission from [34]).