Gene- and cell-based bio-artificial pacemaker: what basic and translational lessons have we learned?

Abstract

Normal rhythms originate in the sino-atrial node, a specialized cardiac tissue consisting of only a few thousands of nodal pacemaker cells. Malfunction of pacemaker cells due to diseases or aging leads to rhythm generation disorders (for example, bradycardias and sick-sinus syndrome (SSS)), which often necessitate the implantation of electronic pacemakers. Although effective, electronic devices are associated with such shortcomings as limited battery life, permanent implantation of leads, lead dislodging, the lack of autonomic responses and so on. Here, various gene- and cell-based approaches, with a particular emphasis placed on the use of pluripotent stem cells and the hyperpolarization-activated cyclic nucleotide-gated-encoded pacemaker gene family, that have been pursued in the past decade to reconstruct bio-artificial pacemakers as alternatives will be discussed in relation to the basic biological insights and translational regenerative potential.

Figure 1

(a) Schematic representation of a monomeric subunit of HCN channels. The approximate locations of the TVGYG motif and the cyclic nucleotide-binding domain are highlighted. Sequence comparison (middle and right) of the S5–S6, the P-loops and the S4 of various HCN and K⁺ channels. Adapted from Siu et al. (b) Summary of the effects of the S3–S4 linker length on HCN1 steady-state activation (V_1/2), a measure of the energetic stability for opening and closing. A strong correlation was observed between the V_1/2 of the channels and their linker length. Schematics representing three HCN1 channels with shorter, wild-type (WT) and longer S3–S4 linkers are given for illustration. Adapted from Tsang et al.

Figure 2

(a) Top: representative tracings of HCN-encoded pacemaker current from Ad-CMV-GFP-, Ad-CMV-WT-HCN1-, Ad-CMV-HCN1-ΔΔΔ- and Ad-CMV-HCN1-QQ253QQ-transduced ventricular CMs. Bottom: automaticity could be recorded only from the Ad-CMV-HCN1-ΔΔΔ-transduced group. Steady-state activation curves indicate that HCN1-ΔΔΔ channels open most easily. Correlation of HCN current to induced pacing rate, MDP and phase 4 slope of HCN1-ΔΔΔ-mediated biopacemaking. (b) Effects of the I_f blocker ZD7288 on WT or engineered HCN1-transduced LV CMs. Spontaneously firing APs recorded from an HCN1-ΔΔΔ-transduced LV CM are abolished after addition of ZD7288. Upon silencing, an electrical stimulus elicits a normal ventricular AP. Adapted from Xue et al.

Figure 3

Left: anterior (left) and posterior (right) views of electroanatomical mapping of (a) control and (b) a SSS swine heart that has been implanted an electronic pacemaker whose wired lead generates heart beat in the RA. (c) SSS animal after Ad-CGI-HCN1-ΔΔΔ injection demonstrate the atrial activation at the injection site in the left atrial (LA). (d) Anterior view after Ad-CGI-HCN1-ΔΔΔ injection during spontaneous LA rhythm (left) and device-supported RA pacing (right). Note that the earliest endocardial activation shifted from the left injection site to the right pacing site during device-supported RA overdrive pacing. Adapted from Tse et al.

Figure 4

(a) Fluoroscopic image showing pacing leads at the RA and RV apex, the decapolar catherter at the roof of RA against the intra-atrial septum and the mapping catheter in the LA during electroanatomic mapping. The site of injection was marked by a surgical clip (red arrow). Reffered patch indicates reference patch for anatomical mapping. (b) Gross pathology showing the injection site at the LA apendage (arrow). (c and d) Sectioned heart demonstrating the expression of GFP with GFP staining (brown) ath low (left) and high (right) magnifications.