Treatment of disorders of the sinus node or the atrioventricular node requires insights into the molecular mechanisms of development and homoeostasis of these pacemaker tissues. In the developing heart, transcription factor TBX3 is required for pacemaker and conduction system development. Here, we explore the role of TBX3 in the adult heart and investigate whether TBX3 is able to reprogramme terminally differentiated working cardiomyocytes into pacemaker cells.

TBX3 expression was ectopically induced in cardiomyocytes of adult transgenic mice using tamoxifen. Expression analysis revealed an efficient switch from the working myocardial expression profile to that of the pacemaker myocardium. This included suppression of genes encoding gap junction subunits (Cx40, Cx43), the cardiac Na⁺ channel (Na_V1.5; I_Na), and inwardly rectifying K⁺ ion channels (K_ir genes; I_K1). Concordantly, we observed conduction slowing in these hearts and reductions in I_Na and I_K1 in cardiomyocytes isolated from these hearts. The reduction in I_K1 resulted in a more depolarized maximum diastolic potential, thus enabling spontaneous diastolic depolarization. Neither ectopic pacemaker activity nor pacemaker current I_f was observed. Lentiviral expression of TBX3 in ventricular cardiomyocytes resulted in conduction slowing and development of heterogeneous phenotypes, including depolarized and spontaneously active cardiomyocytes.

TBX3 reprogrammes terminally differentiated working cardiomyocytes and induces important pacemaker properties. The ability of TBX3 to reduce intercellular coupling to overcome current-to-load mismatch and the ability to reduce I_K1 density to enable diastolic depolarization are promising TBX3 characteristics that may facilitate biological pacemaker formation strategies.