Ectopic Purkinje fibers recruitment leads to permanent structural and functional conductive defects following myocardial infarction in newborn mice

The Purkinje network (PN) is the most distal part of the cardiac conduction system and ensures fast conduction of the electrical impulse through the ventricles. Despite its small volume in the heart, the PN has a disproportionally high pathogenic role. Indeed, defects in PN architecture or electrophysiology trigger conduction blocks and/or ventricular arrythmia associated with poor clinical outcomes. Purkinje fibers (PF) are specialized cardiomyocytes with a low proliferative capacity and are recruited from bipotent cardiomyocyte progenitors throughout development up to the neonatal period. Late PF recruitment is dependent on maximal levels of the transcription factor Nkx2-5. A decade ago, a neonatal regenerative window was identified in mice following myocardial infarction (MI). While damaged contractile myocardium regenerates within weeks, neither PN structure nor function has been investigated in this context. This study aims to determine whether the PN is capable of regeneration in the neonatal period, and whether PN maturation is compromised in the regenerating context. Neonatal MI was performed by permanent ligation of the left coronary artery on one-day-old mice. PN structure and function were characterized by histology and electrocardiogram. PF recruitment was investigated by clonal analysis in wildtype and Nkx2-5 haploinsufficent mice. Damage caused by the MI to the PN is spatially restricted and PFs are not replaced after regeneration, leaving a gap in the network. Surprisingly, we also observed PN hyperplasia (Fig. 1A) in regions that were not directly affected by the MI, attesting to abnormal PN maturation in the context of regeneration. At the cellular level, ectopic PFs have abnormal shapes and gap junction content. Consistently, ventricular conduction was slowed, shown by an increased QRS duration (Fig. 1B). Nkx2-5± mice, which are incompetent for late PF recruitment, do not develop MI-induced hyperplasia. Together with clonal analysis, this indicates that the hyperplastic PN in WT mice arises from ectopic PF recruitment during regeneration. Our study reveals the incapacity of the PN to regenerate after direct damage. We also uncover an extended window of PF recruitment following MI, which leads to permanent structural defects and impaired electrical conduction.