Advances in understanding ventricular conduction system development, physiology, arrhythmogenesis and therapeutics

The ventricular conduction system (VCS) is essential for highly synchronous myocardial excitation and contraction. Developmental and acquired diseases impacting VCS structure and function are responsible for a substantial burden of rhythm disturbances. We highlight five recently published studies that advance our understanding of VCS formation, function, pathophysiology and therapeutics.

Nkx2–5 Loss of Function in the His-Purkinje System

The homeobox transcription factor Nkx2–5 plays an essential role in the cardiac development and mutations are responsible for congenital heart defects, including conduction disturbances. Using a conditional knockout strategy with a Cx40-CreERT2 driver to inactivate Nkx2–5 expression in the VCS of neonatal mice, the authors observed apical VCS hypoplasia and progressive loss of expression of a subset of fast-conduction markers including Cx40 and Hcn4 (Fig 1A).¹ Mutant mice displayed an altered ventricular activation sequence, with progressively reduced QRS amplitude and an RSR’ complex. These excitation defects were associated with progressive ventricular diastolic dysfunction and dyssynchrony, without evidence of fibrosis. These data indicate that primary conduction system disease may induce secondary contractile defects that may be especially responsive to cardiac resynchronization therapy.

Figure 1. Top Stories on the Ventricular Conduction System A:

Nkx2–5 VCS-conditional deletion mice develop Purkinje network hypoplasia. Top Panel: Whole-mount immunofluorescence for Cntn-2 on opened LV from 3-month-old control and Nkx2-5ΔVCS hearts. Lower Panel: Genetic lineage tracing of ventricular Cx40+ cells after Cre induction at E18.5 in control and Nkx2–5ΔVCS mice showing the distribution of YFP+ cells in the Purkinje fiber network indicated by a co-immunofluorescence with Cntn-2 at P20. B: Schematic diagram summarizing the hypothetical relationship between ventricular conduction system and Dbh-derived cardiomyocytes and Dbh+ cardiomyocytes in development and maturation of murine hearts. C: mCntn2–800 systemic injection summary. Left Panel: Schematic representation of workflow for light-sheet microscopy visualization. Right Panel: 3D volumetric analyses of intact hearts following mCntn2–800 systemic injection with high-resolution labeling of the entire CCS. Panel A reproduced from Choquet et al. 2023.¹ Panel B reproduced from Sun et al 2023.⁴ Panel C reproduced from Goodyer et al. 2022.⁵

His bundle pacing and arrhythmia treatment

The excitable gap (EG) is critical for maintenance of reentrant arrhythmias including ventricular tachycardia and fibrillation. A recent study utilized a computational approach to determine whether rapid His bundle pacing (HBP) could modify arrhythmia maintenance or defibrillation efficacy.² The authors found that HPB reduced the EG, modestly improved arrhythmia termination by suppressing reentry and also increased the efficacy of defibrillation, especially in models with bundle branch block². These findings suggest that rapid HBP during reentrant ventricular arrhythmias is a promising new antiarrhythmic and defibrillation strategy². The authors hypothesize that further refinement of HPB, such as dynamic changes in pacing cycle length in response to local electrograms, may improve the effectiveness of this strategy².

The anatomic origin of ventricular arrhythmia in CPVT

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an arrhythmia syndrome characterized by aberrant sarcoplasmic reticulum calcium release that evokes delayed afterdepolarizations. Previous experimental evidence has suggested that Purkinje cells within the VCS are the cellular source of arrhythmia triggers. A recent study using mutant mice with tissue-targeted ablation of the Casq2 gene, coupled with in silico modeling, suggests that it is working ventricular myocytes closely juxtaposed to Purkinje cells that are the triggering cells.³ The authors propose that source-sink relationships at the Purkinje-myocardial junction favor retrograde, but not antegrade conduction of subthreshold excitations by DADs. Moreover, they hypothesize that ectopic beats generated at the Purkinje-myocardial junction may be operative in common acquired diseases including heart failure and following myocardial infarction.

Dopamine beta-hydroxylase expressing cardiomyocytes in the VCS

The extent of transcriptional and functional heterogeneity within the VCS is incompletely characterized. Here, using single-cell and spatial transcriptomic analyses, genetic fate mapping and molecular imaging with computational reconstruction from multiple developmental stages, the authors identified cardiomyocytes expressing Dbh, which encodes dopamine beta-hydroxylase, an enzyme that converts dopamine to norepinephrine (Fig 1B).⁴ Using optogenetic electrophysiological interrogation of Dbh^Cre/ChR2-tdTomato mice, they provide evidence that these Dbh⁺-derived cardiomyocytes are enriched within the VCS and function as part of the ventricular CCS. Electrophysiological characterization of cardiac conduction using ex vivo isolated hearts from cardiac-restricted Dbh conditional knockout mice demonstrated slowed atrioventricular conduction. Furthermore, catecholaminergic-type vesicles were identified in adult cardiomyocytes in the Dbh-cardiomyocyte-rich atrioventricular junction⁴. Whether this catecholamine-associated population of conduction-system enriched cardiomyocytes plays a distinct role in arrhythmogenic disease pathogenesis remains to be determined. Also unknown is the relative physiological significance of catecholamines derived from sympathetic neurons as opposed to Dbh⁺ cardiomyocytes.

In vivo visualization and targeting of the CCS

Iatrogenic damage to the conduction system is not uncommon during invasive cardiac procedures, especially surgeries for congenital heart disease and trans-aortic valve replacements (TAVR). At present there is no method to distinguish the CCS from surrounding myocardium in real-time and guide the operator. Here, leveraging the prior discovery of contactin-2 (Cntn2) as a cell surface marker enriched in the CCS, this team of investigators fused a near-infrared dye to a Cntn2 polyclonal antibody that resulted in labeling and live imaging of the murine CCS following a single intravenous injection of the antibody-dye conjugate (Fig 1C).⁵ Moreover, with the goal of creating a viable human prototype, a fully human monoclonal Fab targeting Cntn2 was generated and used to deliver cargo to the CCS, providing proof-of-principle for targeted delivery of therapeutic compounds to the conduction system⁵. Whether this strategy can be successfully translated into human therapeutics remains uncertain but the approach is promising.