Lineages of the Cardiac Conduction System

Abstract

The cardiac conduction system (CCS) initiates and coordinately propagates the electrical impulse to orchestrate the heartbeat. It consists of a set of interconnected components with shared properties. A better understanding of the origin and specification of CCS lineages has allowed us to better comprehend the etiology of CCS disease and has provided leads for development of therapies. A variety of technologies and approaches have been used to investigate CCS lineages, which will be summarized in this review. The findings imply that there is not a single CCS lineage. In contrast, early cell fate decisions segregate the lineages of the CCS components while they remain connected to each other.

Keywords: cardiac conduction system; cardiac conduction system lineage; cardiac development; cell lineage; genetic fate map; genetic inducible fate map; lineage tracing.

Figure 1

Developmental origin of the cardiac conduction system components. (A) The mature cardiac conduction system (CCS, grey) consists of the sinoatrial node (SAN), the atrioventricular node (AVN) and ring bundles (AVRBs), the atrioventricular bundle (AVB), left and right bundle branches (LBB and RBB, respectively) and the peripheral ventricular conduction system (PVCS). The contributions of the first and second heart field (FHF and SHF, respectively) to the adult myocardium are visualized in pink (FHF-derived) and dark red (SHF-derived); (B) Cardiogenic mesodermal cells of the FHF differentiate and form the cardiac crescent (CC) at E7.5. The primary heart tube (PHT) forms through fusion of the cardiac crescent (E8) and subsequently forms the myocardium of the left ventricle and part of the atrioventricular canal (AVC). Cardiogenic mesodermal cells of the SHF are continuously added to the arterial and venous pole of the heart tube and form the other myocardial components of the heart. The locations of the progenitor cells of the CCS components are depicted by yellow circles for the SV/SAN, red circles for AVN/AVRBs and green circles for AVB/BBs. Abbreviations: IFT, inflow tract; IVR, interventricular ring, OFT, outflow tract; SV, sinus venosus.

Figure 2

Genetic fate mapping relies on recombinase-mediated excision to switch ‘On’ and label cells and their progeny. (A) The ubiquitously active reporter transgene consists of a STOP-gene flanked by two LoxP sites (‘floxed’ STOP-gene) and a gene encoding a marker. Cre recombinase recognizes the two LoxP sites and excises the STOP cassette. The reporter is in the ‘Off’ state, but after Cre-mediated excision switches to the ‘On’ state, expressing the marker and thereby labels the cell; (B) Schematic representation of genetic fate mapping. Only cells that express Cre during development will switch the reporter transgene to the ‘On’ state. As this is a heritable recombination event, all the descendants of these cells will be labeled likewise, independent of Cre expression, and can be characterized.

Figure 3

Retrospective clonal analysis using the transgene nLaacZ. (A) The nLaacZ transgene harbors an intergenic duplication resulting in a truncated coding sequence. A rare spontaneous recombination event removes the duplication, thereby generating a functional LacZ gene encoding nuclear localized β-galactosidase; (B) Schematic representation of nLaacZ-based retrospective clonal analysis visualizing one recombination event (2). Spontaneous recombination of the nLaacZ transgene can occur in any cell and at any time during development and labels a cell and its descendants. The descendants form a clone because they originate from the same cell. Clone size is proportional to the time between recombination and analysis. Labeling of a more undifferentiated cell early in development (1) results in a bigger clone consisting of several cell types compared to labeling at a later developmental stage in a more fate restricted cell (2 and 3). A temporal history of a lineage is reconstructed by characterizing multiple clones that differ in size, distribution and cell type.