Reciprocal repression between Isl1 and Nkx2-5 defines the zebrafish embryonic cardiac pacemaker domain

The cardiac pacemaker initiates each heartbeat and is essential for normal cardiac function. Although the gene regulatory network controlling pacemaker development has been extensively studied in mammals, it remains unclear whether the same regulatory hierarchy governs pacemaker development in other vertebrates. Here, we combined high-speed optical mapping with genetic analysis to define the gene regulatory network that establishes and maintains pacemaker identity in the zebrafish embryo. We show that the LIM-homeodomain transcription factor Isl1 is essential for maintaining pacemaker identity of venous pole cardiomyocytes. Loss of Isl1 causes pacemaker cells to acquire chamber-like characteristics through ectopic nkx2-5 expression, resulting in loss of a dominant pacemaker site and severe bradyarrhythmia. Reducing Nkx2-5 activity largely restores regular rhythm despite persistent loss of other Isl1-dependent markers, indicating that inappropriate Nkx2-5 activity contributes substantially to the electrophysiological defects in isl1 mutants. In contrast, genetic disruption of shox2 does not impair embryonic pacemaker development or alter isl1 expression, suggesting that Shox2 is not an essential component of the zebrafish pacemaker gene regulatory network. Together, our findings establish a zebrafish-specific transcriptional hierarchy in which Isl1 safeguards pacemaker identity by restricting Nkx2-5 independently of Shox2. These results show that a conserved boundary between pacemaker and chamber myocardium can be maintained by distinct upstream factors during vertebrate heart development.