Atrial fibrillation (AF) is the most common cardiac arrhythmia and is associated with significant morbidity (1
). Most individuals with AF have underlying cardiac disease, but approximately 30%–45% with paroxysmal AF and 20%–25% with persistent AF are younger patients with no cardiac abnormalities (5
). Haissaguerre et al. identified ectopic beats arising from the pulmonary veins (PVs) as initiators or arrhythmogenic triggers of AF (7
) which led to PV isolation as an effective, empiric therapy for drug-refractory AF. However, PV isolation is associated with complications and risks, including PV reconnection, which reduces its efficacy (8
). On the other hand, pharmacologic anti-arrhythmic therapy for AF is often ineffective and associated with burdensome side effects (11
While most investigations to date have focused on the contribution of PV myocytes to AF triggers, some evidence supports the existence of nonmyocardial cells within PVs, which may also contribute to these triggers. Recently, interstitial Cajal-like cells have been identified within human PVs (14
). However, the direct contribution of such cells to atrial arrhythmogenesis has not been demonstrated. Still, the implications of identifying a unique cell population that may contribute to atrial arrhythmias are significant, as they may unmask a previously unknown source or mechanism of arrhythmogenesis. In addition to descriptions of new cell types that may contribute to atrial arrhythmias, emerging evidence supports the contribution of atrial ectopy outside the thoracic veins to atrial arrhythmias (16
). Since atrial myocytes do not possess the same cellular electrophysiological properties as those found in PV myocytes, the theories proposed to explain the initiation of PV ectopy are unlikely to account for extrapulmonary vein foci. Therefore, the existence of a distinct cell population that may contribute to atrial triggers from the PVs and extrapulmonary foci may help explain this apparent inconsistency.
Pigmented cells within the murine heart have been observed previously (18
), though their function remains unknown. Interestingly, dermal melanocytes and melanoma lines do express voltage-dependent currents (22
) and under conditions of increased oxidative stress some ionic currents are modified to promote cellular excitability (24
). In the skin, reactive intermediaries such as 5,6-dihydroxyindole (DHI) (27
) are formed in the pheomelanin synthesis pathway. On the other hand, 5,6-dihydroxyindole-2-carboxylic acid (DHICA), which is thought to be less reactive than DHI, is formed in the eumelanin synthesis pathway, diminishing the formation of reactive species (28
). The melanin synthesis enzyme dopachrome tautomerase (Dct) contributes to, but is not required for, melanin synthesis in dermal melanocytes by catalyzing the conversion of L-dopachrome to DHICA (29
). In the absence of Dct, there is decreased DHICA formation with concomitant DHI build up, which promotes the accumulation of reactive species (30
). Furthermore, Dct is important for intracellular calcium regulation; in fact, eumelanin binds calcium with an affinity similar to calmodulin (32
). Given that dysregulation of calcium and reactive species have been described in patients with AF (33
), and the fact that Dct is involved in both calcium handling and reactive species generation, we sought to investigate the role of Dct and Dct-expressing cells in atrial arrhythmogenesis.
In the present study, we characterized Dct-expressing cells in the murine and human heart. Single-cell transcriptional profiling showed that murine Dct-expressing cells in the heart more closely resemble atrial myocytes than dermal melanocytes. Mice lacking functional DCT retained melanocyte-like cells in the heart and were susceptible to atrial arrhythmias. By contrast, animals lacking melanocyte-like cells in the heart failed to demonstrate atrial arrhythmias, even if they also lacked Dct. Isolated cardiac melanocytes were excitable, with similar action potentials to atrial myocytes. However, in the absence of Dct, cardiac melanocytes displayed prolonged repolarization with early afterdepolarizations and frequent calcium oscillations. Dct-positive cells in the murine atrium expressed adrenergic and muscarinic receptors and appeared to be innervated by autonomic nerves. Furthermore, treatment with muscarinic agonists resulted in fewer atrial arrhythmia episodes in mice that lacked cardiac melanocyte-like cells than in littermates that retained cardiac melanocytes, suggesting that the presence of these cells promotes arrhythmogenesis. β-Adrenergic antagonists reduced atrial arrhythmias to a larger extent in mice with cardiac melanocytes, further demonstrating the importance of autonomic regulation of this cell population. Treatment of mice lacking Dct with antioxidants also significantly reduced atrial arrhythmias. These findings support a potential connection between Dct-expressing cells, the clinical syndrome of atrial ectopy initiating AF, autonomic dysregulation, and oxidative stress.