Constitutive cardiomyocyte proliferation in the leopard gecko (Eublepharis macularius)

Although the contractile function of the heart is universally conserved, the organ itself varies in structure across species. This variation includes the number of ventricular chambers (one, two, or an incompletely divided chamber), the structure of the myocardial wall (compact or trabeculated), and the proliferative capacity of the resident cardiomyocytes. Whereas zebrafish are capable of comparatively high rates of constitutive cardiomyocyte proliferation, humans and rodents are not. However, for most species, the capacity to generate new cardiomyocytes under homeostatic conditions remains unclear. Here, we investigate cardiomyocyte proliferation in the lizard Eublepharis macularius, the leopard gecko. As for other lizards, the leopard gecko heart has a partially septated ventricular lumen with a trabeculated myocardial wall. To test our hypothesis that leopard gecko cardiomyocytes routinely proliferate, we performed 5-bromo-2'-deoxyuridine incorporation and immunostained for the mitotic marker phosphorylated histone H3 (pHH3) and the DNA synthesis phase (S phase) marker proliferating cell nuclear antigen (PCNA). Using double immunofluorescence, we co-localized pHH3 or PCNA with the cardiomyocyte marker myosin heavy chain (MHC). We found that ~0.5% of cardiomyocytes were mitotically active (pHH3+/MHC+), while ~10% were in S phase (PCNA+/MHC+). We also determined that cell cycling by gecko cardiomyocytes is not impacted by caudal autotomy (tail loss), a dramatic form of self-amputation. Finally, we show that populations of cardiac cells are slow cycling. Overall, our findings provide predictive evidence that geckos may be capable of spontaneous cardiac self-repair and regeneration following a direct injury.