Defining Cardiac Innervation and Reinnervation During Cardiac Injury

Cardiac nerves regulate essential roles in development, homeostasis, and repair. Yet, the precise architecture of the intact, cardiac nervous system has not been rigorously analyzed. Our goal is to model the intact, mammalian cardiac nervous system and neurovascular network in high-resolution using lineage tracing, tissue clearing, whole-mount imaging, and 3D modeling. Excitingly, our results identify extensive parasympathetic innervation in the cardiac ventricles, challenging the clinical misconception that parasympathetic nerves only innervate the cardiac nodes and are void from the ventricles. Moreover, we demonstrate that parasympathetic and sympathetic nerve axons develop synchronously and are intertwined throughout the ventricles. The cardiac neuronal networks are prone to causing pathology and fatal arrhythmias following an adult myocardial infarction. Interestingly, an infarction in the neonatal mouse results in robust heart regeneration with restored autonomic functions. In this study, we demonstrate that precise reinnervation occurs in the regenerating heart following injury, in stark contrast to the non-regenerating hearts. Remarkably, the regenerating myocardium reestablishes parasympathetic and sympathetic axon bundling, which we define in this study. Mechanistically, we demonstrate that this neuroplasticity is dependent on collateral artery formation, which precedes reinnervation during regeneration. These novel discoveries provide evidence that physiological reinnervation occurs uniquely during neonatal heart regeneration.