Mapping cardiac innervation in the long QT syndrome type 1 transgenic mouse model using whole heart imaging

Cardiac innervation plays a crucial role in maintaining heart function. Abnormalities in cardiac innervation can be associated with arrhythmia, ischemic injury, and dysfunction, as documented in heart transplantation reports. There has been a lack of research on heart innervation patterns in congenital hereditary heart disease, including Long QT Syndrome (LQTS), which is a prevalent form of arrhythmia. By considering this gap, we comparatively analyzed global heart innervation patterns and axon fiber prevalence between wild-type (WT) and the Kcnq1^A340E/A340E mutation-bearing LQTS Type 1 transgenic mouse models. Hearts from WT and Kcnq1^A340E/A340E mice were immunostained with a pan-neuronal marker TUJ1 and imaged using the Lightsheet microscopy. The whole-heart images were processed and binarized to evaluate nerve fiber density, axon fiber diameter, focusing on fibers < 2.5 μm and > 2.5 μm on the dorsal and ventral sides of the heart, as well as branch number, length, and junction numbers. The comparative global innervation analysis of WT and Kcnq1^A340E/A340E transgenic mice hearts did not display a statistically significant difference in the TUJ1 immunoreactive nerve fiber density, analyzed by fluorescence intensity prevalence. Interestingly, the nerve fibers < 2.5 μm were detected to have a lower prevalence in Kcnq1^A340E/A340E mice compared to WT mice on both dorsal and ventral sides. Furthermore, the branch number, branch length, or junction number of global heart innervation between the experimental groups showed similar quantitative values. Notably, the overlay of innervation patterns within and between WT and Kcnq1^A340E/A340E mice hearts revealed a distinct fiber distribution pattern. These findings indicated a unique, fingerprint-like innervation pattern in each heart, independent of the Kcnq1 mutation. Collectively, our data indicated that the nerve fiber diameter distribution in the hearts of Kcnq1^A340E/A340E mice is slightly different from that of WT mice, and that there is a unique innervation pattern in each heart, similar to a heartprint, regardless of the mutation. Deciphering the underlying mechanisms behind ion channel mutations and cardiac innervation patterns by analyzing distinct congenital cardiac diseases awaits future investigation.