Abstract Body (Do not enter title and authors here): Background: Myocardial infarction (MI) initiates an injury response that triggers fibroblast to myofibroblast cell transition and pathological fibrosis, a substrate for arrhythmogenesis. However, there are no methods to image in-vivo development of fibrosis continuously in the murine model other than cardiac MRI which is costly and time-consuming.

Objectives: We aimed to develop a novel 3D method to track myofibroblast activity in the heart in vivo and correlate with electrophysiological changes using optical mapping.

Methods: Mice with the alpha-Smooth Muscle Actin (α-SMA) promoter for expression of the red fluorescent protein DsRed were used to label myofibroblasts. MI was induced by left anterior descending artery ligation in α-SMA-RFP (n=4) and control C57BL/6 mice (n=5). RFP fluorescence was monitored over time using a PerkinElmer® In Vivo Imaging System (IVIS) before and after MI. 3D IVIS imaging with micro-computed tomography (CT) of 3D cardiac structure was then performed. Ex-vivo heart imaging of RFP followed by 3D panoramic optical mapping of electrical activation via Di-4-ANEPPS was performed and action potential duration (APD) maps generated.

Results: After MI α-SMA-RFP mice demonstrate strong myofibroblast RFP signal localized above the heart on IVIS imaging at varying time points from 9 days to 3 months after MI. Myofibroblast RFP signal is not present before MI, and is also absent in control C57 mice. Ex-vivo imaging of hearts confirms myofibroblast RFP signal correlates with the site of infarction as well as increased APD. In α-SMA-RFP mice myofibroblast RFP intensity at the fibrotic infarct site is significantly greater than non-infarcted sites (mean difference 52.9 ± 13.2, p = 0.0088). Control C57 hearts do not demonstrate preferential RFP signal at infarct sites (mean difference -3.28 ± 2.99, p = 0.29). Furthermore, 3D IVIS imaging with CT co-registration demonstrates the localization of RFP signal in space near the heart.

Conclusion: IVIS imaging of α-SMA-RFP is able to track myofibroblast activity in living mice up to three months after MI, and can be combined with optical mapping to study electrophysiology (APD, phase, activation maps). This novel approach can offer a method to monitor myofibroblast dynamics in disease states and test treatments that reduce the pathologic myofibroblast persistence that leads to fibrosis, scarring and arrhythmogenesis.