In vitro Assessment of Cardiac Reprogramming by Measuring Cardiac Specific Calcium Flux with a GCaMP3 Reporter.

Cardiac reprogramming has become a potentially promising therapy to repair a damaged heart. By introducing multiple transcription factors, including Mef2c, Gata4, Tbx5 (MGT), fibroblasts can be reprogrammed into induced cardiomyocytes (iCMs). These iCMs, when generated in situ in an infarcted heart, integrate electrically and mechanically with the surrounding myocardium, leading to a reduction in scar size and an improvement in heart function. Because of the relatively low reprogramming efficiency, purity, and quality of the iCMs, characterization of iCMs remains a challenge. The currently used methods in this field, including flow cytometry, immunocytochemistry, and qPCR, mainly focus on cardiac-specific gene and protein expression but not on the functional maturation of iCMs. Triggered by action potentials, the opening of voltage-gated calcium channels in cardiomyocytes leads to a rapid influx of calcium into the cell. Therefore, quantifying the rate of calcium influx is a promising method to evaluate cardiomyocyte function. Here, the protocol introduces a method to evaluate iCM function by calcium (Ca) flux. An αMHC-Cre/Rosa26A-Flox-Stop-Flox-GCaMP3 mouse strain was established by crossing Tg(Myh6-cre)1Jmk/J (referred to as Myh6-Cre below) with Gt(ROSA)26Sor/J (referred to as Rosa26A-Flox-Stop-Flox-GCaMP3 below) mice. Neonatal cardiac fibroblasts (NCFs) from P0-P2 neonatal mice were isolated and cultured in vitro, and a polycistronic construction of MGT was introduced to NCFs, which led to their reprogramming to iCMs. Because only successfully reprogrammed iCMs will express GCaMP3 reporter, the functional maturation of iCMs can be visually assessed by Ca flux with fluorescence microscopy. Compared with un-reprogrammed NCFs, NCF-iCMs showed significant calcium transient flux and spontaneous contraction, similar to CMs. This protocol describes in detail the mouse strain establishment, isolation and selection of neonatal mice hearts, NCF isolation, production of retrovirus for cardiac reprogramming, iCM induction, the evaluation of iCM Ca flux using our reporter line, and related statistical analysis and data presentation. It is expected that the methods described here will provide a valuable platform to assess the functional maturation of iCMs for cardiac reprogramming studies.