Project description:Direct conversion of fibroblasts to induced cardiomyocytes (iCMs) has great potential for regenerative medicine. Recent publications have reported significant progress, but the evaluation of reprogramming has relied upon non-functional measures such as flow cytometry for cardiomyocyte markers or GFP expression driven by a cardiomyocyte-specific promoter. The issue is one of practicality: the most stringent measures - electrophysiology to detect cell excitation and the presence of spontaneously contracting myocytes - are not readily quantifiable in the large numbers of cells screened in reprogramming experiments. However, excitation and contraction are linked by a third functional characteristic of cardiomyocytes: the rhythmic oscillation of intracellular calcium levels. We set out to optimize direct conversion of fibroblasts to iCMs with a quantifiable calcium reporter to rapidly assess functional transdifferentiation. We constructed a reporter system in which the calcium indicator GCaMP is driven by the cardiomyocyte-specific Troponin T promoter. Using calcium activity as our primary outcome measure, we compared several published combinations of transcription factors along with novel combinations in mouse embryonic fibroblasts. The most effective combination consisted of Hand2, Nkx2.5, Gata4, Mef2c, and Tbx5 (HNGMT). This combination is >50-fold more efficient than GMT alone and produces iCMs with cardiomyocyte marker expression, robust calcium oscillation, and spontaneous beating that persists for weeks following inactivation of reprogramming factors. HNGMT is also significantly more effective than previously published factor combinations for the transdifferentiation of adult mouse cardiac fibroblasts to iCMs. Quantification of calcium function is a convenient and effective means for the identification and evaluation of cardiomyocytes generated by direct reprogramming. Using this stringent outcome measure, we conclude that HNGMT produces iCMs more efficiently than previously published methods.

Project description:Background: Direct cardiac reprogramming of fibroblasts into cardiomyocytes has emerged as one of the promising strategies to remuscularize the injured myocardium. Yet, it is still insufficient to generate functional induced cardiomyocytes (iCMs) from human fibroblasts using conventional reprogramming cocktails, such as our previously published combination consisting of MEF2C, GATA4, TBX5 and microRNA miR-133 (MGT133). Results: To discover potential missing factors for human direct reprogramming, we performed transcriptomic comparison between human iCMs and functional cardiomyocytes (CMs). We identified T-box transcription factor TBX20 as the top CM gene that is unable to be activated by MGT133. TBX20 is required for normal heart development and cardiac function in adult CMs but its role on cardiac reprogramming remains undefined. Here, we found that transduction of MGT133+TBX20 in human cardiac fibroblasts resulted in enhanced reprogramming featured with significantly activated contractility gene programs and signatures more similar to ventricular CMs. Human iCMs produced with MGT133+TBX20 more frequently demonstrated beating and calcium oscillation in co-culture with pluripotent stem cell derived CMs. More mitochondria and higher mitochondrial respiration were also detected in iCMs after TBX20 overexpression. Mechanistically, comprehensive transcriptomic, chromatin occupancy and epigenomic integration revealed that TBX20 localized to the cis-regulatory enhancers of under-expressed cardiac genes, such as MYBPC3, MYH7 and MYL4, to activate gene expression via strengthening the occupancy and co-occupancy of transcription factors. Furthermore, we identified TBX20-regulated enhancers and confirmed the synergistic effect of MGT and TBX20 on enhancer activation. Conclusions: TBX20 promotes cardiac cell fate conversion via direct activating cardiac enhancers. Human iCMs generated with TBX20 showed enhanced cardiac function in terms of contractility and mitochondrial respiration.

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Optimization of Direct Fibroblast Reprogramming to Cardiomyocytes Using Calcium Activity as a Functional Measure of Success

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