Direct Reprogramming of Fibroblasts into Functional Cardiomyocytes

Background: Fibroblasts constitute nearly 50% of the cell mass of the normal heart, though these cells do not contribute to contractility.  Fibroblasts are generally considered terminally differentiated cells incapable of further differentiation into another cell type.

Methods/Results: Microarray analysis was used to identify genes present in rat neonatal cardiac cells, and high-throughput screens identified 3 genes (Gata4, Mef2c, and Tbx5; GMT) capable of inducing expression of cardiomyocyte-specific target genes.  When isolated transgenic rat fibroblasts expressing a GFP expression vector were then exposed to GMT, they were able to express GFP and target genes (including the troponin cTnT, ryanodine receptor Ryr2, and connexin Gja1) present in normal adult cardiomyocytes but not adult fibroblasts.  Importantly, genes present in embryonic myocytes were not induced in fibroblasts, suggesting that it was not necessary for these cells to revert back to pluripotent stems for differentiation.  Mechanistic expression analysis showed that the GMT combination induced transcription by a demethylation.  To confirm that the mechanism was conversion of existing fibroblasts and not activation of rare cardiac stem cells, the authors induced identical changes in dermal fibroblasts.  Finally, fibroblasts induced by GMT were also shown to be spontaneously contractile.  When GMT was infused in vivo into rat hearts, fibroblasts in heart tissue were shown to develop the induced cardiomyocyte phenotype. 

Implications:Evidence exists that damaged myocardium may be replaced when infusion of stem cells.  These data suggest that endogenous cells may instead by activated to replace tissue by infusion of GMT.  Additional advantages is avoidance of tumor formation seen with direct stem cell infusions.  Large amounts of fibroblasts can be easily cultured from endocardial biopsy by routine cell culture techniques, and reprogrammed cells can infused back into the heart without concern for donor mismatch.  The presence of abundant fibroblasts in cardiac tissue ensures a large reservoir of potential progenitor cells. 

Yoshida Y. Recent stem cell advances: induced pluripotent stem cells for disease modeling and stem cell-based regeneration. Circulation 2010 Jul 6; 122(1): 80-7.