505 Conversion of Human Fibroblasts Into Functional Cardiomyocytes by Small Molecules Cao et al Science. 2016;352:1216–1220. C onverting cardiac fibroblasts to cardiomyocytes has been considered as a regenerative strategy for myo- cardial infarction and other disorders. Recently, Cao et al 1 defined a cocktail of 9 chemical compounds with this capa- bility, increasing the likelihood of clinical success. It has taken a while, but the concept that the terminally differentiated state is immutably stable no longer dominates modern biology. In 1938, Hans Spemann contemplated a fan- tastical experiment in which transfer of an egg nucleus could redirect a recipient somatic cell to become pluripotent, and Gurdon 2 made this a reality in 1958 by converting gut epitheli- al cells into whole frogs. Inspired by Gurdon et al, 2 Takahashi and Yamanaka 3 developed the induced pluripotent stem cell (iPSC) technology where 4 transcription factors sufficed in generating PSCs. The last decade has witnessed an explosion in cell fate manipulations. Many somatic cell types were inter- converted by the means of transcription factors and epigenetic and cellular pathway modulators. As one of the most difficult cells to regenerate, cardiomyocyte was successfully generated from fibroblasts by introducing cocktails of transcriptions factors. 4–7 In a recent article in Science, Cao et al 1 reported the successful reprograming of functional cardiomyocytes by small molecules and growth factors. Cao et al 1 devised a 3-step protocol. Human foreskin fi- broblasts or human fetal lung fibroblasts were first treated with 9 compounds (9C: CHIR99021, A83-01, BIX01294, AS8351, SC1, Y27632, OAC2, SU16F, and JNJ10198409) that were the outcome of multiple rounds of focused screen- ing. Next, the cells were incubated with a cardiac induction medium containing previously known cardiogenic factors activin A, BMP4, VEGF, and CHIR99021. Finally, human cardiomyocyte-conditioned medium was used to aid matura- tion. At day 30, 6.6±0.4% cells in the culture were cardiac troponin T positive, a remarkable improvement in efficiency over transcription factor–mediated reprogramming (Figure). The chemically converted cells in many ways resembled CMs derived from human PSCs. They displayed sarcomeric struc- tures, epigenetic signatures, transcriptomes, and electrophysi- ological properties comparable to early-stage cardiomyocytes. This all-chemical approach represents a major advance for cardiac regeneration. From a cell therapy perspective, it is ap- pealing because it circumvents inadvertent genomic modifica- tions that can occur with genetic methods, and the chemical compounds themselves are nonimmunogenic. Other poten- tial advantages include less variability, tighter dose control, and greater cost-effectiveness if translated clinically. From a chemical biology perspective, a deeper understanding of the protein targets of the compounds and the signals they evoke constitute a new angle on understanding cardiac differentia- tion and reprogramming mechanisms. Mechanism of Cardiac Specification 9C appeared to operate exclusively in the first step of repro- gramming. Treatment led to decondensation of closed chroma- tin regions, which was apparent by a decrease in H3K9me3, HP1γ, H3K27me3 marks and an increase in H3K4me3 and H3K27ac marks. These changes correlated with induction of genes that mark embryonic mesoderm, including EOMES, T, MESP1, and KDR that, in the context of an embryo, are expressed before the appearance of cardiac cells. A more de- tailed characterization of the cells induced by 9C will answer whether the cocktail directs fibroblasts to a progenitor state, such as the transient extraembryonic endoderm (XEN)-like cells that appear during chemical reprogramming toward iPSCs. 8 Another important question is whether 9C narrowly specifies cardiac fate or makes cells capable of following mul- tiple lineages, such as mesendoderm progenitors created by genetic reprogramming with MESP1. 9 Such breadth would make 9C broadly important for tissue regeneration. The efficiency of reprogramming was low (≈7%), raising the question of whether 9C acts deterministically, but only a few cells in the starting population are competent to respond, versus whether directed differentiation is stochastic and car- ries a low probability of overall success. In either case, defin- ing the cell types that are competent to respond will also be important for translation. Mechanisms of Chemical Reprogramming All-chemical cocktails have been reported to convert so- matic cell into other cell types, including PSCs, neurons, The All-Chemical Approach A Solution for Converting Fibroblasts Into Myocytes Yu Liu, Mark Mercola, Robert J. Schwartz (Circ Res. 2016;119:505-507. DOI: 10.1161/CIRCRESAHA.116.309146.) © 2016 American Heart Association, Inc. Circulation Research is available at http://circres.ahajournals.org DOI: 10.1161/CIRCRESAHA.116.309146 Commentaries on Cutting Edge Science The opinions expressed in this Commentary are not necessarily those of the editors or of the American Heart Association. Commentaries serve as a forum in which experts highlight and discuss articles (published here and elsewhere) that the editors of Circulation Research feel are of particular significance to cardiovascular medicine. Commentaries are edited by Aruni Bhatnagar & Ali J. Marian. From the Department of Biology and Biochemistry, University of Houston, TX (Y.L., R.J.S.); Department of Medicine and Cardiovascular Institute, Stanford University School of Medicine, Palo Alto, CA (M.M.); and Texas Heart Institute, Texas Medical Center, Houston (R.J.S.). Correspondence to Robert J. Schwartz, PhD, Department of Biology and Biochemistry, and The University of Houston, 4800 Calhoun Rd, Houston, TX 77204. E-mail rjschwartz@uh.edu by guest on September 1, 2017 http://circres.ahajournals.org/ Downloaded from