Transdifferentiation: Akting Like a More Mature Cardiomyocyte

by Cristina Harmelink, Ph.D. - Hatzopoulos Hub Site 11

The World Health Organization lists heart disease as the main cause of death worldwide. In the United States, the American Heart Association estimates that annually almost 1 million people will have a myocardial infarction and roughly 5.7 million people will have heart failure. 1 Both lead to cardiac myocyte death and reduced heart contractility.

In 2012, PCBC (Schneider Hub Site 12 and Srivastava Hub Site 15) investigators have discovered that a resident population of fibroblasts within the adult mouse heart is amenable to in vivo cell fate manipulation. 2, 3 For direct cardiac reprogramming to achieve its therapeutic potential, rapid and efficient production of a large number of completely reprogrammed cardiac-like myocytes must be accomplished.

En route to a more translational application, Zhou et al (Schneider Hub Site 12) tackle some of these challenges in their recent PNAS article by improving upon their lab’s established protocol of reprogramming fibroblasts into induced cardiac-like myocytes (iCMs). In the standard protocol, retroviral expression of four developmentally critical cardiac transcription factors (Gata4, Hand2, Mef2c, and Tbx5; GHMT) drives what once would be considered terminally differentiated fibroblasts toward a cardiac fate both in vitro and in vivo.2   Now, Zhou et al demonstrate that Akt1 not only augments the GHMT speed and efficiency of fibroblast-to-iCM conversion, the resulting iCMs have molecular, morphological, behavioral, and metabolic characteristics more akin to mature cardiomyocytes. Specifically, addition of Akt1 to the GHMT protocol (AGHMT) promotes expression of cardiac structural proteins and an overall transcriptional profile more similar to that of adult mouse ventricular cardiomyocytes. Moreover, AGHMT-derived iCMs are larger, with more bi- or –multinucleated cells and increased mitochondrial activity. Striated muscle fibers, spontaneous beating, and increased calcium flux provide evidence of functional sarcomeres and coordinated calcium signaling. These beating iCMs are also responsive to beta adrenergic stimulation and antagonism. 4

 

Delving into the mechanisms behind AGHMT reprogramming, Zhou et al find that Akt1 appears to enhance the effects of GHMT by mediating IGF1, with mTORC1 and FOXO3A as downstream modulators.4  Biologically this makes sense because myocardial AKT activity is downstream of multiple paracrine signaling cascades that cooperatively regulate chamber formation during cardiogenesis. 5 Perhaps inclusion of other protein kinases regulated by myocyte:non-myocyte interactions would synergize with AGHMT (e.g. ERK1/2). They screened 192 protein kinases to identify AKT and I imagine they found additional promising candidates to combine with the transdifferentiation protocol. This may also help to more completely erase the molecular history of what once was a fibroblast, as the RNAseq data reveal that AGHMT iCMs still maintain elevated expression of some fibrotic genes. This may not be of consequence since the iCMs are behaving very much like mature cardiomyocytes, but at the very least it suggests that the reprogramming protocol can be further optimized. 

It will be interesting to see what effects the AGHMT cocktail has when delivered to mouse hearts post-myocardial infarction. Can Akt expression be controlled to promote cardioprotection and prevent pathological hypertrophy as seen in other studies with prolonged activity? 6  And would AKT also be beneficial in strategies to convert human fibroblasts into therapeutically viable cardiomyocyte-like cells?

For those, like me, who are relatively new to the ins and outs of direct reprogramming and are interested in reading more about it, I found the following reviews helpful: 

Mending broken hearts: cardiac development as a basis for adult regeneration and repair. Mei Xin, Eric N. Olson, and Rhonda Bassel-Duby. Nat Rev Mol Cell Biol. 2013 August ; 14(8): 529-541.

Direct Cardiac Reprograming: Progress and Challenges in Basic Biology and Clinical Applications. Taketaro Sadahiro, Shinya Yamanaka, and Masaki Ieda. Circ Res. 2015; 116:1378-1391.

Citations:

1. Mozaffarian D, Benjamin EJ, Go AS, Arnett DK, Blaha MJ, Cushman M, de Ferranti S, Després JP, Fullerton HJ, Howard VJ, Huffman MD, Judd SE, Kissela BM, Lackland DT, Lichtman JH, Lisabeth LD, Liu S, Mackey RH, Matchar DB, McGuire DK, Mohler ER, Moy CS, Muntner P, Mussolino ME, Nasir K, Neumar RW, Nichol G, Palaniappan L, Pandey DK, Reeves MJ, Rodriguez CJ, Sorlie PD, Stein J, Towfighi A, Turan TN, Virani SS, Willey JZ, Woo D, Yeh RW, Turner MB, Subcommittee AHASCaSS. Heart disease and stroke statistics--2015 update: A report from the american heart association. Circulation. 2015;131:e29-322

2. Song K, Nam YJ, Luo X, Qi X, Tan W, Huang GN, Acharya A, Smith CL, Tallquist MD, Neilson EG, Hill JA, Bassel-Duby R, Olson EN. Heart repair by reprogramming non-myocytes with cardiac transcription factors. Nature. 2012;485:599-604

3. Qian L, Huang Y, Spencer CI, Foley A, Vedantham V, Liu L, Conway SJ, Fu JD, Srivastava D. In vivo reprogramming of murine cardiac fibroblasts into induced cardiomyocytes. Nature. 2012;485:593-598

4. Zhou H, Dickson ME, Kim MS, Bassel-Duby R, Olson EN. Akt1/protein kinase b enhances transcriptional reprogramming of fibroblasts to functional cardiomyocytes. Proc Natl Acad Sci U S A. 2015;112:11864-11869

5. Tian Y, Morrisey EE. Importance of myocyte-nonmyocyte interactions in cardiac development and disease. Circ Res. 2012;110:1023-1034

6. Abeyrathna P, Su Y. The critical role of akt in cardiovascular function. Vascul Pharmacol. 2015

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