Overexpression of Tbx20 in Adult Cardiomyocytes Promotes Proliferation and Improves Cardiac Function After Myocardial Infarction

Abstract

Background Adult mammalian cardiomyocytes (CM) have the potential to proliferate, but this is not sufficient to generate adequate CMs after myocardial infarction (MI). The transcription factor Tbx20 is required for CM proliferation during development and adult CM homeostasis. The ability of Tbx20 overexpression (Tbx20OE) to promote adult CM proliferation and improve cardiac function after MI was examined. Methods and Results Tbx20OE was induced specifically in adult mouse differentiated CMs. Increased CM proliferation and fetal-like characteristics were found in Tbx20OE hearts, compared to controls, without causing pathology 4 weeks after Tbx20OE at baseline. Moreover, Tbx20OE in adult CM after MI significantly improved survival, cardiac function, and infarct size 4 weeks post-MI. Improved cardiac repair, as indicated by increased CM proliferation and capillary density, was observed in the MI border zone of Tbx20OE hearts compared to controls. Expression of proliferation activator (cyclin D1, E1, and IGF1) and fetal contractile protein (ssTNI, βMHC) mRNA was increased, while negative cell-cycle regulators (p21, Meis1) were decreased, in Tbx20OE hearts, compared to controls, under both baseline and MI conditions. Tbx20OE in adult hearts activates multiple pro-proliferation pathways including Akt, YAP and BMP. Interestingly, p21, Meis1, and a novel cell-cycle inhibitory gene Btg2, are directly bound and repressed by Tbx20 with induction of proliferation in neonatal CM. Conclusions Tbx20OE specifically in adult CM activates multiple cardiac proliferative pathways, directly represses cell-cycle inhibitory genes p21, Meis1 and Btg2, promotes adult CM proliferation, and preserves cardiac performance post-MI.

Tbx20 drives cardiac progenitor formation and cardiomyocyte proliferation in zebrafish

“…These findings are in line with previous in vitro studies on neonatal rat cardiomyocytes and in vivo studies on mouse fetal hearts showing that Tbx20 overexpression increases cardiomyocyte proliferation and thickens the compact myocardium [15, 35]. While mammalian cardiomyocytes cease to proliferate soon after birth, Tbx20 overexpression is able to induce cardiomyocyte proliferation in adult mouse hearts and leads to the generation of new cardiomyocytes and improved cardiac repair after myocardial infarction [29, 35]. Tbx20’s potent ability to induce cardiomyocyte proliferation in multiple models, along with the finding that Tbx20 expression is reactivated in the regenerating adult zebrafish heart following injury [26, 36], point to the intriguing possibility of utilizing Tbx20 activation as a therapeutic strategy for cardiac repair.…”

“…Whether these direct targets, along with the pathways they regulate, are mediators of the cardiac expansion activity of Tbx20 in embryonic zebrafish will require further examination. Interestingly, Tbx20 has been shown to regulate adult mouse cardiomyocyte proliferation by directly repressing cell cycle inhibitors [29], suggesting that the mechanisms by which Tbx20 regulates its direct and downstream targets during development are context and stage-dependent. Future genome-wide analyses will help to reveal the complement of binding partners and gene targets involved in Tbx20-mediated cardiac expansion during zebrafish cardiogenesis.…”

Cardiomyocyte Proliferation

“…15 Our lab generated mice with Tbx20 overexpression in adulthood that exhibit increased cardiomyocyte proliferation and repair after injury, characterized by induction of multiple proliferative pathways, repression of inhibitory pathways, and increased fetal characteristics. 16 In the Tbx20 overexpressing mice, we saw alterations in Yap1, Akt and BMP signaling, but we still do not understand how the multiple pathways that control cardiomyocyte proliferation intersect. It is becoming increasingly clear that postnatal induction of cardiomyocyte proliferation requires both induction of proliferative pathways and repression of cell cycle inhibitory pathways.…”

Tbx20 Is an Essential Regulator of Embryonic Heart Growth in Zebrafish

“…Interestingly, in adult murine hearts, Tbx20 overexpression beginning in the fetal period promotes the activation of cardiac proliferative pathways such as BMP2/pSMAD1/5/8 and PI3K/AKT/GSK3β/β-catenin signaling, thereby maintaining adult cardiomyocyte in an immature state in vivo [35]. Very recently, Xiang et al were even able to show that Tbx20 overexpression significantly increased cardiomyocyte proliferation and cardiac repair after induced myocardial infarction (MI) [36]. …”

“…Tbx20 was recently shown to physically bind and thereby repress the cell cycle inhibitory proteins p21 and Meis1 as well as the antiproliferative protein Btg2, thereby activating cardiomyocyte proliferation [36]. Transcriptional profiling of Tbx20-deficient weiches herz mutant zebrafish hearts as well as hearts with stable, cardiomyocyte-specific overexpression of Tbx20 will help us to identify the Tbx20-dependent regulatory transcriptional network that controls cardiomyocyte proliferation.…”