Meis1 Regulates Postnatal Cardiomyocyte Cell Cycle Arrest
The neonatal mammalian heart is capable of substantial regeneration following injury through cardiomyocyte proliferation (Porrello et al, Science 331:1078–1080, 2011; Proc Natl Acad Sci U S A 110:187–92, 2013). However, this regenerative capacity is lost by postnatal day 7 and the mechanisms of cardiomyocyte cell cycle arrest remain unclear. The homeodomain transcription factor Meis1 is required for normal cardiac development but its role in cardiomyocytes is unknown (Paige et al, Cell 151:221–232, 2012; Wamstad et al, Cell 151: 206–220, 2012). Here we identify Meis1 as a critical regulator of the cardiomyocyte cell cycle. Meis1 deletion in mouse cardiomyocytes was sufficient for extension of the postnatal proliferative window of cardiomyocytes and for reactivation of cardiomyocyte mitosis in the adult heart with no deleterious effect on cardiac function. In contrast, overexpression of Meis1 in cardiomyocytes decreased neonatal myocyte proliferation and inhibited neonatal heart regeneration. Finally, we show that Meis1 is required for transcriptional activation of the synergistic CDK inhibitors p15, p16, and p21. These results identify Meis1 as a critical transcriptional regulator of cardiomyocyte proliferation and a potential therapeutic target for heart regeneration.
KeywordsCardiomyocytes Heart injury Regeneration Meis1 Cell cycle
The past decade witnessed a revolution in our understanding of cardiac biology, with groundbreaking research demonstrating that the adult mammalian heart is capable of limited but measurable cardiomyocyte turnover [5, 6, 7, 8, 9]. However, the ultimate goal of complete regeneration of the heart remains elusive. In stark contrast to the adult mammalian heart, we recently demonstrate that the mammalian heart is in fact capable of complete regeneration following apical resection of 15 % of the ventricular myocardium . This remarkable regenerative capacity was associated with robust cardiomyocyte proliferation throughout the myocardium. Moreover, lineage-tracing studies demonstrated that the newly f ormed myocytes were derived from preexisting cardiomyocytes, rather than from a progenitor population. Finally, we showed that cessation of this regenerative phenomenon occurred at postnatal day 7 (P7), which coincides with the developmental window when cardiomyocytes become binucleate and withdraw from the cell cycle [9, 10]. It is unclear whether the loss of this regenerative potential in the adult heart is due to an intrinsic cell cycle block in adult cardiomyocytes or to loss of mitogenic stimuli as the heart ages (or both). Thus, it is important to determine the mechanisms by which the mammalian heart switches off this regenerative capacity in the week after birth.
In an effort to identify genes involved in postnatal regeneration arrest, we performed several gene arrays following MI at multiple postnatal time points. This allowed us to identify Meis1 as one of the few transcription factors that were dysregulated between injury at P1 and injury at P7 and P14. Meis1 has been studied extensively in the hematopoietic system, is required for normal hematopoiesis, and also plays an important role in leukemogenic transformation [11, 12, 13, 14]. What little is known about the role of Meis1 in the heart comes from global KO studies resulting in numerous cardiac defects. However, given that global Meis1 deletion results in embryonic lethality by E14.5, full characterization of the role of Meis1 in cardiomyocytes has been difficult [13, 15, 16]. Despite the role of Meis1 in regulation of hematopoiesis and cardiac development, the mechanism of action of Meis1 remains poorly characterized. Our results indicate that Meis1 expression and nuclear localization in cardiomyocytes coincide with cell cycle arrest. Cardiomyocyte cell cycle exit is associated with downregulation of positive cell cycle regulators (CDK2, CDK3, CDK4, CCND1, and CDK cofactors) and induction of cell cycle inhibitors (CDKI, members of the INK4 and CIP/KIP families) [10, 17, 18, 19, 20]. We identified conserved Meis1 domains in only two key CDK inhibitors, namely, p16 and p21, which are known to regulate all three cell cycle checkpoints. We demonstrated that Meis1 regulates the pattern of expression of these two cell cycle inhibitors following Meis1 knockdown. These results provide role and mechanism of cell cycle regulation by Meis1.
11.2.1 Expression of Meis1 During Neonatal Heart Development and Regeneration
11.2.2 Cardiomyocyte Proliferation Beyond Postnatal Day 7 Following Meis1 Deletion
11.2.3 MI in Meis1 Overexpressing Heart Limits Neonatal Heart Regeneration
11.2.4 Regulation of Cyclin-Dependent Kinase Inhibitors by Meis1
11.3 Future Direction and Clinical Implications
The current study identifies Meis1 as a critical transcriptional regulator of cardiomyocyte cell cycle, upstream of two synergistic CDKI inhibitors. Although the mechanism of activation of Meis1 in the postnatal heart is not quite fully understood, results have demonstrated that Meis1 expression coincides with Hox genes that are known to interact with Meis1, stabilize its DNA binding, and enhance its transcriptional activity. Therefore, it would be important for future studies to define the transcriptional network involved in mediating the effect of Meis1 on postnatal cardiomyocytes. Ultimately, we hope to utilize our understanding of the role and mechanism of Meis1 in cardiomyocyte proliferation to uncover new disease mechanisms and therapeutic approaches for cardiovascular diseases.
We apologize to whose work was not cited in this chapter and for the omission of some discussion points owing to space constraints. This work was supported by grants from the AHA (Grant in Aid) (Sadek), the Gilead Research Scholars Prog ram in Cardiovascular Disease (Sadek), Foundation for Heart Failure Research, NY, and the NIH (1R01HL115275-01) (Sadek).
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