Shalini Muralidhar

Meis1 regulates postnatal cardiomyocyte cell cycle arrest

The neonatal mammalian heart is capable of substantial regeneration following injury through cardiomyocyte proliferation. 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. 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 re-activation 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.

The Oxygen-Rich Postnatal Environment Induces Cardiomyocyte Cell-Cycle Arrest through DNA Damage Response

The mammalian heart has a remarkable regenerative capacity for a short period of time after birth, after which the majority of cardiomyocytes permanently exit cell cycle. We sought to determine the primary postnatal event that results in cardiomyocyte cell-cycle arrest. We hypothesized that transition to the oxygen-rich postnatal environment is the upstream signal that results in cell-cycle arrest of cardiomyocytes. Here, we show that reactive oxygen species (ROS), oxidative DNA damage, and DNA damage response (DDR) markers significantly increase in the heart during the first postnatal week. Intriguingly, postnatal hypoxemia, ROS scavenging, or inhibition of DDR all prolong the postnatal proliferative window of cardiomyocytes, whereas hyperoxemia and ROS generators shorten it. These findings uncover a protective mechanism that mediates cardiomyocyte cell-cycle arrest in exchange for utilization of oxygen-dependent aerobic metabolism. Reduction of mitochondrial-dependent oxidative stress should be an important component of cardiomyocyte proliferation-based therapeutic approaches.

Hypoxia fate mapping identifies cycling cardiomyocytes in the adult heart

Although the adult mammalian heart is incapable of meaningful functional recovery following substantial cardiomyocyte loss, it is now clear that modest cardiomyocyte turnover occurs in adult mouse and human hearts, mediated primarily by proliferation of pre-existing cardiomyocytes. However, fate mapping of these cycling cardiomyocytes has not been possible thus far owing to the lack of identifiable genetic markers. In several organs, stem or progenitor cells reside in relatively hypoxic microenvironments where the stabilization of the hypoxia-inducible factor 1 alpha (Hif-1α) subunit is critical for their maintenance and function. Here we report fate mapping of hypoxic cells and their progenies by generating a transgenic mouse expressing a chimaeric protein in which the oxygen-dependent degradation (ODD) domain of Hif-1α is fused to the tamoxifen-inducible CreERT2 recombinase. In mice bearing the creERT2-ODD transgene driven by either the ubiquitous CAG promoter or the cardiomyocyte-specific α myosin heavy chain promoter, we identify a rare population of hypoxic cardiomyocytes that display characteristics of proliferative neonatal cardiomyocytes, such as smaller size, mononucleation and lower oxidative DNA damage. Notably, these hypoxic cardiomyocytes contributed widely to new cardiomyocyte formation in the adult heart. These results indicate that hypoxia signalling is an important hallmark of cycling cardiomyocytes, and suggest that hypoxia fate mapping can be a powerful tool for identifying cycling cells in adult mammals.

Hypoxic metabolism in human hematopoietic stem cells

BackgroundAdult hematopoietic stem cells (HSCs) are maintained in a microenvironment, known as niche in the endosteal regions of the bone marrow. This stem cell niche with low oxygen tension requires HSCs to adopt a unique metabolic profile. We have recently demonstrated that mouse long-term hematopoietic stem cells (LT-HSCs) utilize glycolysis instead of mitochondrial oxidative phosphorylation as their main energy source. However, the metabolic phenotype of human hematopoietic progenitor and stem cells (HPSCs) remains unknown.ResultsWe show that HPSCs have a similar metabolic phenotype, as shown by high rates of glycolysis, and low rates of oxygen consumption. Fractionation of human mobilized peripheral blood cells based on their metabolic footprint shows that cells with a low mitochondrial potential are highly enriched for HPSCs. Remarkably, low MP cells had much better repopulation ability as compared to high MP cells. Moreover, similar to their murine counterparts, we show that Hif-1α is upregulated in human HPSCs, where it is transcriptionally regulated by Meis1. Finally, we show that Meis1 and its cofactors Pbx1 and HoxA9 play an important role in transcriptional activation of Hif-1α in a cooperative manner.ConclusionsThese findings highlight the unique metabolic properties of human HPSCs and the transcriptional network that regulates their metabolic phenotype.

Harnessing the power of dividing cardiomyocytes

Lower vertebrates, such as newt and zebrafish, retain a robust cardiac regenerative capacity following injury. Recently, our group demonstrated that neonatal mammalian hearts have a remarkable regenerative potential in the first few days after birth. Although adult mammals lack this regenerative potential, it is now clear that there is measurable cardiomyocyte turnover that occurs in the adult mammalian heart. In both neonatal and adult mammals, proliferation of pre-existing cardiomyocytes appears to be the underlying mechanism of myocyte turnover. This review will highlight the advances and landmark studies that opened new frontiers in cardiac regeneration.

Redox Signaling in Cardiac Renewal

SIGNIFICANCE Utilizing oxygen (O2) through mitochondrial oxidative phosphorylation enables organisms to generate adenosine triphosphate (ATP) with a higher efficiency than glycolysis, but it results in increased reactive oxygen species production from mitochondria, which can result in stem cell dysfunction and senescence. RECENT ADVANCES In the postnatal organism, the hematopoietic system represents a classic example of the role of stem cells in cellular turnover and regeneration. However, in other organs such as the heart, both the degree and source of cellular turnover have been heavily contested. CRITICAL ISSUES Although recent evidence suggests that the major source of the limited cardiomyocyte turnover in the adult heart is cardiomyocyte proliferation, the identity and potential role of undifferentiated cardiac progenitor cells remain controversial. Several types of cardiac progenitor cells have been identified, and several studies have identified an important role of redox and metabolic regulation in survival and differentiation of cardiac progenitor cells. Perhaps a simple way to approach these controversies is to focus on the multipotentiality characteristics of a certain progenitor population, and not necessarily its ability to give rise to all cell types within the heart. In addition, it is important to note that cycling cells in the heart may express markers of differentiation or may be truly undifferentiated, and for the purpose of this review, we will refer to these cycling cells as progenitors. FUTURE DIRECTIONS We propose that hypoxia, redox signaling, and metabolic phenotypes are major regulators of cardiac renewal, and may prove to be important therapeutic targets for heart regeneration.

Minor Contribution of Cardiac Progenitor Cells in Neonatal Heart Regeneration

The adult mammalian heart is incapable of regeneration after injury, as shown by the limited amount of cardiomyocyte proliferation and poor neovascularization. We recently showed that neonatal mice have a remarkable ability to regenerate damaged heart after apical resection or myocardial infarction (MI), which includes complete reconstruction of myocardial wall with vascular network [2, 3]. Although lineage tracing showed that the main source of newly formed cardiomyocyte is preexisting cardiomyocytes, it is still possible that there is a minor contribution of other types of cells to the cardiomyocyte. In addition, lineage origin of the newly formed vasculature during postnatal cardiac maturation and neonatal heart regeneration remains unclear (Fig. 50.1).

Corrigendum: Hypoxia fate mapping identifies cycling cardiomyocytes in the adult heart

This corrects the article DOI: 10.1038/nature14582

Erratum: Hypoxia fate mapping identifies cycling cardiomyocytes in the adult heart (Nature (2015) 523 (226-230) DOI: 10.1038/nature14582)

This corrects the article DOI: 10.1038/nature14582

Erratum: Corrigendum: Hypoxia fate mapping identifies cycling cardiomyocytes in the adult heart

This corrects the article DOI: 10.1038/nature14582