Heart disease kills more men and women each year than any other malady. Presently, we know that following cardiac injury in adult mammals, the contractile cells making up the heart, cardiomyocytes, are permanently lost and replaced by fibrotic scar tissue, severely affecting the hearts contractile function and physiology. Furthermore, we also know that cardiomyocytes possess the ability to divide and regenerate following injury in neonatal mammals. However, this regenerative capacity is transient, as cardiomyocytes are essentially incapable of dividing in juvenile and adult mammals. Importantly, we have uncovered one of the cell intrinsic signaling pathways responsible for preventing cardiomyocyte cytokinesis and regeneration in adult mammals. Namely, it is the Hippo-signaling pathway, which plays a central role in maintaining the proper size of organs, like the heart, through restricting cell proliferation. Remarkably, when Hippo-pathway components are knocked out in adult murine cardiomyocytes, they regain the ability to undergo cytokinesis and contribute to regeneration following myocardial function. So, both neonatal and postnatal Hippo-deficient cardiomyocytes are capable of renewing and regenerating, while adult cardiomyocytes can be classified as essentially non-mitotic and non- regenerative. However, the complete molecular mechanisms responsible for imparting neonatal and Hippo-deficient cardiomyocytes their regenerative abilities are unknown. What is more, it is possible that only a small, specialized subpopulation of cardiomyocytes are able to renew and divide post-injury. And that instead of global activation of neonatal cardiomyocyte proliferation, there is an induction of this specialized cardiomyocyte sub-population to expand and regenerate the tissue. Yet, the heterogeneity of neonatal and adult cardiomyocytes remains undetermined, and the distinct cellular identity of proliferative and regenerative cardiomyocytes is unknown. Using innovative techniques in genetics, epigenomics, nuclear RNA sequencing, and single-cell transcriptomic profiling we aim to determine the genes, and signaling pathways responsible for promoting cardiomyocyte regeneration. Specifically, we propose to (1) define the cardiomyocyte-specific transcriptomic and epigenomic landscapes present during cardiac regeneration, as well as their up-stream transcriptional regulators, and (2) determine the heterogeneity and cellular plasticity of cardiomyocytes during regeneration.
Currently, we are aware that neonatal and adult Hippo-deficient cardiomyocytes are capable of renewing and regenerating, while non-mutant adult cardiomyocytes are non-mitotic and non-regenerative. However, the molecular mechanisms and precise cellular identities responsible for driving neonatal and Hippo-deficient cardiac regeneration are unknown. Toward uncovering these molecular programs, we have used epigenomic and transcriptomic profiling techniques to reveal the genes, enhancers, and DNA binding transcription factors that are active during cardiac regeneration. In the proposed research, we will define the transcriptional landscapes and cell identities associated with heart regeneration with the expectation that our studies will provide valuable insight into the mechanisms by which the mammalian cardiomyocytes contribute to tissue regeneration, development, injury response, heart failure, and angiogenesis.
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