Mechanisms of regulating cardiomyocyte chromatin dynamics and regenerative transitions by Tbx20

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Project Summary/Abstract Cardiovascular disease is the leading cause of death in the world, and an adult’s ability to recover after a cardiac injury, such as myocardial infarction (MI) is hindered by the quiescent state of cardiomyocytes (CMs). Anecdotal evidence suggests that pre-adolescent humans can replenish CMs after injury by proliferation of resident CMs. Fetal and neonatal mice can also regenerate CMs after injury, but regenerative potential is lost beyond the first postnatal week. Failed attempts to promote regenerative proliferation in adult mouse CMs by activating robust cell cycle effectors have led to pathological hypertrophy and reduced cardiac function. In contrast, our laboratory demonstrated that conditional induction of Tbx20- a transcription factor critical for fetal CM proliferation- promotes fetal characteristics and restrained proliferation in normal and post-MI adult CMs without impairing cardiac function. While this finding has enormous therapeutic relevance, the mechanism underlying this is unknown, particularly because Tbx20 function throughout CM maturation and regenerative transitions are currently ill-defined. Moreover, increasing evidence suggests that Tbx20 interacts with the SWI/SNF chromatin remodeling component BRG1 to promote the expression of genes related to cell cycle activity in fetal and neonatal CMs, providing another facet to the gene regulatory actions of Tbx20. We hypothesize that Tbx20 cooperates with BRG1 to promote proliferation and fetal characteristics in adult CMs by remodeling CM chromatin into a more immature, fetal-like state, and by opening chromatin regions associated with fetal Tbx20 transcriptional targets. We propose two separate Aims to elucidate the mechanism by which Tbx20 promotes proliferation and fetal reversion in adult CMs. In Aim 1, will utilize an integrative multiomics approach to identify how Tbx20 dynamically regulates gene expression throughout normal CM development and regenerative transitions, including its binding targets and effect on chromatin changes (Aim 1.1). We will characterize the reversibility of this regenerative phenotype using a transient model of Tbx20 induction (Aim 1.2). This will determine whether proliferative, fetal-like CMs can mature into adult CMs following the removal of Tbx20, which is necessary to accommodate the metabolic demands of the adult heart. In Aim 2, we will utilize co-immunoprecipitation and CHIP-qPCR to elucidate whether an interaction between Tbx20 and BRG1 occurs in immature CMs (Aim 2.1). We will also utilize conditional mouse models of Tbx20 induction and Brg1 deletion to determine whether Tbx20 induction after MI promotes proliferation, fetal characteristics, and improved cardiac function via a BRG1-dependent mechanism (Aim 2.2). Understanding gene regulatory mechanisms during CM maturation will advance therapeutic strategies aimed at reactivating fetal gene programs, promoting CM regeneration, and improving cardiac function post-MI. The Aims in this proposal seek to elucidate this in the context of Tbx20, with BRG1 as a potential novel partner in directing the fetal gene program during normal development and regenerative repair.