PROJECT SUMMARY / ABSTRACT Adult human cardiomyocytes have poor capacity for proliferation and functional recovery after heart attack, thus leading to the high morbidity and mortality of cardiovascular diseases. Therefore, there is great need to under- stand the underlying mechanisms of adult cardiomyocyte turnover and maturation. While screening for modula- tors of TEAD1, a core component of the cardiac transcriptional network, my lab identified a novel factor— C5ORF51/RIMOC1—as a potential cardiomyogenic modulator. C5ORF51 (hereinafter referred to as C5x) is a previously uncharacterized protein of nearly unknown function, which appears to play critical roles in the regula- tion of cardiomyocyte homeostasis and heart size. The overarching hypothesis is that C5x, as a novel TEAD1 cofactor, regulates cardiomyogenesis by functioning as a modulator of core cardiac transcription factors. The rationale for the project is that both the ex vivo and in vivo C5x knockout models demonstrate significantly ac- celerated cell cycle activity and cardiomyocyte endowment. Importantly, C5x-cardiomyocyte-knockout mice de- velop massive cardiomegaly and premature mortality, demonstrating a critical role of C5x in maintaining normal cardiac function. RNA sequencing analysis suggests C5x as a gatekeeper for the fetal gene program. We pro- pose two aims to address our hypothesis: [Aim 1] Determine the mechanisms by which C5x modulates tran- scription in cardiomyocytes. [Aim 2] Investigate the role of C5x in adult cardiomyocyte proliferation and matura- tion. Under the first aim, we will investigate the molecular interactive model between C5x and TEAD1. We will further determine the interacting partners of C5x in cardiomyocytes to understand its regulatory network. Under the second aim, we will determine proliferation in the sub-aim 2A and function in sub-aim 2B. Specifically, we will test whether C5x loss is sufficient to induce cell cycle reentry and fetal gene program reactivation in adult, we will also test whether C5x loss is able to induce more cells to enter cell cycle in the cardiac injury models. We will determine the necessity of C5x in cardiomyocyte maturation and function. The research proposed in this application is innovative because this is the first study demonstrating the nonredundant role of C5x in vivo and in heart and illustrating a novel transcriptional modulator in cardiomyocytes. The proposed study is significant because it will uncover a novel regulatory mechanism leveraging cell cycling and maturation. Ultimately, this knowledge has the potential of offering new opportunities for the development of innovative treatment modality to induce cardiogenesis.