Mechanisms underlying maintenance of cardiac chamber identity in zebrafish

PROJECT SUMMARY In the vertebrate heart, the atrial and ventricular chambers contain distinct types of cardiomyocytes with specific molecular, morphological, and physiological properties. Atrial and ventricular specification are thought to occur in the early embryo, and chamber-specific gene expression patterns are apparent before the heart tube forms. However, the early assignment of atrial and ventricular cardiomyocyte fates is not sufficient to ensure commitment to a particular identity: chamber fate decisions seem to be malleable and can be reversible, even after differentiation initiates. Although the stability of chamber-specific characteristics is crucial for effective cardiac function, we do not yet understand the mechanisms responsible for maintenance of cardiac chamber identity. Our research addresses this significant gap in our knowledge by focusing on the genetic pathways that reinforce ventricular cardiomyocyte identity in the embryonic zebrafish heart. Several lines of evidence indicate that the ventricular myocardium harbors considerable plasticity: under certain circumstances, ventricular cardiomyocytes can transform, gradually losing their ventricular traits and simultaneously acquiring atrial traits. Our recent work has demonstrated that a FGF-MEK-ERK signaling pathway plays a pivotal role in the maintenance of ventricular identity, enforcing the retention of ventricular characteristics and suppressing the appearance of atrial characteristics. Building on this discovery, we are now well positioned to illuminate novel aspects of the mechanistic basis for chamber identity maintenance. First, we will elucidate the downstream effectors that mediate the impact of FGF-MEK-ERK signaling on the maintenance of ventricular identity. Our data suggest that dynamic patterns of ERK activation, downstream of FGF signaling, act cell-autonomously within ventricular cardiomyocytes to regulate transcription factors that promote ventricular gene expression and repress atrial gene expression, and we will test this model using real- time biosensors, chimera analysis, and transcriptomics. Second, we will identify factors that confer differential plasticity within distinct ventricular regions. The inner and outer curvatures of the ventricle differ in their inherent malleability; these differences may reflect local dynamics of ERK activity, regional distributions of biomechanical cues, and/or spatially distinct gene expression patterns, and we will test each of these hypotheses. Third, we will examine whether the signals that maintain ventricular identity in zebrafish play similar roles in human cardiomyocytes. Using pharmacological and genetic tools, we will test the hypothesis that FGF-MEK-ERK signaling and its effectors act to reinforce the stability of ventricular cardiomyocyte identity during differentiation of human pluripotent stem cells in vitro. Overall, our studies are likely to provide novel insight into crucial and conserved mechanisms that strike a balance between ventricular commitment and plasticity. Moreover, our results have the potential to inspire innovative strategies for regenerative medicine and to reveal new perspectives on the origins of congenital heart disease.