PROJECT SUMMARY
The governing premise of this application is that cardiac injury response falls along a continuum, depending on
species, developmental stage, as well as therapeutic/experimental manipulation. Lower organisms such as
zebrafish and neonatal mice are capable of complete heart regeneration following partial amputation or ischemic-
induced cardiomyocyte (CM) death. Remuscularization is facilitated by proliferation of pre-existing
cardiomyocytes (CMs). On the other hand, in response to cardiac injury or stress in adult mammals, CMs fail to
proliferate and instead undergo adverse hypertrophic remodeling that can result in cardiac dysfunction. There is
great interest in identifying molecules and signaling cues that guide CMs to undergo a proliferative versus
hypertrophic response so that we can modulate these factors to promote adult mammalian cardiac regeneration
and repair in response to injury. In this proposal we investigate the hypothesis that junctional and
cytoarchitectural proteins are key effectors that regulate how CMs respond to injury. Modulation of these proteins
coordinate the proliferative state of CMs, but also facilitate proper re-integration of CMs with neighboring cells
during cardiac regeneration. In preliminary studies, we initiated a screen for cardiac development and
regeneration in zebrafish mutants that disrupt factors known to regulate cell junctional and cytoskeletal proteins.
The selected proteins have known interactions with Hippo-Yap signaling, a pathway of significant interest for
promoting cardiac regeneration. From this screen, we identified Llgl1 as critical for zebrafish heart
morphogenesis and development, and Shroom3 as a factor that is critical for regulation of CM proliferation during
zebrafish heart regeneration. Subsequently, we found that CM specific deletion of either Llgl1 or Shroom3
impairs intercalated disc integrity which was associated with aberrant CM cell cycle activity in uninjured hearts.
Here, we propose to delineate the cellular and physiological role of Llgl1 and Shroom3 in mammalian CMs during
development and the post cardiac injury response in neonatal and adult mice, and to further investigate the
mechanism by which these proteins interact with Hippo-Yap pathway components. Aims 1 and 2 employ
transgenic mice with CM specific deletion of Llgl1 or Shroom3, respectively, to investigate the role of these two
proteins in cardiac development and the post injury response with emphasis on cell morphology and
establishment of junctional complexes and intercalated discs. We employ the zebrafish model to investigate the
mechanisms underlying Llgl1 and Shroom3 functions, and whether these factors are co-regulated by the Hippo
kinase, Lats1/2. Collectively, our proposed studies will elucidate the role of key effectors of the cardiac injury
response, capable of modulating CM cytoarchitecture and cell-cell junctions to promote functional regeneration
and restore a healthy myocardium.
