Strong as a Hippo's Heart: Biomechanical Hippo Signaling During Zebrafish Cardiac Development

Abstract

The heart is comprised of multiple tissues that contribute to its physiological functions. During development, the growth of myocardium and endocardium is coupled and morphogenetic processes within these separate tissue layers are integrated. Here, we discuss the roles of mechanosensitive Hippo signaling in growth and morphogenesis of the zebrafish heart. Hippo signaling is involved in defining numbers of cardiac progenitor cells derived from the secondary heart field, in restricting the growth of the epicardium, and in guiding trabeculation and outflow tract formation. Recent work also shows that myocardial chamber dimensions serve as a blueprint for Hippo signaling-dependent growth of the endocardium. Evidently, Hippo pathway components act at the crossroads of various signaling pathways involved in embryonic zebrafish heart development. Elucidating how biomechanical Hippo signaling guides heart morphogenesis has direct implications for our understanding of cardiac physiology and pathophysiology.

Keywords: Hippo signaling; Yap1/Wwtr1 (Taz); cardiac development; endocardium; intra-organ-communication; mechanobiology; myocardium; zebrafish.

FIGURE 1

Biomechanical Hippo signaling pathways in the zebrafish endocardium. Biomechanical forces are sensed and transmitted by the mechanosensitive protein Cadherin-5 (Cdh5) and Piezo1/2 ion channels. This turns the Hippo signaling pathway to an inactive state (right) and results in the nuclear translocation of unphosphorylated YAP1/WWTR1 transcription factors where they form a complex with TEAD transcription factors to regulate gene expression. In the absence of biomechanical stimuli, the Hippo signaling pathway is active (left). This initiates a cascade of phosphorylation events on MST1/MST2 and LATS1/LATS2 kinases, which phosphorylate YAP1/WWTR1 proteins that are retained in the cytoplasm or become proteolytically degraded.

FIGURE 2

Hippo pathway-dependent developmental processes during early cardiac development in zebrafish. The zebrafish heart tube at 5 days post fertilization consists of the inner endocardium (blue), myocardium (red), and the surrounding epicardium (brown). The atrioventricular (AV) valve, comprised of myocardial and endocardial cells, and the outflow tract (OFT) valve, comprised of smooth muscle and endothelial cells, have formed and blood flow has a unidirectional flow pattern (dashed red line). The ventricular myocardium has segregated into inner trabecular and outer compact wall layers. Hippo pathways-dependent processes discussed in this review are highlighted in the colored boxes.

FIGURE 3

Hippo signaling pathways during zebrafish cardiac development. (A) Cell junctional complexes (including Cadherin-5, Pecam-1, and Vegfr3/Flt4), stretch-induced ion channels Piezo1/2, or primary cilia act as mechanosensors within the endocardium (blue). Increased junctional forces within the endocardium are sensed by Cadherin-5, which triggers the nuclear localization of Yap1/Wwtr1 and causes endocardial cell proliferation. A compact wall architecture is organized within the myocardium (red). Tension heterogeneity upon myocardial proliferation induces actomyosin enrichment at their apical sides and causes the nuclear localization of Yap1. This triggers delamination of myocardial cells that seed the trabeculation layer. Neighboring myocardial cells activate Notch signaling, which inhibits actomyosin network contractility. Erbb2-mediated nuclear export of Wwtr1 causes myocardial cell lamination into the trabecular layer. (B) Within the outflow tract (OFT) valve, Piezo1 regulates Yap1 nuclear localization within the endothelial (yellow) and smooth muscle cell layers (green). Within the endothelial layer, Piezo1 modulates expression of the mechanosensitive transcription factor klf2a. (C) During epicardial formation, Ift88 interacts with Yap1 within the cytoplasm and forms a complex with Amolt1 to regulate Yap1 activity. Within the nucleus a Yap1-Tead complex activates expression of bmp4, which restricts the growth of proepicardium (purple) and myocardium (red). (D) Biomechanical forces involve hemodynamics, biomechanical coupling, and tension heterogeneity acting within different tissue layers. These mechanical stimuli activate mechanosensitive Hippo pathway signaling.