Retinoic Acid Production by Endocardium and Epicardium Is an Injury Response Essential for Zebrafish Heart Regeneration

Abstract

SUMMARY Zebrafish heart regeneration occurs through the activation of cardiomyocyte proliferation in areas of trauma. Here, we show that within three hours of ventricular injury, the entire endocardium undergoes morphological changes and induces expression of the retinoic acid (RA)-synthesizing enzyme raldh2. By one day post-trauma, raldh2 expression becomes localized to endocardial cells at the injury site, an area that is supplemented with raldh2-expressing epicardial cells as cardiogenesis begins. Induced transgenic inhibition of RA receptors or expression of an RA-degrading enzyme blocked regenerative cardiomyocyte proliferation. Injured hearts of the ancient fish Polypterus senegalus also induced and maintained robust endocardial and epicardial raldh2 expression coincident with cardiomyocyte proliferation, while poorly regenerative infarcted murine hearts did not. Our findings reveal that the endocardium is a dynamic, injury-responsive source of RA in zebrafish, and indicate key roles for endocardial and epicardial cells in targeting RA synthesis to damaged heart tissue and promoting cardiomyocyte proliferation.

Hydrogen peroxide primes heart regeneration with a derepression mechanism

“…In spite of great efforts, the molecular mechanisms underlying heart regeneration remain incompletely understood. Previous reports support that developmental signaling pathways such as FGF, PDGF, retinoid acid, TGFβ and integrin are essential for adult zebrafish heart regeneration [19][20][21][22][23]. Ventricular resection induces expression of fgfr2/4 in the epicardium and their ligand fgf17b in the myocardium, and conditional blocking of the FGF signaling by overexpression of dominant-negative FGFR1 substantially increases fibrin/ collagen deposits and diminishes myocardial regeneration, supporting a notion that the FGF signaling promotes myocardial regeneration through epicardial-myocardial interaction [21].…”

Signaling During Epicardium and Coronary Vessel Development

“…It is now generally accepted that EPDC do not differentiate into CM in the fish heart, 134 -136 but it is also established that the epicardium is necessary for cardiac regeneration to occur, as shown by the requirement for epicardial Raldh2-RA epicardial signaling in CM proliferation during this regeneration. 135 Several studies have analyzed the capacity for cardiac regeneration in mice. 131 A recent report links the cardiac response after mechanical or ischemic injury to Notch activity.…”

Fast revascularization of the injured area is essential to support zebrafish heart regeneration

“…Other tissues such as the epicardium and endocardium are activated upon injury. Similar to the myocardium, following-injury epicardial cells exhibit morphological changes and reexpress developmental genes including raldh2 and tbx18 (11,12). The epicardium gives rise to myofibroblasts and perivascular cells (13,14), and epicardium-derived perivascular and smooth muscle cells support revascularization of the injured area in zebrafish and mammals (15,16).…”