Regenerative responses after mild heart injuries for cardiomyocyte proliferation in zebrafish

Abstract

Background: The zebrafish heart regenerates after various severe injuries. Common processes of heart regeneration are cardiomyocyte proliferation, activation of epicardial tissue, and neovascularization. In order to further characterize heart regeneration processes, we introduced milder injuries and compared responses to those induced by ventricular apex resection, a widely used injury method. We used scratching of the ventricular surface and puncturing of the ventricle with a fine tungsten needle as injury-inducing techniques. Results: Scratching the ventricular surface induced subtle cardiomyocyte proliferation and responses of the epicardium. Endothelial cell accumulation was limited to the surface of the heart. Ventricular puncture induced cardiomyocyte proliferation, endocardial and epicardial activation, and neo-vascularization, similar to the resection method. However, the degree of the responses was milder, correlating with milder injury. Sham operation induced epicardial aldh1a2 expression but not tbx18 and WT1. Conclusions: Puncturing the ventricle induces responses equivalent to resection at milder degrees in a shorter time frame and can be used as a simple injury model. Scratching the ventricle did not induce heart regeneration and can be used for studying wound responses to epicardium.

Cell migration during heart regeneration in zebrafish

“…Endothelial cells are also present in the vascular endothelium, which is the inner layer of blood vessels. After heart injury, regenerating myocardium is re-vascularized (Lepilina et al, 2006;Gonzalez-Rosa et al, 2011;Itou et al, 2014), which supports regenerating cardiomyocytes. Unlike the endocardium, aldh1a2 is not induced in endothelial cells of the aorta or coronary vessels after heart injury (Kikuchi et al, 2011b), suggesting that the injury response is focused to endocardial endothelial cells.…”

“…In addition to significant heart injuries that are useful for studying heart regeneration mechanisms, zebrafish hearts also respond to milder injuries, such as puncturing the ventricle and scratching the ventricular surface (Kikuchi et al, 2011b; Itou et al, 2014). Different cells respond to different degrees in such milder injuries.…”

“…Different cells respond to different degrees in such milder injuries. For instance, scratching induces responses of endothelial cells and epicardial cells without activating cardiomyocyte proliferation, and therefore, could be used to examine the wound response of these cell types (Itou et al, 2014). …”

“…Epicardial cells also respond to various stimuli. For instance, they respond to scratching the ventricular surface, which does not induce cardiomyocyte proliferation response (Itou et al, 2014). Epicardial cells respond and express aldh1a2 even to sham operations, which involves opening the pericardiac cavity without heart injury, or injecting saline into the pericardial cavity (Wills et al, 2008).…”

“…As described above, the epicardial tissue envelops the heart. After resection, injured myocardium is exposed, and is covered by the epicardium by 3 days after resection (Kikuchi et al, 2011a;Itou et al, 2014), suggesting migration of epicardial cells (Fig. 3A).…”