Nrg1 is an injury-induced cardiomyocyte mitogen for the endogenous heart regeneration program in zebrafish.

Matthew Gemberling,Amy L Dickson,Ravi Karra,Kenneth D Poss

doi:10.7554/elife.05871

Abstract

Heart regeneration is limited in adult mammals but occurs naturally in adult zebrafish through the activation of cardiomyocyte division. Several components of the cardiac injury microenvironment have been identified, yet no factor on its own is known to stimulate overt myocardial hyperplasia in a mature, uninjured animal. In this study, we find evidence that Neuregulin1 (Nrg1), previously shown to have mitogenic effects on mammalian cardiomyocytes, is sharply induced in perivascular cells after injury to the adult zebrafish heart. Inhibition of Erbb2, an Nrg1 co-receptor, disrupts cardiomyocyte proliferation in response to injury, whereas myocardial Nrg1 overexpression enhances this proliferation. In uninjured zebrafish, the reactivation of Nrg1 expression induces cardiomyocyte dedifferentiation, overt muscle hyperplasia, epicardial activation, increased vascularization, and causes cardiomegaly through persistent addition of wall myocardium. Our findings identify Nrg1 as a potent, induced mitogen for the endogenous adult heart regeneration program.

Highlights

Myocardial infarction (MI) is a common injury that causes permanent loss of hundreds of millions of cardiac muscle cells, increasing susceptibility to heart failure and sudden death
Using quantitative PCR, we found that nrg1 levels increase after genetic ablation of ∼50% of cardiomyocytes in the adult zebrafish ventricle. nrg1 levels rise above baseline at 3 days post-injury and peak at ∼11-fold above uninjured levels by 7 days, an injury timepoint at which cardiomyocyte proliferation peaks (Wang et al, 2011). nrg1 levels lower to ∼fourfold above uninjured levels by 14 days post injury
We found that nrg1 signals rarely overlapped during heart regeneration with cells positive for fli1a:EGFP or cmlc2: EGFP, which mark vascular endothelial cells and cardiomyocytes, respectively (Figure 1J,K)

Summary

Introduction

Myocardial infarction (MI) is a common injury that causes permanent loss of hundreds of millions of cardiac muscle cells, increasing susceptibility to heart failure and sudden death. 3 days later, we resected ventricles, and found that elevated nrg expression led to an ∼84% increase in the cardiomyocyte proliferation index near the injury site at 7 dpa (n = 15, 18; Figure 2C–E) This finding reveals a mitogenic influence of Nrg signaling on heart regeneration and suggests that nrg levels are a limiting factor for cardiomyocyte proliferation after cardiac injury. ICH caused ectopic wall thickening by 10 wpf, with obvious clone mixing and clone growth in the Z-plane away from the lumen (14/14 iCH, 0/11 controls; Figure 8A–F) These findings indicate that Nrg does not activate a gradual layering process but builds muscle radially with proliferation dynamics that are more reminiscent of injury-induced regeneration. After 6 months of iCH, many large coronary vessels were evident in the ventricle (Figure 6G) Together, this analysis indicates that expression of a single molecule, Nrg, is sufficient to induce and maintain critical aspects of the heart regeneration program that encompass several cell types

Conclusions

Findings

Materials and methods