Unlocking the Secrets of the Regenerating Fish Heart: Comparing Regenerative Models to Shed Light on Successful Regeneration

Abstract

The adult human heart cannot repair itself after injury and, instead, forms a permanent fibrotic scar that impairs cardiac function and can lead to incurable heart failure. The zebrafish, amongst other organisms, has been extensively studied for its innate capacity to repair its heart after injury. Understanding the signals that govern successful regeneration in models such as the zebrafish will lead to the development of effective therapies that can stimulate endogenous repair in humans. To date, many studies have investigated cardiac regeneration using a reverse genetics candidate gene approach. However, this approach is limited in its ability to unbiasedly identify novel genes and signalling pathways that are essential to successful regeneration. In contrast, drawing comparisons between different models of regeneration enables unbiased screens to be performed, identifying signals that have not previously been linked to regeneration. Here, we will review in detail what has been learnt from the comparative approach, highlighting the techniques used and how these studies have influenced the field. We will also discuss what further comparisons would enhance our knowledge of successful regeneration and scarring. Finally, we focus on the Astyanax mexicanus, an intraspecies comparative fish model that holds great promise for revealing the secrets of the regenerating heart.

The Role of Macrophages During Zebrafish Injury and Tissue Regeneration Under Infectious and Non-Infectious Conditions

“…The heart of the adult zebrafish is known to regenerate completely within a few weeks (72,73). It is the first organ that forms during the embryonic development, with the appearance of myocardial and endocardial progenitor cells as early as at 5 hpf (72).…”

Next-generation plasmids for transgenesis in zebrafish and beyond

“…The initial Tol2kit contained the destination vectors pDestTol2pA2 as “empty” destination vector and pDestTol2CG2 that included the myl7 (formerly cmlc2) promoter driving cytoplasmic EGFP in the developing myocardium from late somitogenesis stages on, easily screenable by 24 hpf (Huang et al, 2003; Kwan et al, 2007). However, myl7 -based transgenesis markers are not desirable for transgenes focused on heart development, a significant interest in the field (Bakkers, 2011; Cao and Poss, 2018; Kemmler et al, 2021; Potts et al, 2021; Smith et al, 2021; Yao et al, 2021). Further, the heart becomes increasingly obscured by skin pigmentation in standard zebrafish strains, rendering transgene detection in adults challenging (White et al, 2008).…”

Comparative Study in Zebrafish and Medaka Unravels the Mechanisms of Tissue Regeneration

“…Amongst, extensive knowledge was gained from the zebrafish studies. Since the ground-breaking discovery of zebrafish heart regeneration by Poss et al (2002), researchers have made in-depth investigations to understand the mechanisms of heart regeneration in zebrafish (Marques et al, 2019;Jaźwińska and Blanchoud, 2020;Potts et al, 2021). Briefly, zebrafish hearts mount a robust immune response in the recruitment of macrophages and neutrophils immediately after injury (Lai et al, 2017) and fast revascularization that expands superficially and intraventricularly and serve as the scaffold for CM repopulation (Marín-Juez et al, 2016, 2019.…”

“…Since regenerative capacity exists unevenly among species and their respective organs, an exciting way to uncover the mechanisms of tissue regeneration is by comparing the repair processes in animals with differential regenerative capacities. Such comparisons have been carried out in two ways: "Interspecies" comparing the repair of the same tissue/organ that is regenerative in one species (could also be age or living condition) but non-regenerative in another; Or "inter-organ" comparing two regenerative tissues within the same species to identify a central regenerative program (Potts et al, 2021). However, the regenerative species are usually quite distant in phylogeny, anatomy, and physiology from those non-regenerative ones, such as zebrafish and mice.…”