Towards deciphering variations of heart regeneration in fish

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

Tissue regeneration has been in the spotlight of research for its fascinating nature and potential applications in human diseases. The trait of regenerative capacity occurs diversely across species and tissue contexts, while it seems to decline over evolution. Organisms with variable regenerative capacity are usually distinct in phylogeny, anatomy, and physiology. This phenomenon hinders the feasibility of studying tissue regeneration by directly comparing regenerative with non-regenerative animals, such as zebrafish (Danio rerio) and mice (Mus musculus). Medaka (Oryzias latipes) is a fish model with a complete reference genome and shares a common ancestor with zebrafish approximately 110–200 million years ago (compared to 650 million years with mice). Medaka shares similar features with zebrafish, including size, diet, organ system, gross anatomy, and living environment. However, while zebrafish regenerate almost every organ upon experimental injury, medaka shows uneven regenerative capacity. Their common and distinct biological features make them a unique platform for reciprocal analyses to understand the mechanisms of tissue regeneration. Here we summarize current knowledge about tissue regeneration in these fish models in terms of injured tissues, repairing mechanisms, available materials, and established technologies. We further highlight the concept of inter-species and inter-organ comparisons, which may reveal mechanistic insights and hint at therapeutic strategies for human diseases.

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

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.

Multiple cryoinjuries modulate the efficiency of zebrafish heart regeneration

Zebrafish can regenerate their damaged hearts throughout their lifespan. It is, however, unknown, whether regeneration remains effective when challenged with successive cycles of cardiac damage in the same animals. Here, we assessed ventricular restoration after two, three and six cryoinjuries interspaced by recovery periods. Using transgenic cell-lineage tracing analysis, we demonstrated that the second cryoinjury damages the regenerated area from the preceding injury, validating the experimental approach. We identified that after multiple cryoinjuries, all hearts regrow a thickened myocardium, similarly to hearts after one cryoinjury. However, the efficiency of scar resorption decreased with the number of repeated cryoinjuries. After six cryoinjuries, all examined hearts failed to completely resolve the fibrotic tissue, demonstrating reduced myocardial restoration. This phenotype was associated with enhanced recruitment of neutrophils and decreased cardiomyocyte proliferation and dedifferentiation at the early regenerative phase. Furthermore, we found that each repeated cryoinjury increased the accumulation of collagen at the injury site. Our analysis demonstrates that the cardiac regenerative program can be successfully activated many times, despite a persisting scar in the wounded area. This finding provides a new perspective for regenerative therapies, aiming in stimulation of organ regeneration in the presence of fibrotic tissue in mammalian models and humans.