Mending broken hearts: Cardiac regeneration in zebrafish

Abstract

Cardiovascular disease is the leading global cause of death, accounting for more than 30% of all deaths. It claims more lives than all forms of cancer combined. Bearing these facts in mind, research in this field is direly needed. A heart attack, also known as myocardial infarction (MI), happens when the blood flow through one of the coronary arteries is lost. The result is insufficient supply of heart muscle cells with oxygen and nutrients, causing massive death of one billion or more heart cells. However, there is no significant replacement of the lost tissue; instead, scar tissue is formed. Scarring is essential to initially keep the organ intact, but a non-contractile scar cannot restore cardiac output – i.e. pump a sufficient amount of blood through the body – and can cause arrhythmia (irregularities in the heart beat) that can lead to cardiac arrest. If we could use regenerative therapies to induce regeneration of the heart after a heart attack, we could help thousands of patients. One possibility is to push the heart to heal itself. How? If we understand how a heart regenerates under natural circumstances, we could devise strategies to induce healing in the human heart. One species that naturally regenerates its heart is the zebrafish. Zebrafish are small animals, 2-3 cm long. They are used all over the world in laboratories to study all kinds of mechanisms and processes, ranging from development of a fertilized egg into a mature animal to behavioral studies – but also regeneration is studies intensively. In this thesis, I used the zebrafish to get a better understanding of heart regeneration in a natural setting. Using novel techniques like tomo-seq or single-cell mRNA-sequencing I studied proliferating cardiomyocytes, which are the main cells contributing to myocardial regeneration. I describe how they change the way they generate energy, compared to uninjured heart muscle cells. Interestingly, the way the regenerating heart muscle cells generate energy is the same as in the embryo. Further comparison showed that the proliferating heart muscle cells have become “younger”: they are similar to the embryonic cells and they can be thought of as a sort of stem cell that has the potential to make more cells. Moreover, I also took interest in the involvement of the immune system in the regenerative process. Dynamics of the two most important cells of the innate immune response, macrophages and neutrophils are described and analyzed. I provide detailed information when and in which quantity these two cell types are recruited to the injured heart. Lastly, I identify the time point when inflammation is turned off and pro-repair signals are released from the macrophages at seven days after injury. Overall, the results described in this thesis add a plethora of new information to help understand heart regeneration in a natural setting.