Periodic Reporting for period 4 - CAVEHEART (Heart regeneration in the Mexican cavefish: The difference between healing and scarring)
Période du rapport: 2021-09-01 au 2022-08-31
Coronary artery disease is the single most common cause of death in Europe, accounting for 1.8 million deaths in Europe each year. There are around 7.4 million people living with cardiovascular disease in the UK, more than twice as many people than with cancer and Alzheimer’s disease combined and causing more than a quarter (28 per cent) of all deaths in the UK. Blockage of one of the arteries supplying blood to the heart results in a lack of oxygen in the downstream heart muscle and subsequent death of the starved cells (myocardial infarction/heart attack). The dead heart muscle is replaced by fibrous scar tissue in those fortunate enough to survive the heart attack, providing a necessary solution in replacing necrotic muscle to close the ventricular wall. However, the non-contractile fibrous tissue will never be replaced by new heart muscle, and may cause severe contractile dysfunction, resulting in heart failure and even recurring myocardial infarction. The combination of improved treatments for cardiovascular disease, increased public awareness of the risk factors, and government strategies aimed at helping people live a healthy lifestyle has reduced mortality rates, meaning that the number of people who survive a heart attack is steadily increasing. This impressive improvement in survival rate as well as the increasingly elderly populations, however, also mean more people living with the long term effects of reduced heart function and heart failure, costing the UK economy £9 billion per year, with this number steadily increasing every year.
Complete regeneration of the adult heart after injury is a feature exclusive to a limited number of species, including lower vertebrates such as the zebrafish and salamander. Injury to a zebrafish heart results in a spectacular scar-free regeneration process, with the wound tissue completely being replaced with new, functional cardiac muscle. If we can discover what fundamental mechanisms drive natural heart regeneration in fish, we can directly apply this knowledge to heal the human heart after a heart attack and during heart failure. Differences in inflammation and scarring are thought to be important differences between animals that can regenerate their hearts and animals that cannot, but not much is known about how these processes regulate heart regeneration.
In this project we use Astyanax mexicanus, the Mexican cavefish, as well as zebrafish, to study the mechanisms underlying scarring and inflammation during heart regeneration. Astyanax mexicanus is a single fish species comprising cave-dwelling and surface populations. These two populations arose around 10.000 - 8 million years ago when flooding caused surface fish living in rivers in Northern Mexico to enter the caves. River levels retreated over time and many caves never reconnected to the river and the fish evolved into different cavefish populations, whilst remaining the same species capable of interbreeding with their surface fish counterparts. During their independent evolution in the caves, the fish lost their eyes and pigment, redundant in the absence of light. Instead, they developed other features that helped survive in a cave. We found that, like zebrafish, the river surface fish regenerate their heart, while, cavefish cannot and form a permanent fibrotic scar, similar to the human injury response.
Comparing heart regeneration versus scarring in the same species allows us to focus directly on the mechanisms underlying this difference.
Complete regeneration of the adult heart after injury is a feature exclusive to a limited number of species, including lower vertebrates such as the zebrafish and salamander. Injury to a zebrafish heart results in a spectacular scar-free regeneration process, with the wound tissue completely being replaced with new, functional cardiac muscle. If we can discover what fundamental mechanisms drive natural heart regeneration in fish, we can directly apply this knowledge to heal the human heart after a heart attack and during heart failure. Differences in inflammation and scarring are thought to be important differences between animals that can regenerate their hearts and animals that cannot, but not much is known about how these processes regulate heart regeneration.
In this project we use Astyanax mexicanus, the Mexican cavefish, as well as zebrafish, to study the mechanisms underlying scarring and inflammation during heart regeneration. Astyanax mexicanus is a single fish species comprising cave-dwelling and surface populations. These two populations arose around 10.000 - 8 million years ago when flooding caused surface fish living in rivers in Northern Mexico to enter the caves. River levels retreated over time and many caves never reconnected to the river and the fish evolved into different cavefish populations, whilst remaining the same species capable of interbreeding with their surface fish counterparts. During their independent evolution in the caves, the fish lost their eyes and pigment, redundant in the absence of light. Instead, they developed other features that helped survive in a cave. We found that, like zebrafish, the river surface fish regenerate their heart, while, cavefish cannot and form a permanent fibrotic scar, similar to the human injury response.
Comparing heart regeneration versus scarring in the same species allows us to focus directly on the mechanisms underlying this difference.
We have published our discovery of Astyanax mexicanus, the Mexican cavefish, as a new model for heart development and regeneration research. We found that heart muscle proliferation peaks at similar levels in both surface fish and cavefish one week after injury. However, in cavefish this peak coincides with a strong scarring and immune response and, ultimately, cavefish heart muscle fails to replace the scar. We identified the gene lrrc10 to be upregulated in surface fish compared to cavefish after injury. Similar to cavefish, knockout of lrrc10 in zebrafish impairs heart regeneration without affecting wound heart muscle proliferation. The study of Astyanax mexicanus provides the distinctive advantage to be able to directly compare a natural regenerative and scarring response within one species, without having to correct for inter-species differences. This facilitates the identification of the DNA regions that control heart regeneration versus scarring. Using a technique called Quantitative Trait Locus (QTL) analysis, we have linked the degree of heart regeneration to 3 loci in the genome, identifying novel candidate genes fundamental to the difference between scarring and regeneration. We are still looking further into the results from the QTL analysis and the identified candidate genes. Additionally, we have performed a new extended QTL analysis and have identified a number of genes that we are taking forward for functional validation. We have also extended our comparative analysis to zebrafish to better understand the difference between regeneration and scarring. We found that regenerative capacity differs between different wild-type zebrafish strains and have been able to use this knowledge to identify differences in specific metabolic pathways as a a key factor influencing this difference. Reanalsyses of sequencing data from Astyanax mexicanus confirms this finding. The activity of these metabolic pathways does not link to the amount of cardiomyocyte proliferation, but is important for cardiomyocyte differentiation state. This is an important finding, which has already been disseminated at several conferences and we are now finalising these results for publication.
We have established the Astyanax model for heart regeneration research, which has allowed us to identify a number of key processes during heart regeneration, part is now published or will be published soon, and part of our findings will be the basis of further funding. We have published the first study identifying the differences between Astyanax mexicanus surface fish and cavefish heart development. We have also established and published the Astyanax model for heart regeneration research, identifying mechanisms of interest that will be the focus of our research over the next years. We are in the final stages of preparing a manuscript on our comparative zebrafish work.
The results on the QTL data are slightly delayed as a result of the pandemic, but will have our full focus in the next months to also finalise this data and disseminate.
The project has also provided us with many new findings of interest that will be the focus of new grant applications.
The results on the QTL data are slightly delayed as a result of the pandemic, but will have our full focus in the next months to also finalise this data and disseminate.
The project has also provided us with many new findings of interest that will be the focus of new grant applications.