New insights into heart regeneration after injury in zebrafish

Patients suffering from heart injury, for example following a heart attack, cannot grow back the heart tissue that was lost. This leads to a growing group of patients that does not have a prospect of recovery, leaving them with heart failure. Unlike humans, zebrafish are able to re-grow heart tissue. Insight into the way zebrafish regenerate their hearts on a molecular level may one day be used to help patients recover from heart damage. Researchers from the group of Jeroen Bakkers now published two papers in Development, describing new findings in this field of research.

Live imaging of heart muscle cells

The first paper – published on 13 September 2021 – fills a gap regarding the methods with which heart regeneration can be studied in zebrafish. Previously, heart muscle cell division during regeneration could only be studied using “snapshots” of the process. However, it was not possible to observe it in real time. Until now: the researchers developed a model that allows live imaging of heart muscle cells during heart regeneration. To do so, they cultured slices of the zebrafish heart in a dish. “By using slices instead of the whole organ, the heart muscle cells stayed alive and we were able to observe cell division in culture for the first time,” Hessel Honkoop, first author on the paper explains. “Using confocal microscopy on these slices, we are now able to look at the cell division of heart muscle cells while it is actually happening, which gives us insight into the cellular mechanisms behind heart regeneration.”

Two types of cell division

The researches next looked at the dynamics of sarcomeres – the contractile units of heart muscle cells – during cell division following heart injury. Interestingly, they found that the cells use two types of sarcomere dynamics during division, depending on their location in the heart. While the cells directly adjacent to the injury disassemble their sarcomeres, the cells further from the injury do not. “It was surprising to see these heart muscle cells largely retaining their sarcomeres during division. Contrary to what we previously thought, disassembly of sarcomeres is apparently not essential for cell division,” says Honkoop.

Their model provides the field with a method to also study other processes during heart muscle cell division in heart regeneration. “The real strength of this model is that we can now see what happens live.”

The role of Prrx1b

The second paper – published on 4 October 2021 – describes how the gene prrx1b functions during heart regeneration. “In a previous study, we found that this gene becomes active in zebrafish after heart injury, so we wondered whether it plays an active role in achieving re-growth of heart tissue,” says Dennis de Bakker, co-first author of the paper. In order to find the answer to this question, they shut down the function of Prrx1b and observed that the zebrafish heart did not regenerate after heart injury anymore. “That indicates that Prrx1b plays an important role during zebrafish heart regeneration,” De Bakker explains.

Inducing heart muscle cell production

Next, the researchers also wanted to know how exactly Prrx1b functions during regeneration. “We identified that Prrx1 acts in the outermost layers of the heart, where it is required for the production of Nrg1,” says Mara Bouwman, other co-first author of the paper. “Nrg1 is a growth factor that can induce the production of new heart muscle cells.” The authors provide evidence suggesting that PRRX1 – the human version of the Prrx1b gene – can fulfill a similar role in regulating NRG1 in human heart cells. Additionally, the authors found that Prrx1b contributes to heart regeneration by limiting the scarring response of the heart after injury. “Together, our results show that the zebrafish heart employs Prrx1b to help reduce scarring and stimulate tissue regeneration,” De Bakker concludes.

Patient recovery

Both studies contribute to our knowledge about heart regeneration in zebrafish. Knowing how other species accomplish re-growing their hearts may aid in developing strategies for recovery after heart injury in patients. Future studies should focus on translating the knowledge obtained in these studies to humans.

Publications

Live imaging of adult zebrafish cardiomyocyte proliferation ex vivo. H.Honkoop, P.D. Nguyen, V.E.M. van der Velden, K.F. Sonnen, J. Bakkers. Development, 2021.

Prrx1b restricts fibrosis and promotes Nrg1-dependent cardiomyocyte proliferation during zebrafish heart regeneration. Dennis E.M. de Bakker*, Mara Bouwman*, Esther Dronkers, Filipa C. Simões, Paul R. Riley, Marie-José Goumans, Anke M. Smits, Jeroen Bakkers. Development, 2021.
* These authors contributed equally