Genetic Clue to How the Zebrafish Regenerates its Heart

Professor Jan Philipp Junker. Credit: © Pablo Castagnola, MDC

The ability to regenerate is widespread in the animal kingdom. Some flatforms, for example, can regenerate missing heads or entire bodies from small fragments, whereas salamanders are capable of regrowing limbs.

Zebrafish, a common model for human cellular research, can regenerate entire organs—including its heart. Now, researchers at the Max Delbrück Center for Molecular Medicine in Germany have discovered the regeneration process is significantly aided by connective tissue cells, or fibroblasts.

When a human suffers a heart attack without rapid treatment, heart muscle cells, or cardiomyocytes, become damaged by the lack of oxygen and die off. Since humans can’t produce new cardiomyocytes, the heart can no longer pump as well as it should after the loss of cells and subsequent formation of scar tissue.

Zebrafishes also experience this cell die-off and scaring, but the process stops there. At this point, zebrafishes form new cardiomyocytes, which are capable of contracting—regenerating its own heart.

“We wanted to identify the signals that come from other cells and help drive the regeneration,” said Jan Philipp Junker, head of the Quantitative Developmental Biology Lab at the Berlin Institute for Medical Systems Biology.

To do this, the researchers used single-cell genomics to search for injured heart cells in a zebrafish after induced heart attack that are not present in a healthy zebrafish heart. Junker and his team found three new types of fibroblasts that temporarily enter an activated state. Although the fibroblasts were externally identical to the others that form following scarring, Junker said the activated cells can read a whole series of additional genes that are responsible for forming proteins, including connective tissue factors like collagen 12.

When the researchers genetically turned off the collagen 12-expressing fibroblasts, the zebrafish was unable to regenerate its organs, according to the study published in Nature Genetics.

“It makes sense that fibroblasts are responsible for giving the repair signals,” said Junker. “They form right at the site of injury, after all.”

To identify the source of the activated fibroblasts, Junker’s team produced cell lineage trees using a technique called LINNAEUS, which his lab developed in 2018. LINNAEUS works with genetic scars that collectively act like a barcode for the origin of every cell.

“We create this barcode using CRISPR-Cas9 genetic scissors. If, after injury, two cells have the same barcode sequence, it means they’re related,” explained Junker.

Using the technique, the team identified two sources of temporarily activated fibroblasts: the outer layer of the heart (epicardium) and the inner layer (endocardium). Cells producing collagen 12 were found exclusively in the epicardium.

Even with this information, it is still unclear whether damaged hearts in mammals, like humans and mice, lack the necessary signals or lack the ability to read the signals. If the signals are lacking, medication could eventually be developed to simulate them. However, finding a way to mimic signal interpretation would be much more difficult.

The researchers now want to look more closely at the genes that the temporarily activated fibroblasts read especially often. They know that many of the genes in question are important for releasing proteins into the surrounding area, and these might include factors that also influence cardiomyocytes. According to Junker, initial research suggests the activated fibroblasts don’t just promote the regeneration of the heart—they also help to form new blood vessels that supply the heart with oxygen.