After a heart attack, cardiac muscle cells die because they are deprived of blood and oxygen. In an adult human, those cells represent a dead end. They can’t change their minds about what kind of cell they’ve become.
In newborn babies, as well as in adult fish, the heart can regenerate after injury. Why can’t the human heart be more fishy? At Emory, researcher Jinhu Wang is seeking answers, which could guide the development of regenerative therapies.
“If we want to understand cardiac regeneration in mammals, we can look at it from the viewpoint of the fish,” he says.
A lot of research in regenerative medicine focuses on the potential of stem cells, which have not committed to become one type of tissue, such as brain, skin or muscle. Wang stresses that the ability of zebrafish hearts to regenerate does not originate from stem cells. It comes from the regular tissues. The cells are induced to go back in time and multiply, although their capacity to regenerate may vary with the age of the animal, he says.
Zebrafish hearts are simpler than mammals’: theirs have just two chambers, while ours have four. Nobel Prize winner Christiane Nusslein-Vollhard has promoted the use of zebrafish as a genetic model in developmental biology. Its embryos are transparent, making it easy to spot abnormalities.
Wang’s fish room in the basement of Emory’s Rollins Research Center contains more than 1000 fish tanks, with different sizes of cage for various ages and an elaborate water recycling system. The adult fish eat brine shrimp that are stored in vats in one corner of the lab.
Wang arrived at Emory at the end of 2016, after working with Kenneth Poss at Duke for several years. There he and his colleagues observed that zebrafish can recover from having half of their heart muscle cut out. Check out this video from Howard Hughes Medical Institute.
Wang wanted to develop an alternative method, because performing the resections on many fish was taxing. “The next day, I couldn’t move my hand,” he says.
He developed a model for inducing a heart attack in fish without even touching them. It involves giving them an extra gene that is toxic to cardiac muscle cells. It is only turned on when the anticancer drug tamoxifen is introduced. In this video accompanying Wang’s 2011 paper in the journal Development, after a chemically induced heart attack, fish are sluggish and appear to gasp for breath. But a month later, they have recovered.
More recently, Wang has developed a system for culturing zebrafish hearts outside the body for one month. That’s much longer than any other “in vitro” culture system. He is also investigating the regeneration of coronary blood vessels. Wang provided the image below showing the network of coronary vessels in the zebrafish heart.
Monsters of regeneration
The 2018 movie Rampage, with Dwayne “The Rock” Johnson, introduced many non-scientists to the CRISPR/Cas9 gene editing technology, which Wang and his colleagues have begun to use. Unlike the scientists in the movie, Wang’s lab is not producing giant monsters. They are making fish with fluorescent hearts. CRISPR/Cas9 allows them to work more efficiently, checking fish eggs for proper gene editing in just a few days.
In his work with Poss on zebrafish, Wang demonstrated the importance of the epicardium, the outer layer of the heart. In cardiac development, the epicardium nourishes the underlying muscle, and gives rise to the coronary vasculature. If the epicardium is ablated, the ability of the cardiac muscle to regenerate is impaired.
The zebrafish epicardium displays an amazing ability to regenerate, coming back after 90 percent loss in just two weeks. This video, part of Wang’s 2015 Nature paper and used by permission, demonstrates that ability.
Scientists have known for decades that the hearts of non-mammals/lower vertebrate organisims such as newts or axolotls have a surprising regenerative capacity. However, the discovery that zebrafish hearts can accomplish similar feats put the discovery into a system that could be easily manipulated, with lots of readily available genetic tools, says Wang’s colleague Ahsan Husain.
Other scientists at Emory working with zebrafish include Andreas Fritz and Iain Shepherd in the Department of Biology, studying embryonic and nervous system development.