After a heart attack, the human heart suffers from the loss of heart muscle that has been replaced with a non-contracting scar. Replacing lost heart muscle is something that human hearts do not do terribly well. However the zebrafish heart can easily replace lost cells. New research from Duke University has discovered the properties of the outer layer of the heart known as the epicardium that explains the fish’s incredible ability to regrow heart tissue.
After injury to the heart, the cells in the zebrafish epicardium begin to divide and form new cells that will cover the wound. The epicardial cells also secrete chemicals that prompt muscle cells to grow and divide. These cells also support the production of new blood vessels to carry oxygen to new heart tissues.
The Duke study was published in the May 4 edition of the journal Nature, and reported that when the epicardium is damaged, the entire repair process is delayed until the epicardium heals itself before tending to the rest of the heart. Epicardium-based healing of the heart is dependent on the production of a small, secreted protein called sonic hedgehog. In fact, applying Sonic Hedgehog to the surface of the heart drives the epicardial response to injury.
These results provide a new target to exploit in the quest to help heart patients repair the damage caused by a heart attack, which is a major cause of death and disability in the United States. Over five million Americans are currently in the throes of heart failure, and over 900,000 suffer a heart attack each year.
“The best way to understand how an organ regenerates is to deconstruct it. So for the heart, the muscle usually gets all the attention because it seems to do all the work,” said Kenneth D. Poss, Ph.D., senior author of the study and professor of cell biology at Duke University School of Medicine. “But we also need to look at the other components and study how they respond to injury. Clearly, there is something special about the epicardium in zebrafish that makes it possible for them to regenerate so easily.”
Poss and his coworkers have been studying heart regeneration in zebrafish for the last 13 years. When he worked as a postdoctoral research fellow, he was the first to show that zebrafish could regrow severed pieces of heart tissue. Since that time, his laboratory has found that this regeneration involves the input from the epicardium, that thin layer of cells that covers the heart surface.
“The epicardium is underappreciated, but we think it is important because similar tissues wrap up most of our organs and line our organ cavities,” Poss said. “Some people think of it as a stem cell because it can make more of its own, and can contribute all different cell types and factors when there is an injury. The truth is we know surprisingly little about this single layer of cells or how it works. It is a mystery.”
Poss and his colleagues attempted to identify the characteristics of the epicardium that make it so good at regenerating the heart. Duke postdoctoral fellow Jinhu Wang performed open-heart surgery on live zebrafish, and removed approximately one fifth of the heart muscle. Wang also used genetic tricks to kill off 90 percent of the epicardial cells. Then he measured how well the heart healed at various time points. Wang discovered that removing the epicardium created a clear delay in regeneration, but that regeneration eventually caught up to that of zebrafish with an intact epicardium. Wang’s results suggested that the 10 percent of epicardial cells left were able to rebuild the epicardial layer before moving on to heart muscle.
Intrigued by the finding, Jingli Cao, another postdoctoral fellow in the Poss laboratory, devised a technique to remove hearts from zebrafish and grow them in laboratory culture. In culture, the tiny two-chambered fish hearts continue to beat and behave as if they were still tucked inside the organism.
As before, Cao destroyed most of the epicardial layer of the heart and then they placed the cultured hearts under the microscope every day to capture heart regeneration in action. Cao noticed that the epicardium regenerated rapidly, covering the heart like a wave from the base of one chamber to the tip of the other in just a week or two.
The Poss laboratory then searched for small molecule compounds or drugs that would affect the ability to regenerate. In particular, they screened molecules known to be involved in the development of embryos, like fibroblast growth factors and sonic hedgehog, and discovered that sonic hedgehog was essential for heart regeneration. Poss and others plan to extend such a screen to molecules that could enhance heart repair in zebrafish, and perhaps one day give clues for new treatments in humans with heart conditions.
In a second paper that was published in the April 1, 2015, edition of the journal eLife, Poss and his colleagues found that the epicardium produces a molecule called neuregulin1 that makes heart muscle cells divide in response to injury. When Poss and his coworkers artificially boosted levels of neuregulin1, even without injury, the heart started building more and more heart muscle cells.
“Studies of the epicardium in various organisms have shown that this tissue is strikingly similar between fish and mammals, indicating that what we learn in zebrafish models has great potential to reveal methods to stimulate heart regeneration in humans,” said Poss.