Salk Institute researchers have discovered a way to heal injured hearts by reactivating long dormant molecular machinery found in the heart cells. This significant finding could open the door to new therapies for heart disorders in humans.
These new results were published in the November 6th, 2014 edition of the journal Cell Stem Cell. Although adult mammals don’t normally regenerate damaged tissue, they seem to retain a latent ability to do so. When the Salk team inhibited four different molecules that suppress genetic programs that lead to organ regeneration, they observed a dramatic improvement in heart regeneration and healing in laboratory mice.
These experiments provide proof-of-concept for a new type of clinical treatment in the fight against heart disease, which, according to the US Centers for Disease Control and Prevention, kills about 600,000 people each year in the United States alone.
“Organ regeneration is a fascinating phenomenon that seemingly recapitulates the processes observed during development. However, despite our current understanding of how embryogenesis and development proceeds, the mechanisms preventing regeneration in adult mammals have remained elusive,” says the study’s senior author Juan Carlos Izpisua Belmonte, holder of the Roger Guillemin Chair and primary investigator in the Gene Expression Laboratory and the Salk Institute.
We have within every cell of our bodies, the genes for organ regeneration. For several years, Izpisua Belmonte and his coworkers have attempted to clarify the genes that organism uses during embryonic development and during tissue healing highly regenerative organisms such as the zebrafish.
In 2003, Izpisua Belmonte’s laboratory first identified the signals that precede zebrafish heart regeneration, which they followed-up with a 2010 Nature paper, in which scientists from Izpisua Belmonte’s laboratory described how regeneration occurred in the zebrafish. Rather than stem cells invading injured heart tissue, the cardiac cells themselves reverted to a precursor-like state (a process called ‘dedifferentiation’). Dedifferentiation allowed the cells to proliferate within the damaged tissue.
They next determined if mammals retained any of the molecular players responsible for this kind of regenerative reprogramming. However, such an experiment came with some risks, recalls Ignacio Sancho-Martinez, a postdoctoral researcher in Izpisua Belmonte’s lab.
“When you speak about these things, the first thing that comes to peoples’ minds is that you’re crazy,” he says. “It’s a strange-sounding idea, since we associate regeneration with salamanders and fish, but not mammals.”
What are the things that cause a heart to regenerate in these smaller animals? Extensive work on the regenerating hearts of fish and salamanders failed to reveal anything concrete. Therefore, the laboratory changed its tack. “Instead, we thought, ‘If fish know how to do it, there must be something they can teach us about it,’” says the study’s first author Aitor Aguirre, a postdoctoral researcher in Izpisua Belmonte’s group.
The team focused on microRNAs, which control the expression of many genes. They used an extensive genetic screen for microRNAs that changed their expression levels during the healing of the zebrafish heart and that were found in the mammalian genome.
Their studies uncovered four molecules in particular–MiR-99, MiR-100, Let-7a and Let-7c–that fit their criteria. All were heavily repressed during heart injury in zebrafish and they were also present in rats, mice and humans.
However, in studies of mammalian cells in a culture dish and studies of living mice with heart damage, the group saw that the levels of these molecules were high in adults and failed to decline after the heart experienced injury. Therefore, Izpisua Belmonte’s team used adeno-associated viruses that could specifically infect the heart to target each of those four microRNAs and experimentally suppress their expressing levels.
Injecting these inhibitors into the hearts of mice that had suffered a heart attack triggered the regeneration of cardiac cells, and improved numerous physical and functional aspects of the heart, such as the thickness of its walls and its ability to pump blood. The scarring caused by the heart attack was significantly reduced with treatment compared to controls.
The improvements were still obvious three and six months after treatment–a long time in a mouse’s life. “The good thing is that the success was not limited to the short-term, which is quite common in cardiac regenerative biology,” Sancho-Martinez says.
The new study focused only on a handful of 70 some microRNA candidates that turned up in their initial screen. These other molecules might also play a part in heart cell proliferation, healing scars and promoting the formation of new blood vessels–all processes critical for heart repair, Sancho-Martinez says. The data are available so that other research groups can focus on molecules that interest them.
The next step for Izpisua Belmonte’s team is to move into larger animals and see whether “regenerative reprogramming” can work in larger hearts, and for extended periods after treatment, says Sancho-Martinez. And, although the virus packaging disappeared from the animals’ bodies by 2 weeks after treatment, the scientists are working on a new way to deliver the inhibitors to avoid the need for viruses altogether.