That microRNA-294 (mir-294) beneficially affects heart regeneration was discovered via its presence in embryonic stem cell exosomes. Exosomes are extracellular vesicles, membrane-bound packages of molecules that cells pass between one another. They are interesting to the research community because it is in principle much easier to construct a therapy based on delivery of exosomes harvested from stem cells than it is to deliver those same stem cells. Thus most of the present generation of stem cell therapies may well be replaced in the near future by the delivery of extracellular vesicles, and many research groups are testing exosomes from stem cells to see how well they work to spur greater regeneration.
Most vesicles contain a wide variety of molecules, but in the case of embryonic stem cell exosomes and the injured heart, researchers found that near all of the therapeutic effect was mediated by mir-294. Thus they could go a step further and discard the exosomes as well as the cells. The results of that line of work are noted in today's publicity materials and paper. Applying mir-294 causes adult heart muscle cells to regress into a state more like that of embryonic cells, provoking greater replication and thus greater regeneration. This sort of in-situ reprogramming of cell behavior is growing in popularity in the research community, see the work of Turn.bio for example, though it remains to be seen whether or not it can be made safe enough to be the basis for a near future regenerative therapies.
By adulthood, the heart is no longer able to replenish injured or diseased cells. As a result, heart disease or an event like a heart attack can be disastrous, leading to massive cell death and permanent declines in function. A new study is the first to show that a very small RNA molecule known as miR-294, when introduced into heart cells, can reactivate heart cell proliferation and improve heart function in mice that have suffered the equivalent of a heart attack in humans. "In previous work, we discovered that miR-294 actively regulates the cell cycle in the developing heart. But shortly after birth miR-294 is no longer expressed. The heart is very proliferative when miR-294 is expressed in early life. We wanted to see if reintroducing it into adult heart cells would turn them back to an embryonic-like state, allowing them to make new heart cells."
The researchers tested their idea in mice that had myocardial infarction (heart attack). Mice were treated with miR-294 continuously for two weeks after sustaining myocardial injury. Two months following treatment, the researchers observed noticeable improvements in heart function and a decrease in the area of damaged tissue. Examination of treated heart cells revealed evidence of cell cycle reentry, indicating that the cells had been reactivated, regaining the ability to produce new cells. "The miR-294 treatment reawakened an embryonic signaling program in the adult heart cells. Because of this, the old heart cells were no longer quite like adult cells, but neither were they fully embryonic. In this in-between state, however, they had the ability to make new cells."
Embryonic heart is characterized of rapidly dividing cardiomyocytes required to build a working myocardium. Cardiomyocytes retain some proliferative capacity in the neonates but lose it in adulthood. Consequently, a number of signaling hubs including microRNAs are altered during cardiac development that adversely impacts regenerative potential of cardiac tissue. Embryonic stem cell cycle miRs are a class of microRNAs exclusively expressed during developmental stages; however, their effect on cardiomyocyte proliferation and heart function in adult myocardium has not been studied previously.
In this study, we determine whether transient reintroduction of embryonic stem cell cycle miR-294 promotes cardiomyocyte cell cycle reentry enhancing cardiac repair after myocardial injury. A doxycycline-inducible AAV9-miR-294 vector was delivered to mice for activating miR-294 in myocytes for 14 days continuously after myocardial infarction. miR-294-treated mice significantly improved left ventricular functions together with decreased infarct size and apoptosis 8 weeks after MI. Myocyte cell cycle reentry increased in miR-294 hearts parallel to increased small myocyte number in the heart. Isolated adult myocytes from miR-294 hearts showed upregulation of cell cycle markers and miR-294 targets 8 weeks after MI. Thus ectopic transient expression of miR-294 recapitulates developmental signaling and phenotype in cardiomyocytes promoting cell cycle reentry that leads to augmented cardiac function in mice after myocardial infarction.