Salamanders and zebrafish are among the species that can regenerate limbs and even large portions of vital organs after injury, and the regrown portions of their anatomy are just the same as the original. We mammals cannot do this: we can manage fingernails, occasionally fingertips at a very early age, and portions of the liver, but that is about it. One line of modern regenerative research asks whether it is possible to somehow induce the regenerative biochemistry of salamanders and zebrafish in mammals. Is mammalian incompetence in healing a matter of lost capabilities that originally evolved in a distant shared ancestor species, and thus the necessary biochemistry still exists, but is in some way dormant? The answer to that question, and the more important questions regarding whether or not it will be very hard to make use of anything learned about regeneration in species like salamanders, are only going to be revealed by more research. The present state of knowledge leaves a lot of room for speculation.
In recent years research has pointed to the importance of the immune system in enhanced regeneration: immune cell activity seems key to regeneration in both salamanders and zebrafish, and if suppressed injuries scar rather than regrowing lost tissues. Also some inroads have been made into finding protein machinery such as the ERK pathway that is vital to salamander-like regeneration, but different and less active in mammals. These are all still starting points for later research rather than answers in and of themselves, guideposts that may help focus other scientific groups on the right paths. All in all this is largely a matter of preliminary research aimed at understanding exactly how regeneration works in these species, and trying to do anything with this knowledge is still a fairly remote possibility for much of this work.
That isn't always the case, however. This research group has taken a more aggressive approach, cataloging microRNAs in heart tissue that are shared by zebrafish and mice, finding those that differ in amount between species after heart injury, and then artificially altering mouse microRNA levels to be more like those of the zebrafish. The result is positive, which suggests that this research is a step in the right direction. In particular it adds weight to the idea that at least some shared regenerative mechanisms from the deep evolutionary past remain dormant in mammals rather than lost entirely:
[Researchers] have healed injured hearts of living mice by reactivating long dormant molecular machinery found in the animals' cells, a finding that could help pave the way to new therapies for heart disorders in humans. The new results suggest that although adult mammals don't normally regenerate damaged tissue, they may retain a latent ability as a holdover, like their distant ancestors on the evolutionary tree.
In a [2010 paper], the researchers described how regeneration occurred in the zebrafish. Rather than stem cells invading injured heart tissue, the cardiac cells themselves were reverting to a precursor-like state (a process called 'dedifferentiation'), which, in turn, allowed them to proliferate in tissue. Although in theory it might have seemed like the next logical step to ask whether mammals had evolutionarily conserved any of the right molecular players for this kind of regenerative reprogramming, in practice it was a scientific risk. "When you speak about these things, the first thing that comes to peoples' minds is that you're crazy. It's a strange sounding idea, since we associate regeneration with salamanders and fish, but not mammals."
The team decided to focus on microRNAs, in part because these short strings of RNA control the expression of many genes. They performed a comprehensive screen for microRNAs that were changing in their expression levels during the healing of the zebrafish heart and that were also conserved 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 did not decline with injury. So the team used adeno-associated viruses specific for the heart to target each of those four microRNAs, suppressing their levels experimentally.
Injecting the inhibitors into the hearts of mice that had suffered a heart attack triggered the regeneration of cardiac cells, improving 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 much reduced with treatment compared to controls, the researchers found. The improvements were still obvious three and six months after treatment - a long time in a mouse's life.
Heart failure is a leading cause of mortality and morbidity in the developed world, partly because mammals lack the ability to regenerate heart tissue. Whether this is due to evolutionary loss of regenerative mechanisms present in other organisms or to an inability to activate such mechanisms is currently unclear. Here we decipher mechanisms underlying heart regeneration in adult zebrafish and show that the molecular regulators of this response are conserved in mammals. We identified miR-99/100 and Let-7a/c and their protein targets smarca5 and fntb as critical regulators of cardiomyocyte dedifferentiation and heart regeneration in zebrafish. Although human and murine adult cardiomyocytes fail to elicit an endogenous regenerative response after myocardial infarction, we show that in vivo manipulation of this molecular machinery in mice results in cardiomyocyte dedifferentiation and improved heart functionality after injury. These data provide a proof of concept for identifying and activating conserved molecular programs to regenerate the damaged heart.