Regeneration in Mammals–Ancient Capacity is Not Lost, but Actively Suppressed
It was 1995, and Ellen Heber-Katz ran a busy lab at the Wistar Institute in Philadelphia, at the top of a thriving career in auto-immunity. Her lab housed many mice in common cages, labeling the mice with tiny holes punched in their ears. One day, she discovered that a litter of mice were un-labeled, and she spoke to her post-doc about it. But Lise Clark said she was sure she had punched their ears. So Heber-Katz punched their ears herself, and checked back the next day. She could hardly see the holes she had punched. Within a few days, the ears healed over, smooth skin, seamless cartilage with nary a scar. Mice aren’t supposed to be able to do this.
Many researchers would take this as a minor inconvenience, and contine the project with other mice. But Heber-Katz was more curious than that. She bred the mice that showed extraordinary healing ability, and set about to study what it was that set them apart from other mice.
Heber-Katz couldn’t wait to study the phenomenon in depth. But her colleagues counseled against it, urging her not to waste her expertise in the field of autoimmune disease and transplant rejection. They advised her to pursue the disappearing ear holes as “an aside,” like a hobby. But Heber-Katz knew she had stumbled onto something big, and she just had to go after it full force. “I realized, since I didn’t know anything about wound healing, I had better go to a meeting about it,” she says. So she did, and there an expert told her: “Oh no, mouse ear holes absolutely do not close.” So Heber-Katz kept her finding secret. “I was really on cloud nine,” she says. – Katie Moisse, [Sci Am (2006)]
She bred the mice, eventually crossing four strains to creat the MRL mouse line that heals so well. She found that their hearts heal scarlessly from injury. When humans suffer heart attacks, heart tissue dies and usually the damage becomes permanent. Mice can’t repair their own hearts either, but MRL mice can.
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? [Michael Rae at FightAging.org]
Juan Carlos Belmonte’s Group at the Salk Inst has gotten normal mice to perform the same trick, using a micro-RNA signal to turn normal heart (muscle) cells back into the stem cells from whence they came, so they can make new heart cells. They were able to reactivate
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 in Nature], 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…
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.
In other words, the surprise is that we didn’t lose something in the advance from fish to mammal, rather we acquired a response that suppresses regeneration. The ability to regenerate remains intact in mammals, but it is switched off. This raises the possibility that if we want our bodies to be able to regenerate damaged tissue without scarring, we don’t have to acquire a new mechanism or even re-acquire one that has been lost; all we have to do is to fiddle with biochemical switches. Switching on or off particular genes in particular tissues has become a reliable technology, using any of several techniques (e.g. CRISPR, RNAi, retroviruses).
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.
In the same vein, Heber-Katz’s group has also concluded that the capacity to regenerate has not been lost in mammals, but is actively suppressed. Four years ago, they had already identified a gene called p21 which is defective in their MRL mice. They knocked the p21 gene out of normal mice and discovered that those mice could heal their ears similarly to MRL mice.
Animals capable of regenerating multiple tissue types, organs, and appendages after injury are common yet sporadic and include some sponge, hydra, planarian, and salamander (i.e., newt and axolotl) species, but notably such regenerative capacity is rare in mammals. The adult MRL mouse strain is a rare exception to the rule that mammals do not regenerate appendage tissue…Using the ear hole closure phenotype, a genetically mapped and reliable quantitative indicator of regeneration in the MRL mouse, we show that the unrelated Cdkn1atmi/Tyj/J p21-/- mouse (unlike the B6129SF2/J WT control) closes ear holes similar to MRL mice, providing a firm link between cell cycle checkpoint control and tissue regeneration. [Ref]
Although Heber-Katz’s group was based on genetically engineered mice, there is no reason to expect they could not do the same with normal mice, using one of the three in vivo techniques I mentioned above to shut off the p21 gene temporarily and locally. I know of no one who is trying this in humans, but it seems to me that this technology is ready, and if I were a heart attack patient, I would eagerly volunteer for early trials.
This story of regeneration not being lost but suppressed fits beautifully with the song that I have sung so often on these pages and elsewhere–that there is no fundamental limit to life span in our metabolisms, but that evolution has programmed a fixed length of life for the purpose of stabilizing ecologies.
One thing that doesn’t fit so well is the story that Heber-Katz has been focusing on the last two years: inflammation is an essential part of the regeneration process [ref, ref, ref]. This could be an example of antagonistic pleiotropy. Suppressing inflammation is an important anti-aging strategy, and it may have to be pried apart from wound healing in order to make further progress.
Porpoises and other marine mammals
Adult porpoises can repel a large shark, but are frequently injured in the encounter. It is estimated that 40% of poropoises in the ocean have survived a shark bite, but they don’t carry scars from the event. Porpoise skin and the blubber layer underneath recover in a matter of days from lacerations up to a foot long.
A Georgetown University pediatrician published this information a few years ago in adermatology journal. “Reports of the survival after severe traumatic injury of other marine mammals, such as the southern elephant seal [ref] and the Hawaiian monk seal [ref], suggest that efficient healing of soft-tissue injury might be widespread among marine mammals.”
What do the porpoises have that we don’t have? I think the proper question is likely to be, by what signals have we suppressed the porpoise’s capacity to heal?
This post originally appeared on Josh’s blog Aging Matters here.