April 13, 2011 | 6
Last week, the science community was set a-buzz with a new study that showcased the unique relationship between salamanders and algae. The research, done by Ryan Kerney at Dalhousie University in Halifax, Canada, found that spotted salamander tissues contained algae embedded within them. The exact purpose of this relationship (which begins when the salamanders are only embryos) is not yet known, and the jury is still out if it is truly a symbiotic relationship or tends towards mutualism.
An area of research related to that done by Kerney et al. is one that has so far managed to stay out of reach: cellular regeneration.
True cellular regeneration is known as the "Holy Grail" of scientific and medical research – the ability to regenerate cells, tissues and organs without the need of transplants.
True regeneration is not yet possible in humans, according to Prof. Stephane Roy at the University of Montreal, who is looking at tissue regeneration in vertebrates. Since humans have such a limited ability to regenerate, restricted to our livers and skin, Roy had to find an alternative method to study this evolutionary marvel.
He, therefore, studies a relative of the spotted salamander known as the Mexican salamander, or Abystoma mexicanum, also known as the axolotl.
Not your average animal
The axolotl is a Urodele amphibian, which includes all newts and salamanders. However, unlike the others that leave the water once they are sexually mature, the axolotl stays solely aquatic.
In addition to its regenerative capacity, the axolotl is also a perfect model organism for a feature known as neoteny, where an animal maintains all its juvenile characteristics, such as external gills, despite reaching sexual maturity. This gives them the appearance of wearing an elaborately feathered headdress on the side of their heads, which along with their small lidless black eyes and wide head, guarantees that you will never see an animal quite like them again.
Whereas all Urodele amphibians possess some capacity for regeneration, the axolotl is one-of-a-kind. It can regenerate multiple structures like limbs, jaws, tail, spinal cord, skin and more without evidence of scarring throughout their lives. Axolotls can even receive transplanted organs from other individuals and accept them without rejection.
Sadly, it is listed as being critically endangered by the International Union for the Conservation of Nature and Natural Resources (IUCN) because it is only naturally located in one lake in Mexico, Lake Xochimilco. But, due to its amazing regenerative capacity, the axolotl is in captivity all over the word for medical research or the pet trade.
"We must do what we can for this amazing animal," says Roy, whose lab is one of the few in the world doing this type of research. If successful, it has the potential to help millions of people suffering from severe burns, loss of limbs, and even cancer.
According to Roy, the sheer amount of damage that the axolotl can recover from is unbelievable.
"You can cut the spinal cord, crush it, remove a segment, and it will regenerate. You can cut the limbs at any level – the wrist, the elbow, the upper arm – and it will regenerate, and it’s perfect. There is nothing missing, there’s no scarring on the skin at the site of amputation, every tissue is replaced. They can regenerate the same limb 50, 60, 100 times. And every time: perfect."
As if that isn’t incredible enough, the axolotl is also over 1,000 times more resistant to cancer than mammals. But, there is only so much information one can get from observing the physiological changes that occur during limb amputation before you must look closer.
The next step for Roy in his research on regeneration was taking a deeper look into the genetics of the animals by collaborating with German researchers to create the first transgenic, or genetically modified, axolotls.
"We’re at a point where we can manipulate the genetics of these animals," says Roy. "We can now ask fundamental questions, such as how does regeneration work? What steps are involved?"
In order to understand the mechanisms involved, Roy and his team were venturing into unknown territory. Therefore, like any good explorers, they needed a map. The researchers needed to create a road map showcasing the process step-by-step, so that they could test a whole host of things at the various stages of regeneration.
According to Roy, one of the first steps that occur in the regenerative pathway after amputation is the process known as the de-differentiation of cells.
"What happens is that cells at the site of amputation lose their individual characteristics? They effectively become pluripotent-like cells. It’s like making stem cells where you need it, when you need it!"
These cells then create a structure known as a blastema, which is a rounded cover on the wound where the newly de-differentiated cells congregate. This structure then develops into a fully functional limb, with no sign of the previous damage.
The Regeneration Detectives
Roy and his team have been exploring two compounds involved in the regeneration pathway of the axolotl: p53 and Transforming Growth Factor beta one (TGF-β1).
In humans, TGF-β1 is involved in cell differentiation and proliferation, while p53 is infamous as being mutated in over 50 percent of all human cancers.
"These genes are highly conserved across species," says Roy. "And that means that since we have copies, and the axolotl does too, it is a good way to start looking for connections. Because once we understand how an animal like the axolotl can regenerate, there is a possibility of transferring that to other species."
In a study in 2006, Roy monitored TGF-β1 levels during limb regeneration, and noticed that it was significantly higher up to 48 hours after limb removal, which is precisely when the blastema is formed. But if the levels were blocked using an inhibitor, blastema formation and limb regeneration did not occur.
The other protein that Roy explored was p53, which is involved in many aspects of regeneration, DNA repair, and cancer
For his experiment, completed in 2007, Roy and his associates repeated the same experiment, but with p53 instead of TGF-β1. The results showed that all the limbs treated with a suppressor of p53 showed significantly decreased regenerative development.
But unlike in the TGF-β1 experiment, the blastema did form properly. It appeared that the regenerative pathway was activated to create a blastema, but was inhibited before the de-differentiated cells could be reprogrammed and begin forming a new limb.
Therefore, Roy says that both TGF-β1 and p53 appear to be absolutely essential for limb regeneration.
"But, there is still plenty of more work to be done before we get the whole picture and can apply it to other species, including humans," says Roy.
The Future is Wild
There are myths that humans have the ability to regenerate, and that is really only true to a certain extent.
The obvious example of regeneration is skin, which can heal perfectly if only the dermis (the upper-most layer) is damaged. But, if the lower layers (the dermis) are damaged, scar tissue will grow and cover the area.
According to Roy, the most common belief is that human livers can regenerate, which is not completely true. He says that it’s actually called "compensatory hyperplasia," and the cells simply grow to accommodate the decrease in function.
"When you cut part of the liver out, that part doesn’t grow back. What got left behind will simply grow. You can tell the pieces apart, unlike the regeneration of the axolotl."
However, he noted, the liver is still an amazing organ in which as small as 25 per cent can heal into a healthy and complete liver. But, it cannot heal indefinitely.
Therefore, the capacity of human regeneration as we know it now, is limited.
But, the hope for Roy, and all researchers working on limb regeneration, is that the mechanism from the axolotl can eventually be transferred into another species. However, this will not happen soon.
Roy is ever optimistic, but notes that caution is needed before you start placing orders for new body parts. He believes that with the axolotl’s amazing ability and enough time, anything is possible.
"I believe that we will be able to improve tissue healing for burn victims, help eliminate complications for transplants, and even help eliminate cancer. All of this could happen within the next 10 to 20 years. One day, we will succeed!"
And perhaps, make the impossible, possible.
For further reading on the amazing regenerative capacity of the axolotl, please see:
1) Lévesque M. et al (2007) Transforming Growth Factor Beta Signalling Is Essential for Limb Regeneration in Axolotls. PLoS ONE 2(11) e1227.
2) Roy, S. and Lévesque, M. (2006) Limb regeneration in axolotl: Is it superhealing? TSW Devel.& Embry. 1(S1).
3) Villiard, É. et al. (2007) Urodele p53 tolerates amino acid changes found in p53 variants linked to human cancer. BMC Evolu. Biol. 7(180) http://www.biomedcentral.com/1471-2148/7/180
4) Zambrano, L. et al. (2006) Ambystoma mexicanum. In: IUCN 2009. IUCN Red List of Threatened Species. Version 2009.2. http://www.iucnredlist.org.
About the Author: David Manly is a Canadian freelance science journalist who holds degrees in Biology and Zoology, as well as a Masters of Journalism. Having worked in a lab, he now spends time writing about the wondrous world of animals for Lab Spaces, as well as for his own blog The Definitive Host, and you can always find him on Twitter (@davidmanly). His fascination with reptiles and amphibians stems from visiting his local museum as a child and staring at dinosaur bones for hours on end, transfixed. Now, all those years later, that feeling still is as present as ever.
The views expressed are those of the author and are not necessarily those of Scientific American.