Brain injuries and neurodegenerative diseases are some of the most challenging medical conditions to treat. Every year, millions of people worldwide will suffer from a Traumatic Brain Injury (TBI)¹ and as many as 55 million people will be diagnosed with dementia; this number is predicted to almost triple by 2050². For such an important organ, the human brain is not very good at fixing injuries, meaning any damage or disease could cause permanent disability. But imagine we found a way to help our brains heal. Could the consequences of TBIs and brain degeneration become a thing of the past?

For the Aztec Emperor Moctezuma II, it was probably too late. Once the leader of a prosperous civilisation in ancient central Mexico, he was struck in the head by a single rock thrown from a sling and died shortly after³. Exactly how the emperor died is still under debate, but such an injury in the times of the Aztecs would have likely been impossible to properly diagnose and treat. Even today, brain injuries like this can be complicated, and sadly, the outcome might still be the same. Unfortunately for the Aztecs, Emperor Moctezuma’s death would essentially seal the fate of their empire, confining them, and their legends, to the history books.

Today, we know that ancient Aztec legends contained a hint to treating the unlucky Emperor. They believed that our world (“the Fifth Age”) was created when the Gods sacrificed themselves for the sun to move. This act, they believed, birthed the creation of time and marked the beginning of human history. However, one God, Xolotl, the God of fire and lightning, tried to escape his fate by transforming himself into different forms. One day, he came across Lake Xochimilco, where he transformed into an Axolotl and dove into the lake. Legend has it that Xolotl was found, caught, and killed by the God of the Wind, Ehecatl. In other versions of this legend, he was forgiven and condemned by his brother Quetzalcoatl to forever live his life in the lake as an axolotl⁴.

At this point in the story, you may wonder what this legend has to do with TBIs and a deceased Aztec emperor. Well, it is perhaps ironic that their empire may have fallen because of a TBI, as unbeknown to the Aztecs themselves, within their lands, on their dinner tables, and even spoken about in the legend of Xolotl, were the very creatures that today are helping scientists understand and discover new ways of treating TBIs and brain degeneration: axolotls.

What is an axolotl?

Axolotls (ack-suh-lah-tuhl) are small salamanders (~12 inches) native to Mexico. Their name ‘Axolotl’ roughly translates to ‘water monster’ in the Aztec language Nahuatl. Typically, wild axolotls are brown-grey in colour with prominent, external gills (despite having functioning lungs), underdeveloped limbs and webbed feet⁵.

Actually, you could mistake these smiley salamanders for large tadpoles, and in fact, that’s almost what they are. Most amphibians experience very elaborate changes in their appearance and metabolism as they develop from their aquatic larval stage to their adult form. This process, called ‘metamorphosis’⁶, is what tadpoles undergo to become frogs and is needed because the adults tend to live most of their lives on land.

In contrast, axolotls are strange; they don’t metamorphose unless pushed to, meaning they can remain frozen in a youthful state of neoteny forever⁷– even though reproductively they are adults⁸. Why they remain ‘young’ is still unknown, but scientists think it might be advantageous for these little salamanders to stay aquatic, so perhaps there was no need for them to grow up. Despite their friendly faces, axolotls chow down on little fish, arthropods, insects, molluscs and worms, sucking their unsuspecting victim directly into their stomachs, like a vacuum⁹. But why are these cute little creatures so interesting to scientists?

Regrowing limbs… and brains?

Axolotls are famous for their amazing superpowers of regeneration. In fact when hungry, baby axolotls sometimes chomp off limbs from their nearby siblings. Of course, the limbs will grow back…. eventually. It wasn’t long before scientists recognised the importance of this phenomenon and its potential for human medicine, soon discovering that these super salamanders also regenerate their hearts, eyes, spinal cords, neurons and even their brains.

The human brain is generally made up of two different cell types; neurons and glial cells. Neurons are very important for sending and receiving messages, whilst glial cells provide support and help with connections in the brain. When neurons become damaged, it is very hard for our brains to repair or replace them, meaning we can lose vitally important connections, affecting for example our ability to speak, move or think. If we could work out how axolotls repair their brains, could we find ways to help regenerate a human brain too?

To do so, scientists had to first figure out if axolotls actually restored their brain back to its pre-injury state or whether they just ‘patched’ the damage to solve the problem. For decades, this has been a major gap in our understanding of axolotl brain regeneration, but now scientists finally have the tools to start answering this question. Using a very powerful technique called Single Cell RNA sequencing (scRNA-seq), scientists can count active or inactive genes in individual cells. This means that if you took a group of 100 cells, scRNA-seq would show you a freeze-frame of the processes taking place in each cell at any one time. scRNA-seq can be used to look for patterns of activity or inactivity that could tell scientists how the brain is healing over time.

One recent study began by looking at which cells are found in a piece of the Axolotl brain called the telencephalon¹⁰. In humans, this part of the brain contains the neocortex, which helps to control how we behave and how we think. Using scRNA-seq, the researchers found out which genes are important for making new neurons in this part of the brain as the salamanders matured.

So what happened when they damaged the brain? To do this, the scientists removed a piece of the telencephalon and performed a special type of scRNA-seq (called Div-seq) over the course of 12 weeks. This experiment allowed them to keep track of the newly made cells and record which genes were being turned on and off in them during regeneration. Amazingly, a pattern emerged. Regeneration began with lots of unspecialised ‘progenitor’ cells gathering at the injury site, some of which began healing the wound. Next, progenitor cells began changing into neuroblasts, an ‘in between’ cell type, then finally into more specialised neurons that are important for transmitting brain messages. Importantly, not only were these new neurons the same type that were originally lost when the brain was damaged, but amazingly, the researchers saw connections between the damaged region and the surrounding regions reforming. This means that the axolotl brain could be regenerating back to its pre-injury state.

In an independent study¹¹, another group of scientists used a different approach, called Stereo-seq, to ask a similar question. Stereo-seq, like scRNA-seq, looks for active and inactive genes. However, this approach looks for these changes using fine slices of tissues, for example, a slice of the brain. When analysed, Stereo-seq produces visually stunning pictures of these reactions. But what did they learn? Just like the other study, they found that over time the brain regenerated. New unspecialised cells began appearing at the injury site, followed by healing of the wound and new tissue forming. After around 2 months, the damaged area looked similar to before it was damaged. They also found, unlike the other study, unspecialised cells kept in a dormant (‘sleeping’) state near where the injury happened, became active and probably helped to heal the wound. Basically, unlike humans, axolotls never lost that amazing ability to regenerate their brain cells. But what does this mean for us? When we were developing as embryos, we had this superpower too, so if scientists could work out how the axolotls keep this power turned on as they mature, maybe they can find out how to turn it back on in humans too.

Cellular Atlas of a mouse embryo captured using Stereo-seq. Image created using BioRender.com.

Despite their different approaches, a clear message of hope emerged from both studies: axolotl brains can regenerate and restore themselves back to normal. Imagine if one day we could do the same; fully regenerate a human brain, eradicating the potentially life altering consequences of TBI’s and neurodegeneration. Of course, much more work is now needed to really understand the finer details of these regenerative superpowers. And even if we do, human brains are still very complicated, housing our personalities, memories and feelings. Maybe we can heal the damage but could we find a way to piece back these parts of ourselves? We would hope so, and hopefully, one day in the not-too-distant future, we will find out.

Edited by Zheng-Shan Chong

Copy-edited by Rachel Shannon

1. https://www.who.int/publications/i/item/9789241564793
2. https://www.alzint.org/about/dementia-facts-figures/dementia-statistics/
3. https://thejns.org/focus/view/journals/neurosurg-focus/39/1/article-pE2.xml
4. https://www.ancient-origins.net/myths-legends-americas/xolotl-underworld-dog-god-aztecs-009970
5. https://www.nationalgeographic.com/animals/amphibians/facts/axolotl
6. https://en.wikipedia.org/wiki/Metamorphosis
7. https://en.wikipedia.org/wiki/Neoteny
8. https://academic.oup.com/endo/article/145/2/760/2500402
9. https://www.nationalgeographic.com/animals/amphibians/facts/axolotl
10. https://www.science.org/doi/10.1126/science.abp9262
11. https://www.science.org/doi/10.1126/science.abp9444