Axolotls can regenerate their brains, revealing secrets of brain evolution and regeneration

Theaxolotl(Ambystoma mexicanum) is an aquatic salamander renowned for its ability toregenerate its spinal cord, heart and limbs. These amphibians alsoreadily make new neuronsthroughout their lives. In 1964, researchers observed that adult axolotls couldregenerate parts of their brains, even if a large section was completely removed. But one study found that axolotlbrain regenerationhas a limited ability to rebuild original tissue structure.

So how perfectly canaxolotl's regenerate their brains after injury?

As aresearcher studying regeneration at the cellular level, I and my colleagues in theTreutlein Labat ETH Zurich and theTanaka Labat the Institute of Molecular Pathology in Vienna wondered whether axolotls are able to regenerate all the differentcell typesin their brain, including the connections linking one brain region to another. In ourrecently published study, we created an atlas of the cells that make up a part of the axolotl brain, shedding light on both the way it regenerates and brain evolution across species.

Why look at cells?

Differentcell typeshave different functions. They are able to specialize in certain roles because they each express different genes. Understanding what types of cells are in the brain and what they do helps clarify the overall picture of how the brain works. It also allows researchers to make comparisons across evolution and try to find biological trends across species.

One way to understand which cells are expressing which genes is by using a technique calledsingle-cell RNA sequencing (scRNA-seq). This tool allows researchers to count the number of active genes within each cell of a particular sample. This provides a "snapshot" of the activities each cell was doing when it was collected.

这个工具很有帮助理解the types of cells that exist in the brains of animals. Scientists have used scRNA-seq infish,reptiles,miceand evenhumans. But one major piece of the brain evolution puzzle has been missing: amphibians.

Mapping the axolotl brain

Our team decided to focus on thetelencephalonof the axolotl. In humans, the telencephalon is the largest division of the brain and contains a region called theneocortex, which plays a key role in animal behavior and cognition. Throughout recent evolution, the neocortex hasmassively grown in sizecompared with other brain regions. Similarly, the types of cells that make up the telencephalon overall havehighly diversifiedand grown in complexity over time, making this region an intriguing area to study.

We used scRNA-seq to identify the different types of cells that make up the axolotl telencephalon, including different types ofneuronsandprogenitor cells, or cells that can divide into more of themselves or turn into other cell types. We identified what genes are active whenprogenitor cells become neurons, and found that many pass through an intermediate cell type called neuroblasts—previously unknown to exist in axolotls—before becoming mature neurons.

We then put axolotl regeneration to the test by removing one section of their telencephalon. Using aspecialized method of scRNA-seq, we were able to capture and sequence all the new cells at different stages of regeneration, from one to 12 weeks after injury. Ultimately, we found that all cell types that were removed had been completely restored.

We observed that brain regeneration happens in three main phases. The first phase starts with a rapid increase in the number ofprogenitor cells, and a small fraction of these cells activate a wound-healing process. In phase two, progenitor cells begin to differentiate into neuroblasts. Finally, in phase three, the neuroblasts differentiate into the same types of neurons that were originally lost.

Astonishingly, we also observed that the severedneuronal connectionsbetween the removed area and other areas of the brain had been reconnected. This rewiring indicates that the regenerated area had also regained its original function.

Amphibians and human brains

Adding amphibians to the evolutionary puzzle allows researchers to infer how the brain and its cell types has changed over time, as well as the mechanisms behind regeneration.

When we compared our axolotl data with other species, we found that cells in their telencephalon show strong similarity to the mammalianhippocampus, the region of the brain involved in memory formation, and theolfactory cortex, the region of the brain involved in the sense of smell. We even found some similarities in one axolotl cell type to the neocortex, the area of the brain known for perception, thought and spatial reasoning in humans. These similarities indicate that these areas of the brain may be evolutionarily conserved, or stayed comparable over the course of evolution, and that the neocortex of mammals may have an ancestor cell type in the telencephalon of amphibians.

While our study sheds light on the process of brain regeneration, including which genes are involved and how cells ultimately become neurons, we still don't know whatexternal signalsinitiate this process. Moreover, we don't know if the processes we identified are still accessible to animals that evolved later in time, such as mice or humans.

But we're not solving thebrain evolutionpuzzle alone. TheTosches Labat Columbia University explored the diversity of cell types inanother species of salamander, Pleurodeles waltl范,而实验室广东社科院我dical Sciences in China and collaborators at life sciences companyBGIexplored how cell types arespatially arranged in the axolotl forebrain.

Identifying all the cell types in the axolotl brain also helps pave the way for innovative research in regenerative medicine. The brains of mice and humans havelargely lost their capacityto repair or regenerate themselves.Medical interventionsfor severebraininjury currently focus on drug and stem cell therapies to boost or promote repair. Examining the genes and cell types that allow axolotls to accomplish nearly perfect regeneration may be the key to improve treatments for severe injuries and unlock regeneration potential in humans.

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This article is republished fromThe Conversationunder a Creative Commons license. Read theoriginal article.