September 13th, 2022
Written by: Marissa Maroni
Many students have the opportunity to perform a sheep brain dissection as part of an introductory biology class. During the lab, an instructor will help them identify different parts of the brain and what their functions are. For example, they might learn about the role of the hippocampus in long-term memory and point it out in the sheep’s brain. Imagine you are in this lab and while thinking about a sheep’s brain, you find yourself comparing it to the human brain and its functions. Humans and sheep share similar brain regions but have some obvious differences in behaviors, like different ways of communicating. Now imagine you are looking at a frog brain or a fly brain. How would these compare to ours and what can that tell us about our evolutionary differences among species? How do species-specific brain differences allow each animal to perform different functions? Scientists from the Vienna Biocenter and ETH Zürich aimed to answer these questions in the amphibian axolotl (Ambystoma mexicanum) which you may have seen in TikToks, like the one below1. Not only are they incredibly adorable but they also have the unique ability to regenerate various parts of their body including their brain! This article dives into how the axolotl brain compares to other species and begins to tease apart how axolotls regenerate their brain.
What types of cells are in the axolotl brain and are they like other species?
Scientists were able to investigate what kinds of cells are in the axolotl brain using a technique called, single-nucleus RNA-sequencing (snRNA-seq). The snRNA-seq technique quantifies transcripts which are the middlemen between DNA, the genetic code, and proteins, the workers of the cell. This technique quantifies many transcripts in single nuclei and uses this information to learn more about the cells in a tissue or organism. You can check out this PennNeuroKnow article for more details on single sequencing techniques.
Certain cell types express specific types of transcripts in large amounts, compared to other transcripts. These specific transcripts act like markers or signs to researchers indicating the type of cell, like a neuron! Using their sequencing data in this manner, they identified the following cell types: two types of neurons (the difference between these two neurons is whether they activate or inhibit other neurons), several types of glia (specialized non-neuronal cells responsible for cleaning and protecting the brain), and neuroblasts (cells that eventually develop into neurons). With these newly defined axolotl cell types, the scientists now had information on the amount of many transcripts in individual nuclei and categorized them by cell type.
Next, the scientists compared the transcriptional information about axolotl cell types in the brain to two other species, turtles and mice. With the sequencing data, the researchers can ask questions like, “do axolotl excitatory neurons have high amounts of XYZ transcript similarly to turtle excitatory neurons?” Notably, they found that the axolotl excitatory neurons were transcriptionally similar to the mouse and turtle. They were able to do this same analysis with inhibitory neurons and found again that the axolotl brain was transcriptionally related to inhibitory neuronal cells in turtles and mice. Interestingly, the excitatory neurons across species, although related, were less similar to each other in comparison to inhibitory neurons across species suggesting a greater evolutionary diversity in excitatory neuronal cell types among these species. More broadly, this research provides a snapshot of what cells are in the axolotl brain and shows that at the transcriptional level their brains are more similar to turtles and mice than one might assume!
What can sequencing tell us about brain regeneration in axolotls?
As previously mentioned, axolotls can regenerate parts of their brain. Scientists investigated the brain regeneration process in axolotls using the technique DIV-seq. DIV-seq is a form of snRNA-seq that specifically sequences nuclei from dividing cells, this being a crucial step in regeneration. To perform DIV-seq, they created a brain injury by removing a piece of the axolotl brain. Overtime, they collected nuclei from dividing cells and sequenced them to identify what cell types are present throughout regeneration and what transcripts they express.
A specialized type of glia was found to start the process of regeneration. The specialized glia type is called ependymoglia and they help to generate neurons. Neuroblasts, neuron precursors, came next during the regeneration process. This sequential process illustrates that as the cells divide one type of cell dies off and new cell types begin to form each helping to build back the cells once removed. As the regeneration process progresses, the cell types transition from glia into excitatory and inhibitory neurons. After the completion of the regeneration process, the researchers tracked the connections of newly regenerated neurons. Would these new neurons make the same connections as before? Interestingly, the researchers found the newly regenerated neurons did in fact make the same connections that an uninjured axolotl brain does! Indicating that not only can axolotls regenerate portions of their brain, but they can also form the same connections as prior neurons. This not only gives insight into the process required for brain regeneration but also suggests that the process can reestablish prior connections.
Hopefully this article can convince you of two things. First, axolotl brains are evolutionarily similar to other species and have a regenerative ability that could help scientists to understand how to apply this awesome process in broader scenarios. Second, sequencing technologies are an incredibly powerful tool that give us snapshots of what is being made in cells. If you ever get the chance to meet an axolotl, make sure to commend them for their super regenerative ability.
- Lust, K., Maynard, A., Gomes, T., Fleck, J. S., Camp, J. G., Tanaka, E. M., & Treutlein, B. (2022). Single-cell analyses of axolotl telencephalon organization, neurogenesis, and regeneration. Science, 377(6610), eabp9262.