Hofstenia miamia. Photo: marinespecies.org/CC BY-NC-SA 4.0
Regeneration is a fascinating biological phenomenon. One cannot deny that mammals may have more sophisticated brains than most other animals. However, they are incapable of regenerating body parts. Intriguingly, some worms have solved the mystery behind regeneration. And these worms do not just regenerate body parts; they can regenerate their entire bodies. Irrespective of how many pieces they have been cut up into, each piece contains the information to form the whole worm again.
For decades, researchers have studied the planarian worms Schmidtea mediterranea for their regenerative capabilities. Planaria contain adult stem cells called neoblasts: cells which have the power to go on to form the whole body. However, to delve deeper into questions like when and how these adult stem cells arise in the first place, one needs to study the embryo of these worms. Planaria have always been an excellent model system to study regeneration using adult stem cells. However, the inability to directly visualise or manipulate the live embryo in planaria left a lot of open questions about the origin story of these adult stem cells.
A group of researchers led by Mansi Srivastava at Harvard University tackled these questions using a different but equally fascinating worm called Hofstenia miamia. These acoel worms are also capable of whole-body regeneration using neoblasts, and luckily for the researchers, have a much more accessible embryo.
In a study published late last year in Cell, Mansi’s group was able to directly see the first cells that contain the information for the whole body of the worm.
The stem cells in the adult worm can generate different body systems when required, like the nervous system, or the digestive system. The main questions of the study were if each body system had its unique stem cell origins in the embryo, or if they all had a common origin. Using lineage tracing, they went on to find that they could trace the origin of the adult stem cell population to a single pair of cells in the 16-cell stage of the embryo.
One way to trace the lineage of any cell is to label the cell in some way and to ensure that the label does not fade with time and successive divisions of the cell. A common way to do this is to create a transgenic cell line.
Julian, the first author of this study, used a cell line which expressed a fluorescent photoconvertible protein called Kaede, which made all the cells in the embryo and the hatched worm glow green. UV light causes a conformational change in this protein, making it fluoresce red instead of green, turning the cells red as well. Shining a UV laser on specific cells of the embryo turns the protein in those cells red, and more importantly, the cells stay red as they divide more and more and go on to form different parts of the worm.
This way, one can visually follow the path of these red cells from the embryo right into the hatched worm and get an idea of which cell they began from. Science is replete with stories of serendipity, and it so happens that the cell line used in this study was not originally made for the study at all.
“Lorenzo, one of the authors of this paper, was originally intending on generating a line that marks only the stem cell population in the adult worms. He wanted to use this photoconvertible protein as simply a reporter to mark these neoblasts,” said Julian. “But what he found was that the entire worm was glowing green. The line ended up not being a good tool to mark the adult stem cells, but rather a great tool for my project of tracing the lineage of the cells.”
Julian began systematically shining the UV laser on individual and pairs of cells in the embryo, starting at the 8-cell stage, and followed the red cells to see what they form. He found that different cells went on to form different body parts of the worm. When one specific pair of cells in the 16-cell stage of the embryo, the 3a and 3b micromeres, were turned red, the hatched worms showed red cells in roughly the region where the neoblasts were known to be. It appeared that Julian had found the origin cells of these adult stem cells, at least using lineage tracing.
But he had to confirm if those red cells in the hatched worm were truly the stem cells or not. And for this, he had to put them to the ultimate test: cut up the worm and see if the regenerating tissue had red cells. When he cut up the worm in which the hypothetical stem cell population was glowing red, the regenerating tissue also had red cells, confirming his hypothesis. Finally, Julian used single-cell RNA sequencing to study gene expression in the red cells, to identify their molecular signatures and confirm their molecular identity as neoblasts. He also went to study gene expression at different time points in the embryos and the hatched worms, to get an idea of the molecular trajectory the embryonic cells take to form the adult stem cells.
“What Julian was able to do was definitively tell us which cells of the embryo give rise to stem cells,” said Mansi, the corresponding author of the paper and the Principal Investigator of the lab studying Hofstenia miamia. “Now we are really excited to dig into the mechanistic work of how the stem cells are made.”
Hofstenia miamia is a relatively newer model system to study whole-body regeneration and one that is evolutionarily distant from the planarian worms used commonly in labs. Mansi is very intrigued by the array of questions that it poses in the realm of stem cells and regenerative biology.
“One of the key questions that we are hoping to ask with this is about pluripotency. You and I have stem cells, but they are lineage-restricted; our hair stem cells cannot make intestinal cells or our intestinal stem cells cannot make skin cells. But Hofstenia adult stem cells are collectively pluripotent. There must be something fundamentally different about the genomes of these organisms. What is it, what is different about these organisms is that they have no problem maintaining pluripotency throughout their lifetime, but our cells just cannot do it. We want to understand this difference.”
Rohini Subrahmanyam is a postdoctoral researcher at Harvard University in a neighbouring lab studying human embryo development using organoids.