Stem cells, a stunning biological miracle, can replace, regenerate and repair damaged or lost cells. Most animal and human stem cells can only produce a limited number of cell types. For example, stem cells from gut or hair produce only gut or hair, respectively. The ability to recover almost any lost cell type is known as whole body regeneration and is present in many distantly related invertebrates.
Although adult pluripotent stem cells (aPSCs) have been found in a variety of animal species, including sponges, hydras, planarian flatworms, akoelworms, and even sea squids, no species has figured out how to make them.
Researchers from Harvard University's Department of Organismic and Evolutionary Biology have discovered the cellular mechanism and molecular pathway for the creation of aPSCs in the acoel worm Hofstenia miamia, in a new study published in the journal Cell.
H. miamia, also called the triple-banded panther worm, is a species that uses aPSCs called “neoblasts” to regenerate completely. When you dissect H. miamia, each piece will develop a new body with a mouth and a brain. Because of its regenerative capacity, senior author Professor Mansi Srivastava brought H. miamia together in the field years ago. Back in the lab, H. miamia began to develop large numbers of easily studyable embryos.
Postdoctoral researcher Lorenzo Ricci established a methodology for transgenesis in H. miami in a previous study by Srivastava and co-author. Transgenesis is the process of adding something to an organism's genome that is not typically a component of that genome. Lead author Julian O. Kimura (Ph.D. '22) was able to investigate the origin of these stem cells using this method.
According to Kimura, pluripotent stem cells are a feature of all living things with the ability to regenerate. When an animal is injured, these cells are responsible for replacing lost body components. I thought that by learning how animals like H. miamia produce these stem cells, we could learn more about what enables some animals to regenerate.
Stem cell populations of adult animals share several characteristics, such as expression of the Piwi gene. However, no one has yet determined how these stem cells are created in any species in the first place. According to Srivastava, they were primarily studied in the context of adult animals, and in some species, we have some ideas about how they might function, but not how they were created.
The scientists thought that since the worm pups contain aPSCs, they must be produced during embryogenesis. Ricci used transgenesis to generate a line that introduces the protein Kaede into the cell, making the embryo cells glow fluorescent green.
Kaede is photoconvertible, so when a laser beam with a very specific wavelength is pointed at a green color, it turns red. The individual green cells of the embryo can then be laser hit to change their color from green to red.
"We've invented a really new method in the lab using photo-transformed transgenic animals," said Srivastava, for determining the fates of embryonic cells. By monitoring the development of embryos, Kimura used this technique for lineage tracing.
Kimura observed the growth of the embryo dividing from a single cell into many. The early division of these cells is characterized by stereotyped division, meaning that cells from embryo to embryo divide in exactly the same way, allowing for consistent cell naming and research. This situation has led to the idea that each cell serves a different function. For example, in the eight-cell stage, it is possible that the cell in the upper, left corner produces one particular tissue, while the cell in the lower, right corner produces a different tissue.
Kimura methodically photo-transformed each early embryonic cell to determine its function, creating a complete fate map at the eight-cell stage.
He then observed the cells, when the worm had grown into an adult, while still having red labeling. Kimura was able to locate each cell by repetitive tracking on a large number of embryos.
He discovered a very special pair of cells that gave rise to cells that looked like neoblasts at the sixteen-cell stage of the embryo. “We were extremely excited about this,” Kimura continued.
We needed to not only find cells that resembled neoblasts in appearance, but also show that cells behaved similarly to neoblasts.
To be sure, Kimura tested this particular group of cells, known as 3a/3b, in H. miami. In order for cells to become neoblasts, they must have all the known characteristics of stem cells. Do the offspring of these cells produce new tissue during regeneration? The researchers discovered that indeed only the offspring of these cells produced new tissue during regeneration.
The degree of gene expression in stem cells, which must express hundreds of genes, is another feature that sets them apart.
To see if 3a/3b fit this feature, Kimura used a sorting machine to separate the red and green cells in the progeny such that 3a/3b glowed red and all other cells glowed green. He then used single-cell sequencing techniques to find out which genes were expressed in the red and green cells. This information showed that only the offspring of 3a/3b cells and the offspring of any other cells did not match stem cells at the molecular level.
To Kimura, this was clear evidence that the stem cell population in our system has a cellular origin.
The ability to capture cells as they develop and identify the genes responsible for their formation is possible by knowing the biological origin of stem cells, which is very important.
By constructing a huge dataset on embryonic development at the single cell level, Kimura determined which genes are expressed in each cell in embryos from the beginning to the end of development. It gave the transformed 3a/3b cells some more time to grow, but did not allow them to reach the incubation stage. He then used the extraction method to capture these cells. By doing this, Kimura was able to determine exactly which genes were specifically expressed in the cell lineage that gave rise to the stem cells.
According to Kimura, “our analysis shows a collection of genes that could be crucial regulators for the development of stem cells.” "Homologues of these genes serve important functions in human stem cells, and this applies to all animals."
“This is an astonishing story because he found it when he graduated,” Srivastava said of Julian. “I started my lab with the intention of understanding how stem cells form in the embryo.”
The mechanism by which these genes operate in Hofsthenia miamia stem cells will be further investigated by researchers, helping to reveal how nature has created a way to produce and maintain pluripotent stem cells.
By understanding the molecular regulators of aPSCs, the researchers will be able to compare these methods across species and show how pluripotent stem cells evolve throughout the animal kingdom.
Günceleme: 18/01/2023 15:22