Worms that regenerate themselves
Flatworms called planarians are fated for their exceptional regenerative abilities. But how do the neoblast stem cells that can replace missing or injured organs know, in inverted commas, what to replace. Stowers scientist, Carolyn Adler spoke to Chris Smith. She has found a very clever way to remove discreetly just one organ which is allowed to map the unique gene that drives just the regeneration of a missing pharynx.
Carolyn - We studied these flatworms. For hundreds of years now, people have observed that even tiny fragments of these animals have the ability to regenerate entire animals after they've been amputated. This regenerative capacity depends entirely on the activity of a population of cells that are distributed throughout the animal that are stem cells. These cells are constantly dividing, and can give rise to all of the organs that make up the animal. But the big question that we really wanted to address is how these stem cells kind of know what is missing after an amputation or injury has occurred.
Chris - So, how did you attack that? What did you do to try to understand what the cells do and what the signals are that are driving them?
Carolyn - I developed an assay where it could selectively remove one single organ. We take some animals and I take off the water that they're normally living in and replace it with a concentrated solution containing sodium azide. The initial reaction is, the animals writhe around. They slowly stop moving over the course of 5 to 7 minutes. This large pharynx emerges from the ventral side of the animal. And with gentle pipetting, we can detach the pharynx from the rest of the animal.
Chris - So basically, the animal then has the option to use its stem cells to regenerate it. That's what you're studying.
Carolyn - Yes, so this, what we call chemical amputation leaves a small wound at the border between the pharynx and the intestine. And then the stem cells and the remainder of the animal are left with the task of recognising that this injury has occurred and then regenerating that organ.
Chris - And so, how did you then try to study what the stem cells were doing and what genetic programmes they were running in response to that injury and then the regenerative effort that they went through to replace the missing pharynx?
Carolyn - In order to understand the changes in the remainder of the animal after removal of the pharynx, we first performed expression profiling experiments to identify which genes would be upregulated after removal of the pharynx. We reasoned that these genes would be involved in pharynx regeneration. So then, we knocked down each of these genes one at a time and asked whether the pharynx either could or could not regenerate.
Chris - And so, what's the bottom line? What did you find?
Carolyn - We identified 20 genes that are required for regeneration of the pharynx. Among this population of 20 genes, some genes seem to be required very generally for stem cell function. Other genes, including the forkhead transcription factor FoxA, are required specifically for regeneration of the pharynx, but not for regeneration of any other organs.
Chris - Playing devil's advocate for a moment, what happens if you were to take a pharynx and stitch it onto the animal? So, it still had a pharynx. It was in the wrong place. Would it regenerate another one in the right place? Is there some kind of signal that comes from the missing pharynx that says, "I'm not here anymore" and that's what generates the response, and therefore, would a transplantation fool that response?
Carolyn - That's an excellent question and we think that something like that may be true for these animals. What we observed was that after removal of the pharynx, the expression of this transcription factor, FoxA, increased in the stem cells. What that means is that normally, when the pharynx is present, it could be preventing upregulation of FoxA within the stem cell population.
Chris - So, are you sort of saying that different organs have their own genes which signal, "I want you to make a new pharynx. I want you to make a new intestine. I want you to make a new nervous system" and when you injure a discreet bit of the organism, discreet genetic programmes which is on just to regenerate exclusively that body part?
Carolyn - Our evidence suggests that that may be the case. In our field, there is quite a bit of interest in understanding how the stem cell population is really organised and directs its outputs in response to injury and how that is hierarchically organised at the cell biological level.