Studying stem cells in marine invertebrate
It sounds like a grisly experiment, but if you chop off the head of the marine animal Hydractinia echinata it doesn’t die. Instead it re-grows a head within a few days. That power to regenerate tissue throughout its life is why SFI-funded researcher, Dr Uri Frank (http://www.nuigalway.ie/zoology/frank/), believes it could offer a new model to study stem cells.
Stem cell research is one of the most innovative areas of biomedicine. These ’master cells’, which can develop into a variety of cell types, have immense potential in treating disease and degeneration in the human body - but sourcing, understanding and applying them can raise ethical and biological questions.
Attempts to understand the role of stem cells in ageing and disease to date have primarily been conducted using cells from mammals: mice and humans. However, Dr Uri Frank at NUI Galway is proposing that the invertebrate Hydractinia echinata - a relative of jellyfish, sea anemones, hydras and corals - would be cheaper, easier to work with and free of ethical restrictions. It would also provide new insights into the biology of stem cells and their evolutionary origin, and possibly open up new perspectives for research that are not feasible in mammals.
Humans and mice have ’pluripotent’ stem cells only in the embryo at pre-implantation stages, so sourcing them is technically difficult and ethically controversial.
However, Hydractinia, like many other basal invertebrates, maintains a population of ’pluripotent’ stem cells throughout its life. This is why you could chop off its head and it will regenerate a new one in a few days. “The immediate question that arises is why do some invertebrates maintain their pluripotent cells throughout life?” says Dr Frank. “And that question could be turned on its head - why don’t humans keep their pluripotent cells? Why do they lose them?”
Scientists believe that all animals living today, including invertebrates and humans, are the descendants of a single common ancestor that lived hundreds of millions of years before the times of the dinosaurs. If this is true, invertebrate stem cells should be very similar to their human counterparts and studying them may provide information on human stem cells.
“Possibly, this animal maintained its stock of pluripotent cells, like Hydractinia does, but as evolution progressed, for some reason humans and some other animals lost that trait,” says Dr Frank. “There is a theory, as yet unproven, that the loss of these cells was the price that had to be paid for increasing complexity.”
The hydroid animals that Dr Frank works on are morphologically simple, and their stem cells can be studied at any developmental stage. Furthermore, Dr Frank argues that while working with human or mouse stem cells is mainly restricted to cell cultures, Hydractinia is small and translucent and so enables the observation of experimentally labelled stem cells in the living animal.
Dr Frank’s SFI-funded projects include one that centres on characterising the general stem cell population of these invertebrate hydroids. Hyrdactinia contains some stem cells that can differentiate into any cell type, and others that can differentiate only into particular cells but not to others. Both types of stem cell look the same morphologically, so Dr Frank is seeking molecular markers for each cell population.
The other project looks at the function of the Wnt signaling pathway in stem cell decision-making. Some cells use the Wnt protein to signal to other cells how to behave, and it plays a role in embryonic development, but also in stem cells.
Dr Frank has shown that Wnt signals where to make the head in the Hydractinia embryo, but it also instructs stem cells to self-renew. Wnt signaling is similar in mammals, and he wants to further study the role of this pathway in what he proposes is a potentially powerful invertebrate model for stem cell research.