At any one time, there are 700 African clawed frogs or Xenopus laevis living in one of the Crick’s aquatics room, part of our biological research facility (BRF). The frogs here are mostly used as a breeding colony, providing embryos for use in research by two groups at the Crick. Despite playing a key role in hundreds of scientific advances, the profile of frogs used in research has dropped recently, overshadowed by more well-known research animals like the mouse, the fruit fly and the zebrafish.
Over the past sixty years, frogs have played a starring role in a number of Nobel-prizewinning experiments. Today’s field of stem cell research was made possible by John Gurdon’s work cloning a frog by taking nuclei from cells in the intestine of a tadpole and placing them each in frog eggs. The eggs grew into new tadpoles and proved that mature cells (like the one in the original tadpole) still contain all necessary information to grow an organism from scratch.
Crick group leader and Wellcome’s Director of Science Jim Smith has worked with Xenopus throughout his career and is a strong advocate for them. His work with frogs has focused on the very earliest stages of development, identifying the mechanisms that cause cells to become different parts of the embryo.
“I would argue that we’ve learnt more about the principles of vertebrate development from amphibia than from any other vertebrate class,” says Jim. “There are many reasons for this; the females lay lots of eggs, the embryos develop outside the body, they have a simple ‘fate map’ that allows one to predict what different cells will eventually become, they are large and easy to dissect, you can culture them in a simple salt solution, and they don’t increase in size as they develop.”
It’s particularly useful that the frog embryos stay the same size throughout the early stages of their development, because it means that any substance added to an embryo won’t become diluted by the frog increasing in mass as it develops.
Camille Bouissou is a senior laboratory research scientist in Jim’s group and is currently working on a project made possible by this characteristic. It was started by former Crick postdoc George Gentsch, who found that a common biological technique used to study gene function could have unintended side effects.
The technique involves injecting antisense morpholino oligonucleotides, molecules that can be designed to ‘knockdown’ or reduce the activity of a specific gene, to allow researchers to see what role the gene plays. While using this technique to complement genetic experiments in frogs, George found that injecting these molecules was causing the embryos’ immune systems to kick into action.
“It is very important that we understand potential side-effects of Morpholinos,” explains Camille. “If they are causing this response in the immune system, it might mean that results obtained are muddier than we thought and we might have to reconsider how we use them. Morpholinos can still be a hugely valuable technique, but we should make sure that we’re getting the full picture when we’re using them.”
Camille is investigating whether the composition of the Morpholinos affects the severity of the immune response. She explains that if the same experiment was performed in a mouse, the embryo would have to be implanted back into the mother to continue developing, and as the embryo became larger the effective concentration of the Morpholino would decrease.
She is currently injecting thousands of frog embryos with Morpholinos that should not target any known gene. George’s original findings suggested that these injections should still cause an immune response in the frog embryos, even though they aren’t modifying specific genes.
Because frogs lay so many eggs, Camille is able to perform these experiments on a huge scale relatively quickly. For work like this, the experiments need to be replicated as many times as possible to confirm whether the immune system response is a consistent and reproducible side effect of the Morpholinos or a fluke.
Perhaps the most useful feature of frog development is that it is so fast. Jim explains: “If you want to see how a certain substance affects development, you can add it to a frog embryo and often see results within a day.”
Rebecca Jones is a PhD student studying a family of genes that isn’t well understood, other than that they play a role in certain cancers. But one of these genes has been found to also affect development in frogs. When too much of it is present, tadpoles grow two spines and two heads.
“This gives a really straightforward and visual way to check the activity of the gene. We can add something to the frog embryos that we think might affect the activity of the gene, wait a day or two and then look at which ones have developed this second axis,” explains Rebecca. “There isn’t another animal we could do this in. It’s incredibly fast and makes checking gene activity much simpler.” To get to the same stage of development in mice, it would take up to a week and require much more complicated imaging techniques to check on the embryos, involving more invasive procedures for the animals.
While the Xenopus at the Crick are generally used to study the very earliest steps in development, Jim says that their importance shouldn’t be underestimated in other areas of biomedicine: “They have played a role in so many discoveries. We’ve found things out about the cytoskeleton, the cell cycle, cell movement, cell fate, chromatin, apoptosis, and mesoderm and neural inducing factors. You name it - Xenopus have helped us to find out more about it.”