Imagine you are trying to solve a puzzle depicting a mesmerizing scene. Now, picture doing it without any guide, with no reference picture of the final image on the box. Besides, it’s not just a small and easy puzzle but one with 5000 pieces. That is the challenge scientists face when diving into the mysterious world of neurodevelopmental disorders like Autism Spectrum Disorder (ASD).
Let’s break that down. The brain is a complicated organ, and as we learned in the previous posts of this series, it gets even more complicated in the case of ASD. There is a whole community of microbial tenants in your gut playing along as well, making the list of questions we have to answer to understand ASD even longer – hence the 5000 pieces you have to decipher and connect. Unfortunately, we also have no clue what the completed puzzle picture will look like – there are huge gaps in our knowledge of ASD that we have to fill before we can see the whole image and understand the whole biology of ASD.
You might think: “But we do have a reference picture: every person with ASD!” There is a truth in that statement – we can observe the behaviour of people with ASD, and we can do blood tests and MRI scans – but these are merely individual pieces of the puzzle, giving some details but not all of the connections. We cannot understand the disorder in its entirety when only looking at the pieces separately.
Especially because the brain is involved, studying the mechanisms of the disorder becomes more challenging – at least when avoiding the need to break open a patient’s skull. In ancient times, scientists studied the brain in patients with head injuries or during surgical procedures, but I hope we all agree that is a less-than-ideal scenario to say the least, and clearly not the solution for filling in the gaps in the puzzle. Luckily, researchers have found other ways to get a peek inside – they have alternatives for human brains and I don't have to take you through the outdated methods! However, if you do want to learn more about the history of brain research, take a look at this review.
Scientists have unravelled several easier puzzles, each depicting only a small part of the bigger human puzzle. Let me introduce you to the main characters depicted on these easier puzzles: animal models. In this post, I will explain how these animals offer glimpses into the mysterious scene of the human brain and take you through some of the remarkable discoveries that have emerged from the complicated task of task of putting together the puzzle of ASD. I will guide you through a roadmap for animal research, explaining some important choices researchers have to make before using animal models.
The first choice: Do you need an animal model or is there an alternative?
As you might remember from my previous post, cause-and-effect relationships in ASD remain poorly understood; deciphering the cause-and-effect relationships – how and why the separate pieces fit together – is like untangling a complex web of threads. Well, scientists love nothing more than a challenge, and so, many researchers started studying these connections. When you want to find the answers to these types of questions, the first and probably most important decision you need to make is whether you really need to use an animal model. There are a lot of different models that can be used in research, and not all involve animals. Certain animal studies might reveal a chunk of a thousand puzzle pieces at once, but many research questions only need a few details of the picture to be answered – and so those questions can be studied by zooming in on only one or two puzzle pieces, for example cells in a test tube. There are many cell lines available for research (Do you want to learn more about using cells in research? Take a look at Alex’ post!). Nowadays, researchers can even grow mini brains or mini guts with microbes in the lab to study all the different brain and gut cells and their jobs in ASD. In this way, scientists can put together a chunk of maybe fifty or even one hundred puzzle pieces, without needing to use animals!
As researchers, we want to use lab animals to improve life – for example by understanding ASD to improve the lives of people with ASD. At the same time, animals also have a moral status, so we want to cause them the least discomfort possible. Therefore, researchers ask themselves critical questions before using animals in their studies, for example: “Is the purpose of this study worth all those animal lives?” and “Is there a way that this research can be performed without using animal models?” These are important ethical questions that researchers consider in terms of animal welfare, and there are tight regulations and rules that researchers have to follow to make sure animals are treated in a humane way and harm is minimized. For instance, as a scientist you have to ensure the well-being of the animals during your study as animals are living organisms able to feel pain.
The second choice: Who will be the subject of your picture?
If you have decided that it’s absolutely necessary to use animal models to answer your research question, then it’s time to choose what animals you will use. As I described above, trying to figure out an animal puzzle is often more ethically acceptable than directly experimenting on humans, and has fewer limitations than working with just some cells on a petri dish. Those are the main reasons the final picture of the animal puzzle is often used in research as a reference picture to put together part of the human puzzle.
For this to work, the animal image needs to be representative for the human picture. In other words, when you want to study a disorder in an animal, you have to make sure that the puzzle pieces – the details – that you will put together by using the animal, are also present in the human puzzle. In this way, you can translate the animal puzzle pieces you put together to pieces in the bigger human puzzle. In other words, you make sure that you build yourself a reference picture that is as realistic as possible.
As I want to solve the puzzle of the brain and the gut microbiota in ASD, as well as the communication between these two – the gut brain axis (take a look at my post about the gut microbiota in ASD) – these are the organs that I would study in an animal model. Therefore, the brain and the intestine of the animal model I choose have to be analogous to the human brain and intestine. In ASD research, commonly used animal models range from zebrafish to mice. Mice, for example, are surprisingly similar to humans in many ways and they are really nice for studying behaviour. Scientists have designed a lot of tests to measure changes in behaviour in mice, like mazes and sociability tests. Likewise, zebrafish might not look anything like humans, but their genetics are fairly similar to ours. Fun fact: zebrafish are transparent during their development, making them perfect for studying developmental disorders like ASD as more puzzle pieces are immediately visible!
The third choice: Do you want to add props?
I am especially interested in the details that mice can give us. But how do you study ASD in mice? As we want to understand ASD in humans and find treatments for humans, studying a normal mouse won’t give us all the answers. This brings me to the third decision: Should you give some human-like accessories to your animals to make the animal puzzle even more representative for the human puzzle?
Recently, researchers started using some really cool tools to study the role of the microbiota in ASD using mice. For this, they need a very special prop: poop. After putting the poop of children with ASD into mice, the mice start showing ASD symptoms! Can you guess why? It seems to all come down to the gut microbiota! Many gut microbes are present in your poop and those can be transplanted into mice via a poop transplantation. By giving the mice this special prop (human poop), researchers can measure the effects of the ASD dysbiosis on behaviour.
In the future…
Many of the details and connections I explained in this series were discovered using animal models. Chunks of the puzzle deciphered in the mice can be translated to the bigger human picture, giving us glimpses of the big human puzzle, helping us to understand our own bodies.
Animal models can be used not only for research into the development of ASD, but also for research into treatments for ASD. For example, what would be the effect of changing the composition of the microbiota in very specific ways? One way to do this is by a poop transplantation. Just like how a poop transplantation can change the behaviour of mice, it could also impact the behaviour of people with ASD when we, in this case, rebalance their microbial community by transplanting microbiota from a healthy gut into their gut (check out Alex’ article about poop transplants!).
But first, before we start transplanting poop large-scale, we need more research, including research in animals. It’s just like trying out a gluten-free recipe for the first time. Before you serve it to your friend with a gluten allergy, you might want to do a trial run to make sure you won’t contaminate the dish with any gluten – ensuring a safe and pleasant evening for your guest. The same goes for new treatments; scientists first perform experiments in animal models to make sure not only that it works, but also that it’s safe.
The possibilities in research continue to grow. Maybe in the future, we won’t need as many animals to solve the puzzles. Scientists are trying to find other options for studying complex puzzles as well (like the mini brains and intestines) to reduce their negative impact on animal welfare. But for now, they are the main characters of this story, helping us find answers we wouldn’t be able to find without them!
Further reading.
Do you want to read more about the use of zebrafish in Biomedical research? I recommend reading this article:
Choi, TY., Choi, TI., Lee, YR. et al. Zebrafish as an animal model for biomedical research. Exp Mol Med. 2021. https://www.nature.com/articles/s12276-021-00571-5#Sec4
Language.
As I'm not a native English speaker, I used Chat GPT for checking grammar and asking for synonyms. This helped me improve my language and find the right words in English to convey what I wanted to explain to you.
Figures.
The figure shows a mini-gut (small intestinal organoid from mouse, adapted from CanCell - Centre for Cancer Cell Reprogramming), a mini-brain (cerebral organoids model from human brain, adapted from Lancaster, M. et al. 2013) and a cell line (an iPSC-derived dopaminergic neuron cell line, adapted from Applied StemCell, Inc.).
The painting of the girl is based on the painting of the Dutch painter Johannes Vermeer: Girl with the pearl earring.
Acknowledgements.
I’m extremely grateful to Alex for all her help throughout my project. I could not have embarked on this blog writing journey without her feedback, enthusiasm and guidance. I am also thankful to Kristin Denzer for her mentorship as my Honours coordinator and for her feedback on my writing. Lastly, I would like to extend my sincere thanks to Lucía Peralta Marzal, my lab supervisor, for the amazing internship experience that laid the foundation for this project and for her insightful feedback on my writing.