Using brain organoids to uncover the role of sialic acids
Brain organoids provide a means for researchers to study different mechanisms in the developing brain, and to investigate how they function at the molecular level. We spoke to Professor Martin Røssel Larsen about how he is using organoids to investigate the role of sialic acids in early brain development, and the long-term consequences of any malformations.
A type of negatively charged monosaccharide, sialic acids play an important role in intermolecular and intercellular interactions, and in the development of neurons. For example, the presence of a post-translational modification (PTM) called polysialic acid (PSA) on neural cell adhesion molecules (NCAM) prevents neurons from interacting with each other. “These cells are therefore effectively pushed away from each other by the negative charge of PSA, and then free to migrate. This is very important in early brain development, where cells migrate to organise and generate axons, dendrites, and other cell types,” explains Martin Røssel Larsen, Professor of Molecular Biology at the University of Southern Denmark (SDU), where he leads a research group. As Principal Investigator of several research projects based at SDU, Professor Røssel Larsen aims to build a deeper picture of the role of sialic acids in early brain development, work in which he is also considering the brains of other species.
“The biggest difference between humans and chimpanzees is sialic acid,” he continues. “There is an enzyme that takes human sialic acid and puts on an OH group, then you get the sialic acid we find in monkeys.”
Brain organoids
As humans we all have the gene that leads to the addition of an OH group, although it doesn’t function due to a mutation. The project team is working to correct that specific gene in stem cells, then the plan is to investigate whether the introduction of monkey-type sialic acid leads to changes in the human brain, using brain organoids grown from induced pluripotent stem cells.
“These organoids can be thought of as minibrains reflecting early brain development. They represent just one part of the brain, the cortical layer,” outlines Professor Røssel Larsen. Researchers are using sophisticated imaging techniques in this work. “If we change the sialic acid, do we get a different morphology of our mini-brains? Can we correlate that to the size of the cortical layers? It is believed that the size of the cortex is related to the learning and memory differences that we see between monkeys and
humans,” continues Professor Røssel Larsen. “To study this specific molecular change between human and chimpanzees we aim at introducing the mutation of this gene in chimpanzee stem cells, so it doesn’t function. Then we make organoids and look at whether that mutation leads to changes in the size of the cortex and further change in proteins and their PTMs, especially sialylation.”
“If we change the sialic acid, do we get a different morphology of our mini-brains? Can we correlate that to the size of cortex? Many people believe that the size of cortex is related to the learning and memory differences that we see between monkeys and humans.”
Researchers in the project are using imaging techniques as well as mass spectrometry to look at changes in sialic acid in the cortical layers, then relate it back to the function of specific proteins. A further strand of research involves working with nerve terminals – these are small, active compartments which are sometimes called synaptosomes. “We can stimulate these nerve terminals, and they will release neurotransmitter, then they will take up a vesicle again and perform synaptic transmission repeatedly,” explains Professor Røssel Larsen. A method has now been
developed to isolate nerve terminals from brain organoids, meaning they can now be taken into a human system, which opens up new avenues of investigation, says Professor Røssel Larsen. “We can manipulate stem cells, make brain organoids from these, and then see if there is a change in the nerve terminals,” he outlines. “We’re looking at whether we can see changes if we manipulate enzymes that add sialic acids - sialyltransferases. How is sialic acid used in this very sophisticated brain system? Is it a way of fine-tuning interactions? It is likely that protein-protein interaction is heavily regulated by sialic acid on cell surfaces.”
The hope is that this research will uncover morphological changes in the brain that can be related to changes in sialic acid, while Professor Røssel Larsen also hopes to identify the targets of sialic transferases in the brain.
By knocking down for example the 2,6 sialyltransferases - another distinct difference
in the sialic acid biology between human and chimpanzees - Professor Røssel Larsen and his team hope to see exactly what sites and what proteins it changes. “We can then find substrates for these sialyltransferases, and correlate this with any morphological changes in the brain,” he explains. Sialic acids are also thought to play a major role in immunity and in the progression of certain conditions, including certain forms of cancer and schizophrenia, another topic that Professor Røssel Larsen is addressing in his research. “We think that if we want to look at diseases like schizophrenia, we need to move away from only focusing on the genes to look at what is present in the brain at the time with respect to the building blocks that do the work in the cell, the proteins,” he outlines.
“In our interdisciplinary DEVELOPNOID project we have taken plasma from patients with schizophrenia and controls. We then reprogrammed the blood cells from patients to stem cells, and we are now growing brain organoids to investigate underlying molecular mechanisms leading to schizophrenia using ‘Omics technologies and imaging.”
Role of sialic acids
This research could lead to new insights into the underlying mechanisms behind schizophrenia and other diseases, and ultimately the development of new, more effective treatments. The project’s research at this stage is largely fundamental in nature however, with the team focused primarily on uncovering the role of sialic acids in the brain. “If we can identify the role of sialic
acids in the brain it would be a very big achievement, it would be really exciting,” says Professor Røssel Larsen. This is still a relatively neglected area of research, and Professor Røssel Larsen hopes that the project’s work will stimulate further interest and development. “We hope there will be a renewed focus on PTMs like glycosylation, which have been overlooked for a long time. Not many people work on glycosylation in Denmark, and not a lot is known about its true function, partly because the technology is not there yet,” he explains. “We want to contribute to continued development, and to use brain organoids to investigate other diseases beyond schizophrenia. Brain organoids are a very effective model system, they cover more ground than 2-d systems.”
The brain is comprised a lot of different paths that communicate with each other, yet how this communication occurs is not well understood. More sophisticated brain organoid models could help researchers gain a fuller picture. “It is possible now to make different brain organoids from several parts of the brain, then put them together and let them grow together. Then you will see that the neurons start migrating in and out of the different organoids,” says Professor Røssel Larsen. A lot of attention in the research group will be focused in future on developing methods to characterise PTMs, in particular glycosylation, which is a topic of deep interest to Professor Røssel Larsen. “It’s very difficult to measure and analyse changes in the glycostructure, but it’s also very fascinating,” he stresses.
The RoLe of siALic Acids in e ARLy BRAin deveLoPMenT
interdisciplinary project deveLoPnoid
Project objectives
Protein glycosylation is important for communication between cells and cell migration, mechanisms essential for development of the brain. A big difference between us and chimpanzees is the sugar molecule, sialic acids (SA), on cell surface proteins. SA is involved in neural development, and in the present project we will investigate the role of SAs in early brain development using brain organoids, multi-omics and imaging.
Project funding
The role of sialic acids in early brain development project is funded by The Independent Research Fund Denmark.
Project Partners
• Prof. Kristine Freude, Department for Veterinary and Animal Science, University of Copenhagen, DK.
• Dr. Madeline A. Lancaster, MRC Laboratory of Molecular Biology, Cambridge Biomedical Campus, Cambridge, UK.
• Prof. Jonathan Brewer, Department of Biochemistry and Molecular Biology, University of Southern Denmark, DK. contact details
Project Coordinator, Professor Martin Røssel Larsen
Department of Biochemistry and Molecular Biology
University of Southern Denmark (SDU) Campusvej 55; Odense M - DK-5230
T: +45 6550 1872
e: mrl@bmb.sdu.dk
W: https://dff.dk/ansog/stottet-forskning/ podcasts/celler-med-sukker-pa
Røssel
He is internationally recognized for his work in developing methods for characterising post-translational modifications (PTMs) of proteins and in bridging biological mass spectrometry and biomedical research.
