Scientists from New Zealand and Australia have been using sheep to study Huntington’s Disease (HD). These sheep were genetically modified to contain the human Huntington gene with the mutation that causes HD. They were created in 2006 by a team led by Professor Russell Snell from the University of Auckland. Since then, these sheep have been extensively studied with the hope they will shed light on the mechanisms involved in HD.
Until at least six years of age, these HD sheep don’t lose any brain cells. Even at ten years of age, HD sheep have no movement problems. However, they do have early signs of HD. These include problems such as a build-up of Huntington’s protein in their brain, changes to their day-night activity cycle and changes to many small molecules found in the brain, liver and blood. This makes these sheep an ideal model for studying the earliest phase of HD which begins before symptoms appear. This is the stage when medical intervention is most likely to be effective.
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One way to see what is going on in the brain is to measure the levels of many molecules, particularly proteins, to see what the cells in the brain are doing. This had already been done with HD sheep. Another method is to look at RNA. RNA is a molecule very similar to DNA. DNA stores all the information needed to make a human body and keep it alive. The information in DNA is stored in units called genes. Each gene carries the instructions for making a different protein, and it is these proteins that do most of the work in the cell.
Different cells have different functions in the body. This means they sometimes need different proteins. Which proteins the cell needs can also change over time, depending on what the cell is doing. To change which proteins are being produced and how many of each are being made, the cell uses a molecule called RNA. An RNA molecule is created to match the instructions of a particular gene. The RNA molecule then travels to the part of the cell that makes proteins, where its instructions are used to make the specific protein coded for by that gene. When cells want more of a particular protein, they send a signal to make more RNA copies of that gene.
One way to see what a cell is doing is to collect all the RNA molecules present at one time. This allows us to see what the cell is trying to achieve and provides a proxy measurement for all the proteins in the cell. Until recently, RNA could not be sampled from an individual cell. Instead, it would be collected from a group of cells and these cells would be assessed collectively. This was helpful for seeing what was going on in a particular organ. However, there are usually many different types of cell present that are performing different roles, so this method can only give a general idea about what is going on.
One of the major advancements in biotechnology in recent years is the ability to sample RNA from individual cells. This is a much more accurate method that allows us to tease out which cell type is doing what. RNA had already been sampled in bulk from HD sheep brains, but once the new technology became available, Andrew Jiang, a PhD student at the University of Auckland, conducted a study to see if more information could be found by looking at RNA from individual cells.
To do this, he took cells from 6 HD sheep and 6 healthy sheep that were all five years of age. The cells were taken from the part of the brain most affected in HD. The cells were then separated so the RNA could be individually extracted. In total, there were 28,234 cells that were deemed high quality and used for the final analysis. Thirteen different cell types were identified. Some of these were different types of neurons, which are the brain cells that send signals. Others were support cells that help the neurons function and stay healthy.
The first finding was that there was less of a certain type of support cell, called oligodendrocytes, in the HD sheep brains compared to healthy sheep. These cells wrap around part of each neuron and help them conduct signals more efficiently. This may mean that neurons in the HD sheep are not working as effectively as they could be.
The rest of the findings largely relate to the most important type of neuron in HD. These are called Medium Spiny Neurons (MSN). These are the first cells that are lost in the disease. Their main job in the brain is to help coordinate movement. There were many differences in RNA molecules in the MSNs between HD and healthy sheep. Most of these differences were in pathways that involve connections and communication between brain cells. Overall, it looked like there was more communication between brain cells in HD sheep.
While more communication might sound like a good thing, it can be harmful if neurons are overactive. Overactive neurons can tire out and get sick. In extreme cases, this can lead to the death of neurons. One major theory in HD research is that too much of a signalling molecule called glutamate is overexciting MSNs, eventually leading to the death of these cells.
The glutamate theory was supported by this study. RNA from genes involved in receiving glutamate were over-represented in MSNs in HD sheep. RNA from genes for processing glutamate were also over-represented, showing the neurons were working hard to remove the extra glutamate. One of the main support cells in the brain, called astrocytes, were also working hard to help remove extra glutamate.
Although there was some evidence that this extra glutamate was making the neurons sick, there was also evidence that the brains of HD sheep were managing to stay on top of the problem. The mutant Huntington’s gene in sheep is less active than it is in humans. It only produces about 10% - 20% of the RNA seen in humans. This is why the HD sheep don’t have symptoms as bad as humans.
This is actually very helpful, because it extends the phase of the disease prior to symptoms. This allows us to see what is happening at the earliest stages, before neurons start to die. By looking at the ways the cells are compensating for the problem early on, we can see what is helpful. It is possible that medication could be developed to target these pathways to protect the neurons from death.
This single cell RNA study is the first ever done in HD sheep. It supports existing theories while showing what is going on in brain cells in much more detail. It also shows that the HD sheep are a valuable model for the earliest stages of HD, and that they will be an ideal model to test medications designed to prevent HD.
Check out the original research article here:
Jiang, A., You, L., Handley, R. R., Hawkins, V., Reid, S. J., Jacobsen, J. C., ... & Snell, R. G. (2024). Single nuclei RNA-seq reveals a medium spiny neuron glutamate excitotoxicity signature prior to the onset of neuronal death in an ovine Huntington’s disease model. Human Molecular Genetics, ddae087.
https://academic.oup.com/hmg/advance-article/doi/10.1093/hmg/ddae087/7679787
About the Author
Natasha is a postdoctoral scientist who works with sheep models of Alzheimer’s Disease (AD), with the hope of finding a method of prevention. Her PhD project was to create a sheep model of Alzheimer's Disease using CRISPR-Cas9 gene editing technology. Her job now is to assess the model to prove that it will be a useful tool in AD research. This work was funded by a combination of the Freemason’s of New Zealand, the University of Auckland and Brain Research New Zealand. Before starting her PhD, she completed a Bachelor of Science at Massey University, majoring in genetics and zoology. She then went on to complete a Master’s Degree with First Class Honors in Evolutionary Biology, studying the natural hybridization of different species of tree weta, a native New Zealand insect, and how that affected the gene pool of each species. She has wide ranging interests across the biological sciences, and is passionate about learning, research, and education.
