Here we share with you some recent work out of the Centre for Brain Research in New Zealand, in partnership with École Polytechnique Fédérale de Lausanne in Switzerland, investigating the role of huntingtin protein (HTT) aggregates in Huntington’s disease (HD). This study was carried out using human brain tissue generously bequeathed to the Neurological Foundation Human Brain Bank in New Zealand, making this a unique study with important implications for scientists using non-human tissues to study HD. The results identified the best tools for studying human HTT protein, as well as uncovering new relationships between HTT aggregates and several hallmarks of HD in an under-studied region of the brain.
Glossary
Aggregate: clump of proteins (in this case clumps of mutant HTT)
Antibody: small molecule used as a label to help us see proteins in tissue under the microscope
HTT: Huntingtin protein
MTG: Middle temporal gyrus, the region of the brain looked at in this study
TMA: Tissue microarray, a technique for studying many different tissues samples at once
Antibodies are important tools for neuroscience research because they allow scientists to specifically label proteins from the brain such as HTT. These proteins are tiny and therefore invisible to the naked eye, however, labelling proteins with antibodies allows them to be seen under the microscope. For this project, a wide range of antibodies were provided by our Swiss collaborators and tested at the Centre for Brain Research, with each antibody labelling the HTT protein in a slightly different way.
Surprisingly, only a quarter of the antibodies tested in this study were able to label HTT in human brain tissue. This finding shows how important it is to use human brain tissue to study human brain diseases, rather than relying on animal or cell representations to capture the complexity of the human HD state.
Cartoon depiction of an antibody binding the HTT protein at a specific site, and the resulting image down the microscope. The antibody labeling allows us to see HTT aggregates (clumps of HTT protein) in brain tissue.
Once the best antibodies were identified, they were used to label HTT on human brain tissue microarrays (TMAs). Making human brain TMAs is a new technique developed in New Zealand and described here: https://www.hdyo.co.nz/research/2021/10/7/novel-research-techniques-putting-new-zealand-researchers-on-the-world-stage. Briefly, a 2mm sample of very thin brain tissue is taken from up to 60 different brains and attached to the same glass surface. This technique helps limit the use of precious brain tissue, minimizes the usage of experimental resources, and ensures that all 60 brains are treated the exact same throughout the labelling process. TMAs help save time in the lab and may improve our ability to quickly translate our findings from an academic context into therapies for HD patients.
Tissue was used from the middle temporal gyrus (MTG) region of the brain for this study. The MTG is rarely studied in HD because it is one of the least-affected regions, however, this may help us to uncover the important first steps of the disease.
The next step was to count the labelled HTT aggregates and compare the numbers between people with and without HD. The results showed that HTT aggregates were not correlated with the degree of brain cell loss in the MTG, but HTT aggregates were significantly correlated with CAG repeat length (the mutation that causes HD) and the age of symptom onset in HD. These results suggest a critical relationship between the aggregation of mutant HTT and the clinical onset of HD.
This study was the first of its kind to test a wide range of HTT antibodies in a large cohort of human brains, providing new insights into the disease processes of HD. By identifying a group of accurate antibodies for human HTT, this project will serve as the starting point for many exciting new studies of the human HD brain at the Centre for Brain Research, so stay tuned to keep up with the latest work from the lab.
Want to read more? Check out the article published in Neurobiology of Disease here: https://doi.org/10.1016/j.nbd.2022.105884
About the Author
Florence is a recipient of the Woolf Fisher Scholarship and a first-year PhD student at the University of Cambridge. Her current project aims to investigate α-synuclein aggregation in Parkinson's disease, with an emphasis on human brain tissue analysis. Florence completed her undergraduate education at the University of Auckland in New Zealand, completing a Bachelor of Science in Biomedical Science and a first-class Honours degree in Biomedical Science. Her Honours work involved immunohistochemical analysis of Huntington's disease human brain tissue at the Centre for Brain Research in New Zealand.
