Scientists headed by a team at the University of Cambridge, University College London, the Francis Crick Institute, and Polytechnique Montréal have, for the first time, directly visualized and quantified the protein clusters that are believed to trigger Parkinson’s disease (PD). The team developed an imaging technique, Advanced Sensing of Aggregates–Parkinson’s Disease (ASA–PD), that allowed them to see, count, and compare in post-mortem brain tissue the protein assemblies, known as oligomers, which are proposed as early drivers of pathogenesis of Parkinson’s disease. The new development, one of the team noted, is “like being able to see stars in broad daylight.”
Describing their work as a major advance in the study of the world’s fastest-growing neurological disease, the authors suggest their results could help to unravel the mechanics of how Parkinson’s spreads through the brain and support the development of diagnostics and potential treatments.
“The only real way to understand what is happening in human disease is to study the human brain directly, but because of the brain’s sheer complexity, this is very challenging,” said research co-lead Sonia Gandhi, MD, PhD, professor from the Francis Crick Institute. “We hope that breaking through this technological barrier will allow us to understand why, where, and how protein clusters form and how this changes the brain environment and leads to disease.”
Gandhi and colleagues reported on their findings in Nature Biomedical Engineering, in a paper titled “Large-scale visualization of α-synuclein oligomers in Parkinson’s disease brain tissue,” in which they concluded, “Using ASA–PD, each stage of the protein aggregation pathway present in disease brain can now be measured.”
PD is a progressive neurodegenerative disorder that initially causes the loss of dopaminergic neurons in the substantia nigra, resulting in a movement disorder consisting of tremors, bradykinesia, and rigidity, the authors explained. Around 166,000 people in the U.K. live with Parkinson’s disease, and the number is rising. By 2050, the number of people with Parkinson’s worldwide is expected to double to 25 million. While there are drugs that can help alleviate some of the symptoms of Parkinson’s, such as tremor and stiffness, there are no drugs that can slow or stop the disease itself.
For more than a century, doctors have recognized Parkinson’s by the presence of large protein deposits called Lewy bodies. “Pathologically, PD is characterized by neuronal loss accompanied by the accumulation of microscale α-synuclein aggregates called Lewy bodies and Lewy neurites.” It’s these structures that form the basis of PD diagnostic staging criteria. These large species are formed by smaller soluble protein nanoscale assemblies, called alpha-synuclein oligomers, that have long been considered the likely culprits for Parkinson’s disease to start developing in the brain. And while scientists have suspected that these smaller, earlier-forming oligomers may cause damage to brain cells, until now, these oligomers were simply too small to see—just a few nanometres long. “Until now, this hypothesis has remained controversial, at least in part because it has not been possible to directly visualize nanoscale assemblies in human brain tissue,” the investigators pointed out.
“Lewy bodies are the hallmark of Parkinson’s, but they essentially tell you where the disease has been, not where it is right now,” said research co-lead Steven Lee, PhD, professor at Cambridge’s Yusuf Hamied Department of Chemistry. “If we can observe Parkinson’s at its earliest stages, that would tell us a whole lot more about how the disease develops in the brain and how we might be able to treat it.”
The new technology, ASA-PD, developed by Lee and colleagues, uses ultra-sensitive fluorescence microscopy to detect and analyse millions of oligomers in post-mortem brain tissue. “We combined autofluorescence suppression with single-molecule fluorescence microscopy, which together enable the detection of nanoscale α-synuclein aggregates,” they stated.
Since oligomers are so small, their signal is extremely weak. ASA-PD maximizes the signal while decreasing the background, dramatically boosting sensitivity to the point where individual alpha-synuclein oligomers can be observed and studied.
“This is the first time we’ve been able to look at oligomers directly in human brain tissue at this scale: it’s like being able to see stars in broad daylight,” said co-first author Rebecca Andrews, PhD, who conducted the work when she was a postdoctoral researcher in Lee’s lab. “It opens new doors in Parkinson’s research.”
For their reported study, the team examined post-mortem brain tissue samples from people with Parkinson’s disease and compared them to healthy control (HC) individuals of similar age. “By enhancing the sensitivity of traditional immunofluorescence techniques, combined with our analytical approach, we have been able to detect and characterize over 1.2 million α-synuclein aggregates,” the team further explained.
They found that oligomers exist in both healthy and Parkinson’s brains. The main difference between disease and healthy brains was the size of the oligomers, which were larger, brighter, and more numerous in disease samples, suggesting a direct link to the progression of Parkinson’s disease. The team also discovered a subclass of oligomers that appeared only in Parkinson’s disease patients, which could be the earliest visible markers of the disease, potentially years before symptoms appear. “Notably, PD samples contained a shifted subpopulation of bright nanoscale assemblies largely absent from the HCs,” they stated. “The presence of this ‘disease-specific’ shift was detected in PD cases from different brain banks, disease stages, immunofluorescent labels, and antigen-retrieval methods.”
Research co-lead Lucien Weiss, PhD, professor at Polytechnique Montréal, further commented, “Oligomers have been the needle in the haystack, but now that we know where those needles are, it could help us target specific cell types in certain regions of the brain … This method doesn’t just give us a snapshot. It offers a whole atlas of protein changes across the brain, and similar technologies could be applied to other neurodegenerative diseases like Alzheimer’s and Huntington’s.”
In their discussion, the team concluded, “Our findings are consistent with the hypothesis that misfolded α-synuclein readily forms a continuum of larger nanoscale aggregates that eventually give rise to the microscale structures traditionally associated with the disease … We also note that the ASA–PD method is widely applicable to other neurodegenerative diseases, where the role of protein aggregation remains largely unresolved.”
