Parkinson’s disease is often described as a single disorder, but a new analysis of donated human brain tissue suggests that its defining protein pathology can vary dramatically from one patient to another. Researchers at the Mayo Clinic found substantial differences in the amount, chemical modification, aggregation behavior and “seeding” activity of alpha-synuclein, the protein that accumulates in the brains of people with Parkinson’s disease. The findings raise the possibility that patients who receive the same clinical diagnosis may nevertheless harbor biologically distinct forms of disease—an observation that could reshape efforts to develop precision treatments and molecular tests.

The study, published in Acta Neuropathologica, examined postmortem samples from 63 people of European descent, including patients with neuropathologically confirmed Parkinson’s disease and individuals without Lewy body pathology. The Parkinson’s cases were divided according to age at disease onset and clinical duration. Early-onset disease was defined as beginning before age 60, while late-onset disease began after age 60. The late-onset group was further divided into fast-progressing cases, with disease duration of less than five years, and slow-progressing cases, with disease lasting more than 10 years. This design allowed the team to compare molecular features not only between patients and controls, but also among clinically recognizable forms of Parkinson’s disease.

Alpha-synuclein is normally associated with nerve terminals, where it is thought to help regulate synaptic vesicles—the small membrane packages that carry neurotransmitters between neurons. In Parkinson’s disease, however, the protein can misfold and assemble into fibrils and larger inclusions known as Lewy bodies and Lewy neurites. These deposits are a pathological hallmark of Parkinson’s and related synucleinopathies. Misfolded alpha-synuclein can also act as a molecular “seed”: when introduced into cells, it can recruit normally folded alpha-synuclein and convert it into new aggregates. This templating process is sometimes described as prion-like, although it does not imply that Parkinson’s disease spreads between people. Instead, it refers to the ability of a misfolded protein to propagate through connected cells and brain regions.

To isolate disease-associated protein species, the researchers dissected tissue from the cingulate gyrus, a limbic cortical region involved in motor integration, cognition and neuropsychiatric function. They separated proteins according to their solubility, focusing particularly on detergent-insoluble material, which is enriched for tightly packed aggregates. The team measured total alpha-synuclein and alpha-synuclein phosphorylated at serine 129, or pSer129. This chemical modification is strongly enriched in Lewy pathology and is widely used as a laboratory marker of abnormal alpha-synuclein. Using a bead-based immunoassay called AlphaLISA, the researchers found that pSer129-alpha-synuclein was significantly elevated across all Parkinson’s groups compared with controls. Total alpha-synuclein, by contrast, was especially high in late-onset Parkinson’s disease, exceeding levels in both early-onset cases and controls.

The researchers then tested whether the brain-derived material could trigger aggregation in living cells. They used engineered human kidney cells containing two versions of alpha-synuclein, one tagged with cyan fluorescent protein and the other with yellow fluorescent protein. When alpha-synuclein molecules come into close proximity during aggregate formation, energy can transfer between the fluorescent tags, producing a measurable FRET signal. Flow cytometry allowed the researchers to quantify this response across thousands of cells, using an integrated measure that combined the proportion of FRET-positive cells with the intensity of their signal. Every Parkinson’s sample produced more seeding activity than control material, but the most striking result was the variation among individual patients. Seeding activity differed by more than tenfold across cases, and that spread was greater than the average differences separating the predefined clinical subgroups.

The cell experiments also suggested that the signal reflected alpha-synuclein itself rather than a nonspecific toxic effect of brain extracts. When the researchers removed alpha-synuclein from selected homogenates using an antibody attached to magnetic beads, alpha-synuclein levels fell by an average of 64 percent and the integrated FRET signal declined by about 71 percent. Cell viability did not differ significantly between groups, indicating that the stronger fluorescence response was not simply caused by acute cell death or generalized cellular stress. High-content imaging provided an independent confirmation: Parkinson’s samples generated more intracellular puncta, or aggregate-like structures, than control samples. Slow-progressing late-onset cases showed a greater aggregate burden than fast-progressing cases, although much of the variation still appeared to be driven by individual biology.

The relationship between chemical modification and aggregation was particularly revealing. Across Parkinson’s cases, higher pSer129-alpha-synuclein levels correlated positively with stronger FRET seeding activity. Lewy body counts in the cingulate cortex also correlated with both pSer129 levels and the cellular seeding readout. These associations suggest that phosphorylation at serine 129 tracks with the abundance of biologically active aggregate species, but they do not prove that phosphorylation initiates disease or directly causes propagation. Alpha-synuclein can be phosphorylated in healthy neurons in response to normal neuronal activity, and some evidence indicates that serine-129 phosphorylation may occur after aggregation or even inhibit certain forms of fibril formation. In this study, therefore, pSer129 is best interpreted as a molecular correlate of pathology and aggregation dynamics rather than as a definitive trigger.

The team next used a cell-free seed amplification assay known as RT-QuIC, short for real-time quaking-induced conversion. In this technique, a minute amount of patient-derived seed is mixed with purified, normally folded human alpha-synuclein. Repeated shaking and incubation encourage any seed present to convert the soluble protein into fibrils. The reaction contains thioflavin T, a fluorescent dye whose signal rises as amyloid-like structures accumulate. The time required for fluorescence to cross a predefined threshold—the lag time—provides an indirect measure of how efficiently aggregation begins. Parkinson’s samples crossed the threshold substantially faster than controls, with a mean lag time of 26.9 hours compared with 45.1 hours. The assay produced an area under the receiver operating characteristic curve of 0.98; in this dataset, a lag time below 34.7 hours corresponded to 93 percent sensitivity and 100 percent specificity.

RT-QuIC also exposed differences in the physical stability of the amplified aggregates. After amplification, the researchers treated the products with proteinase K, an enzyme that digests exposed or loosely structured protein regions, and analyzed the remaining fragments by gel electrophoresis and silver staining. Products generated from Parkinson’s samples were more resistant to digestion than those from controls at every time point, indicating that the seed-competent assemblies had different structural properties. However, the researchers did not find a single protease-resistance pattern that cleanly separated early- from late-onset disease or fast- from slow-progressing cases. The results instead point to molecular heterogeneity between individuals. Because amplified fibrils can differ from the original brain-derived aggregates, the assay does not provide a complete structural portrait of pathology in the brain, but it offers a reproducible way to compare the biochemical behavior of disease-associated seeds.

Genetic variation may contribute to this diversity. Two of the three patients with the strongest combination of FRET seeding activity and pSer129-alpha-synuclein carried the APOE E3/E4 genotype, and the APOE4 allele was more common in the fast-progressing late-onset group than in the slow-progressing group. APOE4 is best known as a risk factor for Alzheimer’s disease, but previous research has linked it to alpha-synuclein aggregation, neurotoxicity and more aggressive synucleinopathy. The current findings are suggestive rather than conclusive: the study was not designed to establish that APOE4 causes stronger seeding, and the most extreme molecular profiles were not all explained by that genotype. Age-related cellular decline, genetic variants, mitochondrial dysfunction, protein-clearance capacity and other environmental or biological factors could all influence the shape that alpha-synuclein takes in a particular brain.

The implications extend beyond a new way of dividing Parkinson’s disease. Clinical categories such as early onset, late onset, rapid progression and slow progression remain useful, but they did not fully capture the molecular differences measured in this study. That conclusion aligns with emerging biological frameworks, including SynNeurGe and the Neuronal alpha Synuclein Disease Integrated Staging System, which seek to classify Parkinson’s disease using evidence of pathogenic alpha-synuclein, neurodegeneration, genetic risk and clinical features. A future diagnostic system might combine seed amplification, phosphorylation patterns, genetic information and imaging rather than relying on symptoms alone. Such tests could eventually help predict disease trajectory, identify patients most likely to benefit from alpha-synuclein-targeting therapies, or reveal why an intervention works in one patient but fails in another. The present study is based on a relatively small, postmortem cohort with incomplete detailed clinical information, so larger longitudinal studies will be needed to determine whether these molecular signatures can predict outcomes during life. For now, the message is clear: Parkinson’s disease may share a name and a pathological protein, but the molecular behavior of that protein can be remarkably personal.

The study employs single-cell RNA sequencing (scRNA-seq), a revolutionary technology that dissects the genetic activity of individual cells rather than bulk tissue. This allows researchers to classify distinct immune cell subtypes circulating in the blood and to capture their unique gene expression profiles with unprecedented precision. Tétreault and her team have identified that specific immune cells in Parkinson’s patients are not only activated but exhibit upregulated expression of genes linked to cellular stress responses. These findings suggest that the peripheral immune system may bear a molecular signature that mirrors, or perhaps even influences, the neurodegenerative processes occurring in the brain.

This molecular signature, comprised of a constellation of overexpressed genes associated with immune activation and stress response pathways, offers a novel biomarker profile for Parkinson’s disease. Such a profile holds the promise of transforming the current diagnostic paradigm, which largely relies on clinical observation and symptom-based criteria. Early and accurate diagnosis remains one of the most pressing challenges in managing Parkinson’s, and the identification of blood-based biomarkers opens the door to minimally invasive testing methods that could detect the disease at much earlier stages than ever before.

Crucially, the study’s findings also pave the way toward better differential diagnosis. Parkinsonian syndromes such as progressive supranuclear palsy (PSP) and multiple system atrophy (MSA) often present overlapping motor symptoms, making clinical distinction difficult. The unique immune cell gene expression signatures identified in this study provide a molecular fingerprint capable of distinguishing true Parkinson’s disease from its phenotypic mimics, a capability that could significantly improve treatment specificity and patient outcomes.

The research cohort consisted of 14 individuals diagnosed with Parkinson’s disease, 6 patients with related Parkinsonian syndromes, and 10 healthy controls. By comparing these groups, Tétreault’s team could robustly define the genetic signatures associated specifically with Parkinson’s. Importantly, the study confirmed that immune activation was a hallmark of Parkinson’s, while different immune profiles characterized other Parkinsonian disorders. These discoveries underscore the value of peripheral immune biomarkers in clinical contexts and in the stratification of patients for inclusion in clinical trials testing novel therapeutics.

In practical terms, this immune-focused approach may ultimately enable neurologists to monitor disease progression and therapeutic response through simple blood tests, circumventing the need for more intrusive and expensive diagnostic tools such as neuroimaging or cerebrospinal fluid analysis. Furthermore, this paradigm shift highlights immune pathways as potential targets for the development of disease-modifying treatments, broadening the scope beyond traditional dopamine-centered therapies.

Beyond diagnostics, the study advances foundational scientific knowledge by providing an open-source atlas of immune cell subtypes found in both healthy individuals and Parkinson’s patients. This atlas is a valuable resource for the broader scientific community and will facilitate further investigations into the interplay between systemic immunity and neurodegeneration. By mapping the immune landscape at single-cell resolution, the study sets a new benchmark for understanding how peripheral immune cells participate in central nervous system diseases.

The clinical significance of this work is underscored by the growing prevalence of Parkinson’s disease. In Canada alone, nearly 110,000 individuals were living with Parkinson’s in 2024, with projections estimating this number to rise to approximately 150,000 by 2034. As the population ages, such conditions will exert substantial strain on healthcare systems worldwide, heightening the urgency for early diagnosis and novel treatment strategies.

Martine Tétreault’s collaboration with Gaël Moquin-Beaubry, Lovatiana Andriamboavonjy, and Sébastien Audet, who contributed as co-first authors, highlights the multidisciplinary effort required to tackle complex neuroimmune interactions. The study’s publication in the prestigious journal Brain further cements its importance and lays the groundwork for future investigations into immune mechanisms underpinning neurodegenerative disorders.

Funding support from the Courtois Foundation and the Weston Family Foundation, along with technical and clinical expertise from neurologists at the University of Montreal Hospital Research Centre (CRCHUM), was instrumental in the successful completion of this research. The researchers also express gratitude to the patients and their families, whose participation was vital to the study’s insights.

In sum, this research marks a significant advance in Parkinson’s disease biology, linking peripheral immune dysregulation to the disease’s molecular fabric. The ability to pinpoint an immune gene expression signature in blood not only promises enhancements in diagnosis but also offers fresh avenues for therapeutic intervention. By harnessing innovative single-cell sequencing technology, Tétreault and colleagues illuminate a path toward precision medicine approaches in neurodegenerative diseases, potentially transforming patient care in the years ahead.

Subject of Research: Human tissue samples

Article Title: Mapping the peripheral immune landscape of Parkinson’s disease patients with single-cell sequencing

News Publication Date: 26-May-2025

Web References: http://dx.doi.org/10.1093/brain/awaf066

References: Moquin-Beaudry, G., Andriamboavonjy, L., Audet, S., et al. Mapping the peripheral immune landscape of Parkinson’s disease patients with single-cell sequencing. Brain, 26 May 2025.

Image Credits: CHUM

Keywords: Parkinson’s disease, Neurodegenerative diseases, Medical diagnosis, Biomarkers