The rise began far earlier than comparable blood biomarkers. Levels of pTau217 (a marker of tau pathology), NfL (an indicator of axonal damage) and GFAP (a sign of astrocyte activation) did not diverge clearly between groups until the late thirties or early forties. β-synuclein started moving in the third decade of life — more than seven years earlier.

All four biomarkers showed a steeper climb around age 40, roughly a decade before the median age at symptom onset in the cohort, which stood near 50.

Does the signal track memory and brain changes?

The researchers went beyond simple blood tests. Elevated β-synuclein levels tied to worse scores on tasks adapted for people with Down syndrome, including immediate and delayed recall.

Brain imaging backed up the blood findings. Sixty participants with Down syndrome had undergone MRI, and 42 had received FDG-PET scans, a technique that maps glucose metabolism. Higher β-synuclein tracked with smaller gray matter volume and reduced metabolism in regions vulnerable to the disease: the temporal, parietal and prefrontal cortex. Even in asymptomatic participants, elevated levels tied to lower volume in the fusiform gyrus, a temporal-lobe structure involved in face and object recognition.

What are the limitations?

The study is cross-sectional. Investigators estimated the timeline of changes by comparing people of different ages, rather than following the same individuals for decades.

The authors caution that trisomy 21 — the genetic cause of Down syndrome — may itself shape β-synuclein levels. Part of the early rise may reflect neurobiology tied to the extra copy of chromosome 21 rather than Alzheimer's pathology specifically. The neuroimaging analyses are flagged as exploratory and will need replication in larger groups.

Why does this matter for clinical research?

A blood biomarker that flags synaptic damage decades before symptoms could reshape prevention trials. Blood draws are cheaper and easier to repeat than lumbar punctures or PET scans, making it practical to monitor large at-risk populations.

"If we want to intervene increasingly early in Alzheimer's disease, we need to understand the sequence of changes that occurs long before dementia. β-synuclein may offer us a window into what is happening at the synapses during a period of the disease that until now has been difficult to study through blood," said Alba Cervantes, first author of the study and a researcher at IR Sant Pau.

Her supervisor, Dr. Olivia Belbin, who heads the Molecular Neurodegeneration group at IR Sant Pau and is the corresponding author, framed the broader stakes: "One of the major challenges in Alzheimer's research is understanding what is happening in the brain many years before the first symptoms appear. Gaining a more in-depth understanding of these earliest changes is essential to identifying when synapses begin to fail and to opening the door to studying interventions at increasingly early stages of the disease."

What's next?

The team plans longitudinal studies to confirm the proposed timeline and test whether a single β-synuclein measurement can predict cognitive decline for an individual patient. Such work could eventually help select trial participants at the very earliest stages of disease — and possibly, in time, guide treatment decisions in the clinic.