Alpha-Synuclein on Trial
At the World Parkinson Congress in Phoenix, two leading neuroscientists staged a “courtroom battle” over one of medicine’s most consequential questions: what actually drives Parkinson’s disease?
The information in this article is for educational purposes only and is not intended as medical advice. Always consult a qualified healthcare professional for medical questions.
PHOENIX — The courtroom metaphors began almost immediately.
Leonidas Stefanis, a Greek neurologist who has spent much of his scientific career studying alpha-synuclein, rose first. Addressing the audience as though they were a jury, he promised to present “a series of irrefutable pieces of evidence” proving that one small protein sits at the centre of Parkinson’s disease. Alpha-synuclein, he declared, possessed “the motive, the opportunity and the capability to perpetrate and propagate the disease.”
Roger Barker, the Cambridge neurologist arguing the opposing side, waited patiently before delivering his opening line. His opponent, Barker noted dryly, had promised spirits at the debate the previous evening.
“They are not here,” he said. “So can you trust him?”
The audience in the room laughed. But the subject under discussion at the World Parkinson Congress 2026 was anything but light. Parkinson’s disease is now the fastest-growing neurological disorder in the world, with cases more than doubling globally over the past generation. No therapy has yet succeeded in reliably slowing or halting the disease itself. For millions of patients and families, the stakes could hardly be higher.
The debate in Phoenix was nominally about molecular biology. In reality, it exposed something larger and more uncomfortable: a field increasingly aware that it may not fully understand the disease it has spent decades trying to decode.
The very existence of the session was revealing. This year’s congress introduced a new format called “Controversy”, explicitly designed to stage unresolved scientific disputes in public. Such openness is unusual in biomedical research, where consensus is often presented externally even when deep disagreements persist internally. Here, however, disagreement took centre stage - not merely over details, but over the organising theory of Parkinson’s disease itself.
The courtroom framing was not merely rhetorical. The several hundred people packed into the hall - a mix of neurologists, neuroscientists, clinicians, industry researchers and people living with Parkinson’s disease themselves - were assigned the role of jury. Throughout the session, the audience was repeatedly asked to weigh the evidence presented by both sides. By the end of the debate, a vote was conducted both digitally and by show of hands. The result was close to a dead heat, reflecting just how divided the field remains. But Barker’s side - arguing that alpha-synuclein is not the central piece of the Parkinson’s puzzle - appeared to edge out a narrow victory.
That outcome itself was telling. The audience in the room was not composed of casual observers. These were people deeply immersed in Parkinson’s disease, many of them having devoted years - or entire careers - to understanding it.
The rise of a suspect
Alpha-synuclein was not always famous. For years, it was simply one among thousands of proteins found in neurons. That changed in the late 1990s when researchers discovered that Lewy bodies - the abnormal clumps long considered the pathological hallmark of Parkinson’s disease - consisted largely of misfolded alpha-synuclein.
Then came the genetic discoveries. Rare families with inherited Parkinson’s disease were found to carry mutations in the SNCA gene, which encodes alpha-synuclein. More strikingly, some families carried extra copies of the gene itself. Those with duplications developed Parkinson’s. Those with triplications often developed earlier and more aggressive disease.
For researchers like Stefanis, this became foundational evidence. A modest increase in alpha-synuclein levels appeared capable of producing Parkinson’s disease directly.
“There is no other genetic cause of PD that does that,” Stefanis argued. “Only a relatively slight increase in dose-dependent fashion, and you develop Parkinson’s.”
Over time, alpha-synuclein evolved from pathological marker to master hypothesis. The protein appeared everywhere researchers looked: in familial Parkinson’s, in sporadic Parkinson’s, in dementia with Lewy bodies and increasingly in biomarker studies attempting to detect disease before symptoms emerge.
The hypothesis became progressively more ambitious. Perhaps alpha-synuclein did not merely accompany disease. Perhaps it spread through the nervous system in a prion-like fashion, seeding further misfolding as it moved from neuron to neuron.
The implications were enormous. If Parkinson’s disease could be reduced, at least partly, to a propagating protein pathology, then the disease might finally become tractable: measurable through biomarkers, targetable through antibodies and interruptible through molecular therapies.
Pharmaceutical companies invested heavily. So did governments, charities and academic institutions. For years, alpha-synuclein became the gravitational centre of Parkinson’s research.
The prosecutor’s case
Stefanis’ presentation reflected just how much evidence has accumulated around the protein. His case was methodical, layered almost like a legal brief.
First came genetics. Rare SNCA mutations produce classical Parkinson’s disease. Gene dosage matters. More alpha-synuclein produces more severe disease.
Then pathology. According to Braak-style staging models, Lewy pathology appears years before motor symptoms emerge, often beginning in structures linked to smell, autonomic regulation and REM sleep behaviour disorder.
“Lewy body pathology antidates clinical Parkinsonism,” Stefanis said.
He also highlighted newer evidence suggesting that classical Lewy bodies may represent only the visible tip of a much larger pathological iceberg. Recent studies using advanced imaging and antibody techniques have identified oligomeric alpha-synuclein species and extensive synaptic pathology even before overt Lewy body formation.
“This hidden pathology has been uncovered more recently,” he said.
One of Stefanis’ strongest arguments involved the new generation of seed amplification assays, which can detect pathological alpha-synuclein in cerebrospinal fluid, skin biopsies and potentially blood samples. These assays are increasingly able to identify abnormal alpha-synuclein years before classical Parkinson’s symptoms emerge.
“It’s something that occurs early,” Stefanis said, arguing that the biomarker evidence had “cemented this neuropathological notion that alpha-synuclein pathology comes before and correlates with disease manifestations.”
Perhaps most importantly, Stefanis did not present Parkinson’s as a single disease beginning in a single way. Mitochondrial dysfunction, lysosomal failure, inflammation and environmental factors may all contribute, he argued. But many of these pathways appear to funnel into pathological alpha-synuclein biology, which then becomes self-propagating.
“There are other factors,” he said, “but they really have to funnel into this main driver of pathology, which is pathological alpha-synuclein.”
In that formulation, alpha-synuclein is not necessarily the spark. It is the engine.
The defence responds
Barker’s strategy was different. Rather than denying the importance of alpha-synuclein, he attacked the idea that it occupies a uniquely privileged position in Parkinson’s disease.
His critique began with genetics. Yes, SNCA mutations exist, he acknowledged. But they account for only a tiny fraction of Parkinson’s cases.
“These account for 0.1 per cent of people with Parkinson’s,” Barker said. “So we can ignore those.”
Genome-wide association studies implicate many genes, he argued, not just alpha-synuclein. And in some populations, alpha-synuclein does not even emerge strongly in the data.
“So it clearly can’t be the central piece of the puzzle,” Barker said.
But Barker’s deeper challenge concerned causation itself. Parkinsonism can occur without classical Lewy pathology. Conversely, substantial alpha-synuclein pathology can exist in individuals without clinical Parkinson’s disease.
“We have a beautiful double dissociation,” Barker said. “Parkinsonism can be found without alpha-synuclein, and alpha-synuclein pathology can be found without Parkinson’s.”
The point cut directly at the conceptual core of the alpha-synuclein hypothesis. A pathological hallmark is not necessarily the same thing as a disease driver.
Barker repeatedly returned to what he views as a cautionary precedent: Alzheimer’s disease. For decades, Alzheimer’s research focused intensely on beta-amyloid. The logic seemed overwhelming — strong genetics, overwhelming pathology, clear biomarker signals. Yet despite enormous effort, therapies removing amyloid have yielded only limited clinical benefit.
“We target that protein. Makes no difference,” Barker said, referring first to Huntington’s disease and then drawing parallels to Alzheimer’s and Parkinson’s.
That therapeutic disappointment hangs heavily over the entire Parkinson’s field. Multiple anti-alpha-synuclein antibody trials have failed to produce convincing disease-modifying results.
“If you try and target alpha-synuclein,” Barker said, “nobody seems to get better.”
Supporters of the alpha-synuclein hypothesis argue interventions may simply be arriving too late, after pathology has already spread widely through neural systems. Critics counter that the failures expose a deeper problem with the theory itself.
A field between paradigms
What emerged in Phoenix was not simply disagreement over one protein, but evidence of a discipline caught between competing models of disease.
One model — represented broadly by Stefanis — sees Parkinson’s as fundamentally organised around pathological alpha-synuclein biology. The other — articulated by Barker — sees Parkinson’s as a far more heterogeneous systems disorder involving multiple interacting pathologies and vulnerabilities.
Increasingly, the field appears to occupy an uneasy middle ground between those positions.
Few researchers now deny alpha-synuclein’s importance. The evidence tying the protein to Parkinson’s pathology is too extensive. Yet confidence in single-protein explanations has weakened. The failure of disease-modifying therapies, combined with growing recognition of biological heterogeneity, has pushed many researchers toward more pluralistic models involving aging, immunity, metabolism, vascular biology, mitochondrial dysfunction, lysosomal stress and gut-brain interactions.
In that emerging framework, alpha-synuclein may still matter profoundly. Just perhaps not alone.
The unresolved verdict
As the debate closed, the courtroom imagery remained strangely apt. Stefanis had presented alpha-synuclein as the prime suspect in Parkinson’s disease — present early, biologically capable and increasingly detectable before symptoms emerge. Barker had argued that the case remained largely circumstantial.
“Alpha-synuclein,” Barker concluded, “whilst being on the scene of the crime, is not the only one who is there.”
Both men are respected scientists. Both are colleagues. Both clearly enjoyed the theatricality of the exchange. But beneath the humour sat a reality neither side disputed: Parkinson’s disease remains unsolved.
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