Seeing Invisible Oligomers: Rethinking α-Synuclein Pathology Through Proximity Ligation Assay.

Parkinson's disease (PD) and multiple system atrophy are defined by α-synuclein (αSYN)-positive inclusions - Lewy bodies (LBs) and glial cytoplasmic inclusions - yet mounting evidence indicates that these inclusions represent only a fraction of disease-relevant pathology. αSYN exists in dynamic conformational states, and soluble oligomeric assemblies, often undetectable by conventional immunohistochemistry, are increasingly implicated as key neurotoxic species. The αSYN proximity ligation assay (αSYN-PLA) enables in situ detection of widespread non-inclusion oligomeric pathology across synucleinopathies. Notably, PLA studies in LRRK2-associated PD demonstrate abundant oligomeric αSYN even in cases lacking LBs, challenging the centrality of inclusions in defining disease. In this Perspective, we outline the principles of αSYN-PLA, discuss antibody strategies and structural implications of detected species, review current evidence - including LB-negative LRRK2-PD - and consider temporal dynamics and clinical applications of PLA-positive aggregates as biomarkers and therapeutic targets. © 2026 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.