Imaging nanoscopic co-assemblies of α-synuclein and Tau in human samples

α-synuclein (αS), amyloid-β (Aβ), and Tau aggregates are hallmarks of many neurodegenerative diseases, yet the prevalence and nature of their nanoscopic co-assemblies remain poorly understood. Here, we adapt a single-molecule pull-down (SiMPull) assay to profile αS, Aβ, and Tau co-assemblies across post-mortem brain tissue homogenates, soluble brain fractions, human serum, cerebrospinal fluid, and saliva. We show that αS-Tau co-assemblies are abundant across all sample matrices, accounting for ~10-70% of total αS aggregates in both Parkinson's disease (PD) and control samples. Notably, αS-Tau co-assemblies in soluble brain fractions from controls exhibit higher Proteinase K (PK) resistance than those from PD cases. Biophysically, Tau accelerates αS aggregation in simple in vitro systems, but this effect is suppressed in complex environments. Together, this work establishes a framework for probing co-assemblies, revealing that αS-Tau species possess heterogeneous biophysical features and are unlikely to act as a single, uniformly toxic species in PD pathology.