Synphilin-1 as a modulator of aSyn assembly

Alpha-synuclein (aSyn) is an intrinsically disordered protein that undergoes phase-separation and is associated with several neurodegenerative conditions. However, the function and the pathological role of aSyn are still elusive. Here, we modeled different types of aSyn assemblies in living cells, and developed a model that reports on gel and solid-like inclusions based on the coexpression of aSyn and synphilin-1 (Sph1). We identified striking morphological differences between aSyn-aSyn and Sph1-aSyn assemblies, characterized by distinct antibody recognition patterns, resistance to Proteinase K treatment, and protein mobilities. Importantly, we showed that the interaction between Sph1-aSyn can be manipulated, altering inclusion size and number. Sph1-aSyn interactions were central for inclusion formation and localization, and that inclusions include lysosomes and AP-1 vesicles, consistent with previous studies in human brain tissue. In total, we provide novel insight into the biology of protein aggregation, shedding light on potential therapeutic strategies that extend beyond conventional targets. Deciphering the role of Sph1 and other aSyn-interacting proteins on aSyn biology and pathobiology will be essential for treating synucleinopathies.