Synphilin-1 modulates alpha-synuclein assembly, release and uptake.

Alpha-synuclein (aSyn) is an intrinsically disordered protein involved in phase separation and several age-associated neurodegenerative disorders, including Parkinson's disease. However, its function and pathological role remain elusive. Here, we modeled different aSyn assemblies in living cells by exploiting its interaction with synphilin-1 (Sph1). We developed a model that reports on gel- and solid-like inclusions through coexpression of aSyn and Sph1. Distinct morphological differences emerged between VN-aSyn + aSyn-VC and VN-Sph1 + aSyn-VC assemblies, showing unique antibody recognition, proteinase K resistance, and protein mobilities. The VN-Sph1 + aSyn-VC interaction could be manipulated to alter inclusion size and number. These inclusions also contained lysosomes and AP-1 vesicles, aligning with observations in human brain tissue. Our study offers new insight into aSyn aggregation and release, highlighting the importance of Sph1 and other aSyn-interacting proteins in synucleinopathies, which involve diverse copathologies only now beginning to be understood.