Synphilin-1A is a Phosphoprotein Phosphatase 1-Interacting Protein and Affects PPP1 Sorting to Subcellular Compartments

Lewy bodies (LBs) are synphilin-1 (Sph1)-containing aggregates and histological hallmarks of Parkinson's disease. Therefore, understanding processes which modulate the aggregation of Sph1, or its isoform Sph1A, will contribute to our understanding of LBs formation. Protein phosphorylation promotes aggregation, but protein phosphatases with activity towards Sph1 have not been described. The present study documents the identification of a novel Sph1A/phosphoprotein phosphatase 1 (PPP1) complex and unravels its regulatory effect on Sph1A aggregation. Using yeast co-transformation and overlay blot assay, the interaction between Sph1A and PPP1 was mapped to the Sph1A RVTF motif. Then, Sph1A overexpression in human embryonic kidney 293 cells demonstrated that Sph1A specifically targets endogenous PPP1 isoforms to inclusion bodies and that Sph1A/PPP1 complex disruption enhances inclusion bodies formation. Finally, as Sph1A interacted with PPP1CC2, a PPP1 sperm-specific isoform, Sph1 and Sph1A expression was addressed in male germ cells by qRT-PCR, revealing high expression levels in round spermatids. Together, these observations established Sph1A as a novel PPP1-interacting protein able to affect PPP1 sorting to subcellular compartments and Sph1A/PPP1 complex as a negative modulator of LBs formation. Contrarily, in physiological conditions, Sph1 isoforms are pointed as putative participants in vesicle dynamics with implications in neurotransmission and spermiogenesis.