PPM1H loss alters intrasynaptic alpha-synuclein localization in a LRRK2-dependent manner

Mutations in the LRRK2 gene are a major genetic cause of Parkinson's disease (PD) and induce hyperphosphorylation of RAB GTPases through increased LRRK2 kinase activity. The phosphatase PPM1H counteracts this activity by selectively dephosphorylating LRRK2-phosphorylated RAB proteins. Here, we used primary PPM1H knockout neurons to investigate how loss of PPM1H and the resulting hyperphosphorylation of RAB proteins affect the distribution of alpha-synuclein (aSyn), a key protein in PD pathology, at presynaptic terminals. We found that PPM1H deficiency significantly increased the distance between aSyn and the presynaptic marker Bassoon, indicating displacement of aSyn away from the presynapse. Pharmacological LRRK2 kinase inhibition rescued this phenotype, restoring intrasynaptic aSyn positioning to wild-type values. These findings show that loss of PPM1H alters aSyn localization in a LRRK2 kinase activity-dependent manner, suggesting that elevated levels of LRRK2-phosphorylated RAB proteins disrupt the nanoscale organization of aSyn at the presynapse.