Formation of seeding-competent α-synuclein aggregates in parkin-deficient iPSC-derived human neurons

Loss-of-function mutations in PARK2 (parkin) cause early-onset familial Parkinson's disease (PD) and may also contribute to sporadic PD. While Lewy bodies, enriched in aggregated phosphorylated α-synuclein (α-Syn), are typical in PD, their presence in PARK2-mediated PD remains debated. Using human isogenic PARK2^-/- induced pluripotent stem cell-derived neurons, we investigated α-Syn pathology under parkin deficiency. PARK2^-/- neurons showed elevated intracellular aggregated and total α-Syn levels, increased α-Syn release, and higher levels of aggregation-inducing α-Syn seeds. These neurons also displayed more pSer129 α-Syn⁺ inclusions, which were further enhanced by α-Syn preformed fibril (PFF) exposure. Moreover, we identified synaptic loss in the PARK2^-/- neurons, exacerbated by PFF treatment, and dysregulated Ca²⁺ homeostasis consistent with enhanced activity of the smooth endoplasmic reticulum Ca²⁺-ATPase (SERCA). Our data provide an important contribution to the debate on the role of α-Syn in the pathology of PARK2-related PD and challenge the view of PARK2-related PD as a non-synucleinopathy.