Parkinsons disease microglia induce endogenous alpha-Synuclein pathology in patient-specific midbrain organoids.

The accumulation of misfolded alpha-synuclein and the loss of dopaminergic neurons are hallmarks of Parkinsons disease (PD), contributing to the development of synucleinopathies. Although considerable progress has been made in understanding α-synuclein's role in PD pathology, the precise mechanisms involved remain unclear. Human midbrain organoids (hMOs) have emerged as valuable models for studying PD, yet the lack of microglia limits the ability to investigate neuroimmune interactions. Recent studies show that integrating microglia into hMOs enhances neuronal maturation and functionality. Here, we generated a human midbrain assembloid model by incorporating iPSC-derived microglia into midbrain organoids from healthy control individuals and a PD patient carrying the SNCA triplication (3xSNCA) mutation. Our results show that 3xSNCA microglia alone are sufficient to induce early, endogenous formation of phosphorylated alpha-synuclein (pS129) pathology in the absence of exogenous fibril seeding. This PD-pathology emerged as early as day 50 of culture and was not observed in models lacking microglia. These findings highlight a critical role for patient-derived microglia in driving α-synuclein pathology and provide a physiologically relevant platform for studying early neuroimmune mechanisms in PD and testing potential therapeutic strategies.