Transcellular propagation of fibrillar α-synuclein from enteroendocrine to neuronal cells requires cell-to-cell contact and is Rab35-dependent

Parkinson's disease (PD) is a neurodegenerative condition featured by motor dysfunction, death of midbrain dopaminergic neurons and accumulation of alpha-synuclein (alpha Syn) aggregates. Growing evidence suggests that PD diagnosis happens late in the disease progression and that the pathology may originate much earlier in the enteric nervous system (ENS) before advancing to the brain, via autonomic fibers. It was recently described that a specific cell type from the gut epithelium named enteroendocrine cells (EECs) possess many neuron-like properties including alpha Syn expression. By facing the gut lumen and being directly connected with alpha Syn-containing enteric neurons in a synaptic manner, EECs form a neural circuit between the gastrointestinal tract and the ENS, thereby being a possible key player in the outcome of PD in the gut. We have characterized the progression and the cellular mechanisms involved in alpha Syn pre-formed fibrils (PFFs) transfer from EECs to neuronal cells. We show that brain organoids efficiently internalize alpha Syn PFF seeds which triggers the formation of larger intracellular inclusions. In addition, in the enteroendocrine cell line STC-1 and in the neuronal cell line SH-SY5Y, alpha Syn PFFs induced intracellular calcium (Ca2+) oscillations on an extracellular Ca2+ source-dependent manner and triggered alpha Syn fibrils internalization by endocytosis. We characterized the spread of alpha Syn PFFs from enteroendocrine to neuronal cells and showed that this process is dependent on physical cell-to-cell contact and on Rab35 GTPase. Lastly, inhibition of Rab35 increases the clearance of alpha Syn fibrils by redirecting them to the lysosomal compartment. Therefore, our results reveal mechanisms that contribute to the understanding of how seeded alpha Syn fibrils promote the progression of alpha Syn pathology from EECs to neuronal cells shifting the focus of PD etiology to the ENS.

Automated summary

Parkinson's disease is a neurodegenerative disease that can originate in the enteric nervous system and spread to the brain. Enteroendocrine cells in the gut may play a key role in the development of Parkinson's disease. Alpha-synuclein is involved in this process.