Quantitative study of alpha-synuclein prion-like spreading in fully oriented reconstructed neural networks reveals non-synaptic dissemination of seeding aggregates

The trans-neuronal spread of protein aggregates in a prion-like manner underlies the progression of neuronal lesions in the brain of patients with synucleinopathies such as Parkinsons disease. Despite being studied actively, the mechanisms of alpha-synuclein (aSyn) aggregates propagation remain poorly understood. This hinders the development of therapeutic approaches aiming at preventing the spatial progression of intracellular inclusions in neural networks. To assess the role of synaptic structures and neuron characteristics in the transfer efficiency of aggregates with seeding propensity, we developed a novel microfluidic culture system which allows for the first time to reconstruct in vitro fully oriented and synaptically connected neural networks. This is achieved by filtering axonal growth with unidirectional "axon valves" microchannels. We exposed the presynaptic compartment of reconstructed networks to well characterized human aSyn aggregates differing in size: Fibrils and Oligomers. Both aggregates were transferred to postsynaptic neurons through active axonal transport, albeit with poor efficiency. By manipulating network maturity, we compared the transfer rate of aggregates in networks with distinct levels of synaptic connectivity. Surprisingly, we found that transfer efficiency was lower in mature networks with higher synaptic connectivity. We then investigated the seeding efficiency of endogenous aSyn in the postsynaptic population. We found that exposure to Fibrils, and not Oligomers, resulted in low efficiency trans-neuronal seeding which was restricted to postsynaptic axons. Finally, we assessed the impact of neuron characteristics and aSyn expression on the propagation of aSyn aggregates. By reconstructing chimeric networks, we found that neuron characteristics, such as the brain region from which they originate or aSyn expression levels, did not significantly impact aggregates transfer, and observed no trans-neuronal seeding where the presynaptic population did not express aSyn. Overall, we demonstrate that this novel platform uniquely allows the quantitative interrogation of original aspects of the trans-neuronal propagation of seeding pathogenic entities.