Potential mechanisms of neuron-to-neuron transmission of synuclein. Alpha synuclein can be released into the extracellular space via (1) leakage from injured cells with compromised membrane integrity. Extracellular synuclein could then directly translocate the cell membrane and gain access to neighboring neurons (2). Synuclein can also be transmitted from cell-to-cell via conventional exocytosis and endocytosis (3). Synuclein can be packaged into exosomes which are released and taken up by surrounding cells (4).Tunneling nanotubes can form a direct connection between two cells potentially allowing synuclein to transfer freely from one cell to another (5). Finally, synuclein could be transmitted by direct synaptic contact (6).