Synaptic attenuation by human alpha-synuclein depends on two amino acids in its C-terminal tail

Alpha-synuclein is a protein primarily expressed in the central and peripheral nervous systems that is firmly implicated in Parkinson's disease and other neurodegenerative diseases termed the synucleinopathies. In post-mortem analyses, macromolecular aggregates of alpha-synuclein are observed in surviving neurons. Consequently, significant research effort has been invested in understanding the properties of alpha-synuclein, with the vast majority focused on the human form. Notwithstanding its high evolutionary conservation, inter-species differences have been noted, and particularly, that while mouse alpha-synuclein fibrillizes in vitro faster than the human form, it is the latter that is more neurotoxic. In light of the synaptic hypothesis of the synucleinopathies, which posits that synaptic dysfunction precedes neurodegeneration, we investigated whether overexpressed human and mouse alpha-synuclein exert distinct effects on neurotransmission. We found that while human alpha-synuclein attenuates synaptic vesicle recycling and disperses the vesicles in synapses of cultured mouse neurons, surprisingly, the mouse protein does not. To explore the basis for these differences, we created chimeric constructs between the two. We report that the two amino acids D121-N122 in the C-terminal tail of human alpha-synuclein are sufficient to discriminate between the distinct synaptic phenotypes of the human and mouse forms, highlighting their functional significance.