Glutamatergic synaptic resilience to overexpressed human alpha-synuclein

Alpha synuclein (aSyn) is an abundant protein that, in the brain, is concentrated in neuronal presynaptic boutons and associates with synaptic vesicles. aSyn is strongly linked with Parkinson's disease (PD) due to its accumulation in pathognomonic inclusions in neuronal cells, including in glutamatergic neurons. While increased expression of wild-type (WT) aSyn due to multiplications of the SNCA gene, or the expression of mutant forms of aSyn, can cause familial forms of PD, it is still unclear whether increased levels of aSyn alone are sufficient to impair synaptic function. Previous studies have used experimental systems that do not allow systematic characterisation of presynaptic physiology. To address this gap in research, we used lentiviral vectors to overexpress human aSyn (haSyn) in continental and autaptic glutamatergic neurons from rodent hippocampus and systematically analysed their presynaptic function. Virally-transduced neurons exhibited levels of expression of haSyn that mimic those associated with triplications of the SNCA gene in PD patients (2-fold increase in total aSyn), as determined using quantitative immunofluorescence imaging and immunoblots. Neuronal toxicity, neuronal morphology, and SNAP-25, a presynaptic protein, were not altered in continental cultures. Finally, a systematic characterization of autaptic neurons expressing haSyn exhibited no significant difference in any parameter of synaptic function, including basal properties of evoked and spontaneous neurotransmitter release, and synaptic plasticity compared to neurons infected with a control virus. These results indicate that rodent glutamatergic neurons are resilient to aSyn overexpression. In conclusion, our findings suggest neurotoxicity associated with aSyn overexpression is not universal, and that a deeper understanding of aSyn biology and pathobiology is necessary.