Phosphorylation and Truncation of α-Synuclein do not trigger Parkinsonian Readouts in A53T-SNCA Mice

Abstract Parkinson’s disease is a multisystem disorder, in its prodrome characterized by a broad spectrum of non-motor symptoms, including olfactory deficits and REM sleep behavior disorder, that emerge years before the classical motor symptoms develop. Accordingly, a growing number of studies aim to generate mouse models exhibiting α-Synuclein pathology that recapitulate this prodromal phase and its progression to motor stages. This study investigated whether transgenic bacterial artificial chromosome mice carrying the human α-Synuclein gene - with the A53T point mutation, two single nucleotide polymorphisms and a Rep1 polymorphism - can capture features of prodromal and late motoric Parkinson’s disease through mutant α-Synuclein overexpression. Over a 24-month period, 20 heterozygous A53T-SNCA mice and 21 wild-type mice were longitudinally assessed for both non-motor and motor symptoms associated with Parkinson’s disease. EEG-EMG and local field potential recordings were performed to evaluate rapid eye movement sleep behavior disorder and stimulus-evoked neuronal activity disturbances, respectively. Additionally, we performed a behavioral phenotyping including the buried food seeking test and discriminations test for olfactory function, along with Rotarod and CatWalk assessments to evaluate motor performance. Terminal neuropathology was examined by immunohistochemistry, western blotting and a two-step direct immunoassay to correlate pathology with functional outcomes from the longitudinal study. Characterization of the final pathology in heterozygous A53T-SNCA mice revealed a SNCA transgene dose-dependent overexpression of α-Synuclein monomers, exhibiting Serine129 phosphorylation and C-terminal truncation, in the olfactory bulb, striatum and cortex. However, no SDS-resistant higher-molecular-weight α-Synuclein species [≥ 198kDa] were detected unlike those observed in the Parkinson’s disease brain sample. In addition, under our testing conditions, we were unable to identify early measurable signs of olfactory dysfunction or rapid eye movement sleep behavior disorder. Moreover, they maintained their motor performance up to 24 months, and showed no substantial loss of dopaminergic neurons, compared to wild-type mice. In summary, our results demonstrate that overexpression of Serine129 phosphorylated and C-terminally truncated α-Synuclein monomers in heterozygous A53T-SNCA mice is insufficient to drive mature fibrillar α-Synuclein aggregation, pronounced dopaminergic neurodegeneration or Parkinsonian (non-)motor symptoms. This transgenic model therefore highlights the limited ability of these post-translational modifications to initiate pathogenic processes relevant to prodromal and advanced Parkinson’s disease.