α-Synuclein Pathology and Reduced Neurogenesis in the Olfactory System Affect Olfaction in a Mouse Model of Parkinson's Disease

Parkinson's Disease (PD) is characterized by multiple symptoms including olfactory dysfunction, whose underlying mechanisms remain unclear. Here, we explored pathological changes in the olfactory pathway of transgenic (Tg) mice of both sexes expressing the human A30P mutant alpha-synuclein (alpha-syn) (alpha-syn-Tg mice) at 6-7 and 12-14 months of age, representing early and late-stages of motor progression, respectively. alpha-Syn-Tg mice at late stages exhibited olfactory behavioral deficits, which correlated with severe a-syn pathology in projection neurons of the olfactory pathway. In parallel, olfactory bulb (OB) neurogenesis in alpha-syn-Tg mice was reduced in the OB granule cells at 6-7 months, and OB periglomerular cells at 12-14 months, respectively, both of which could contribute to olfactory dysfunction. Proteomic analyses showed a disruption in endo-and exocytic pathways in the OB during the early stages which appeared exacerbated at the synaptic terminals when the mice developed olfactory deficits at 12-14 months. Our data suggest that (1) the alpha-syn-Tg mice recapitulate the olfactory functional deficits seen in PD; (2) olfactory structures exhibit spatiotemporal disparities for vulnerability to alpha-syn pathology; (3) alpha-syn pathology is restricted to projection neurons in the olfactory pathway; (4) neurogenesis in adult alpha-syn-Tg mice is reduced in the OB; and (5) synaptic endo-and exocytosis defects in the OB may further explain olfactory deficits.Significance StatementOlfactory dysfunction is a characteristic symptom of Parkinson's disease (PD). Using the human A30P mutant alpha-synuclein (alpha-syn)-expressing mouse model, we demonstrated the appearance of olfactory deficits at late stages of the disease, which was accompanied by the accumulation of alpha-syn pathology in projection neurons of the olfactory system. This dysfunction included a reduction in olfactory bulb neurogenesis as well as changes in synaptic vesicular transport affecting synaptic function, both of which are likely contributing to olfactory behavioral deficits.