α-Synuclein Expression in the Mouse Cerebellum Is Restricted to VGluT1 Excitatory Terminals and Is Enriched in Unipolar Brush Cells

alpha-Synuclein has a crucial role in synaptic vesicle release and synaptic membrane recycling. Although its general expression pattern has been described in the cerebellum, the precise cerebellar structures where alpha-synuclein is localized are poorly understood. To address this question, we used alpha-synuclein immunohistochemistry in adult mice cerebellar sections. We found that alpha-synuclein labels glutamatergic but not glycinergic and GABAergic synaptic terminals in the molecular and granule cell layers. alpha-Synuclein was preferentially expressed in parallel and mossy fiber synaptic terminals that also express vesicular glutamate transporter 1 (VGluT1), while it was not detected in VGluT2-positive climbing fibers. alpha-Synuclein was particularly enriched in lobules IX and X, a region known to contain a high density of unipolar brush cells (UBCs). To elucidate whether the alpha-synuclein-positive mossy fibers belong to UBCs, we double-labeled cerebellar sections with antibodies to alpha-synuclein and UBC-type-specific markers (calretinin for type I and metabotropic glutamate receptor 1 alpha (mGluR1 alpha) for type II UBCs) and took advantage of organotypic cerebellar cultures (in which all mossy fibers are UBC axons) and moonwalker mice (in which almost all UBCs are ablated) and found that both type I and type II UBCs express alpha-synuclein. In moonwalker mutant cerebella, the alpha-synuclein/VGluT1 immunolabeling showed a dramatic decrease in the vestibulocerebellum that correlated with the absence of UBC. alpha-Synuclein appears to be an excellent marker for intrinsic mossy fibers of the VGluT1 subset in conjunction with UBCs of both subtypes.