Differential Expression of Alpha-Synuclein in Hippocampal Neurons

alpha-Synuclein is the major pathological component of synucleinopathies including Parkinson's disease and dementia with Lewy bodies. Recent studies have demonstrated that alpha-synuclein also plays important roles in the release of synaptic vesicles and synaptic membrane recycling in healthy neurons. However, the precise relationship between the pathogenicity and physiological functions of alpha-synuclein remains to be elucidated. To address this issue, we investigated the subcellular localization of alpha-synuclein in normal and pathological conditions using primary mouse hippocampal neuronal cultures. While some neurons expressed high levels of alpha-synuclein in presynaptic boutons and cell bodies, other neurons either did not or only very weakly expressed the protein. These alpha-synuclein-negative cells were identified as inhibitory neurons by immunostaining with specific antibodies against glutamic acid decarboxylase (GAD), parvalbumin, and somatostatin. In contrast, alpha-synuclein-positive synapses were colocalized with the excitatory synapse marker vesicular glutamate transporter-1. This expression profile of alpha-synuclein was conserved in the hippocampus in vivo. In addition, we found that while presynaptic alpha-synuclein colocalizes with synapsin, a marker of presynaptic vesicles, it is not essential for activity-dependent membrane recycling induced by high potassium treatment. Exogenous supply of preformed fibrils generated by recombinant alpha-synuclein was shown to promote the formation of Lewy body (LB) -like intracellular aggregates involving endogenous alpha-synuclein. GAD-positive neurons did not form LB-like aggregates following treatment with preformed fibrils, however, exogenous expression of human alpha-synuclein allowed intracellular aggregate formation in these cells. These results suggest the presence of a different mechanism for regulation of the expression of alpha-synuclein between excitatory and inhibitory neurons. Furthermore, alpha-synuclein expression levels may determine the efficiency of intracellular aggregate formation in different neuronal subtypes.