Subcellular localization of wild-type and Parkinson's disease-associated mutant α-synuclein in human and transgenic mouse brain

Mutations in the alpha-synuclein (alpha SYN) gene are associated with rare cases of familial Parkinson's disease, and alpha SYN is a major component of Lewy bodies and Lewy neurites. Here we have investigated the localization of wild-type and mutant [A30P]alpha SYN as well as beta SYN at the cellular and subcellular level. Our direct comparative study demonstrates extensive synaptic colocalization of alpha SYN and beta SYN in human and mouse brain. In a sucrose gradient equilibrium centrifugation assay, a portion of beta SYN floated into lower density fractions, which also contained the synaptic vesicle marker synaptophysin. Likewise, wild-type and [A30P]alpha SYN were found in floating fractions. Subcellular fractionation of mouse brain revealed that both alpha SYN and beta SYN were present in synaptosomes. In contrast to synaptophysin, beta SYN and alpha SYN were recovered from the soluble fraction upon lysis of the synaptosomes. Synaptic colocalization of alpha SYN and beta SYN was directly visualized by confocal microscopy of double-stained human brain sections. The Parkinson's disease-associated human mutant [A30P]alpha SYN was found to colocalize with beta SYN and synaptophysin in synapses of transgenic mouse brain. However, in addition to their normal presynaptic localization, transgenic wild-type and [A30P]alpha SYN abnormally accumulated in neuronal cell bodies and neurites throughout the brain. Thus, mutant [A30P]alpha SYN does not fail to be transported to synapses, but its transgenic overexpression apparently leads to abnormal cellular accumulations.