Subcellular localization of wild-type and Parkinson's disease-associated mutant alpha-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 o f 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 cellula r and subcellular level. Our direct comparative study demonstrates extensiv e synaptic colocalization of alpha SYN and beta SYN in human and mouse brai n. In a sucrose gradient equilibrium centrifugation assay, a portion of bet a SYN floated into lower density fractions, which also contained the synapt ic vesicle marker synaptophysin. Likewise, wild-type and [A30P]alpha SYN we re found in floating fractions. Subcellular fractionation of mouse brain re vealed that both alpha SYN and beta SYN were present in synaptosomes. In co ntrast to synaptophysin, beta SYN and alpha SYN were recovered from the sol uble fraction upon lysis of the synaptosomes. Synaptic colocalization of al pha SYN and beta SYN was directly visualized by confocal microscopy of doub le-stained human brain sections. The Parkinson's disease-associated human m utant [A30P]alpha SYN was found to colocalize with beta SYN and synaptophys in in synapses of transgenic mouse brain. However, in addition to their nor mal presynaptic localization, transgenic wild-type and [A30P]alpha SYN abno rmally accumulated in neuronal cell bodies and neurites throughout the brai n. Thus, mutant [A30P]alpha SYN does not fail to be transported to synapses , but its transgenic overexpression apparently leads to abnormal cellular a ccumulations.