Motor dysfunction and gliosis with preserved dopaminergic markers in human α-synuclein A30P transgenic mice

Abstract

α-Synuclein is a major component of Lewy bodies (LBs) in the substantia nigra and cortex in Parkinson’s disease (PD) and dementia with Lewy bodies (DLB), and in glial inclusions in multiple systems atrophy (MSA). Mutations in α-synuclein have been associated with autosomal dominant forms of PD. We investigated the clinical and neuropathological effects of overexpression of human α-synuclein, α-synuclein A30P, and α-synuclein A53T under the control of the hamster prion protein (PrP) promoter; 5–15× endogenous levels of protein expression were achieved with widespread neuronal, including nigral, transgene expression. High expression of α-synuclein A30P in the Tg5093 line was associated with a progressive motor disorder with rigidity, dystonia, gait impairment, and tremor. Histological analysis of this line showed aberrant expression of the protein in cell soma and progressive CNS gliosis, but no discrete Lewy body-like α-synuclein inclusions could be identified. Biochemical analysis demonstrated α-synuclein fragmentation. Despite strong expression of the transgene in the nigra, there was no specific deterioration of the nigrostriatal dopaminergic system as assessed by quantitation of nigral tyrosine hydroxylase (TH) containing neurons, striatal TH immunoreactivity, dopamine levels, or dopamine receptor number and function. Lower expressing lines had no specific behavioral or histopathological phenotype. Thus, high expression of mutant human α-synuclein resulted in a progressive motor and widespread CNS gliotic phenotype independent of dopaminergic dysfunction in the Tg5093 line.

Introduction

α-Synuclein is a highly conserved 140 amino acid presynaptic phosphoprotein implicated physiologically in synaptic plasticity and pathologically in several neurodegenerative disorders, including Parkinson’s disease (PD), dementia with Lewy bodies (DLB), multiple system atrophy (MSA), and Alzheimer’s disease (AD) [6], [15], [19], [24], [25], [27], [39], [45], [46], [49], [50]. Two distinct pathogenic mutations (A53T and A30P) in the α-synuclein gene, located on chromosome 4, have been identified in several families with autosomal dominant PD [31], [39], and α-synuclein accumulates in Lewy bodies (LBs) and Lewy neurites in both familial and sporadic PD and DLB [24], [45], [50], as well as in glial inclusions in MSA [49].

These findings suggest a pathological role for α-synuclein aggregation in these disorders [18]. The mechanisms by which α-synuclein forms neuronal and glial inclusions and becomes associated with neuronal degeneration, or by which α-synuclein mutations result in hereditary forms of PD, are unknown. Structurally, the α-synuclein protein contains seven copies of a degenerate 11 amino acid repeat which predicts four amphipathic alpha helices, structurally similar to exchangeable apolipoproteins such as apolipoprotein E [15]. The alpha helical domains may mediate reversible interactions with lipid domains including synaptosomal membranes [12], [25], [36]. α-Synuclein can form filamentous aggregates in vitro, with kinetics modified by PD associated mutations, catecholamines, and by nucleation with truncated α-synuclein [7], [8], [9], [11], [17], [44].

We tested the hypothesis that overexpression of mutant human α-synuclein in mouse brain induces clinical and neuropathological traits resembling those of human Lewy body disorders. We produced transgenic mice expressing human wild-type (wt), A53T, and A30P α-synuclein in a C57B/6jxSJL F1 hybrid background under the control of the hamster prion protein (PrP) promoter. High expression of α-synuclein A30P resulted in a progressive motor disorder associated with aberrant expression of the protein in cell soma, α-synuclein fragmentation and progressive CNS gliosis, but without specific anatomical or biochemical disruption of the nigrostriatal dopaminergic system.