Defining α-synuclein species responsible for Parkinson's disease phenotypes in mice.

Parkinson's disease (PD) is a neurodegenerative disorder characterized by fibrillar neuronal inclusions composed of aggregated α-synuclein (α-syn). These inclusions are associated with behavioral and pathological PD phenotypes. One strategy for therapeutic interventions is to prevent the formation of these inclusions to halt disease progression. α-Synuclein exists in multiple structural forms, including disordered, nonamyloid oligomers, ordered amyloid oligomers, and fibrils. It is critical to understand which conformers contribute to specific PD phenotypes. Here, we utilized a mouse model to explore the pathological effects of stable ß-amyloid-sheet oligomers compared with those of fibrillar α-synuclein. We biophysically characterized these species with transmission EM, atomic-force microscopy, CD spectroscopy, FTIR spectroscopy, analytical ultracentrifugation, and thioflavin T assays. We then injected these different α-synuclein forms into the mouse striatum to determine their ability to induce PD-related phenotypes. We found that ß-sheet oligomers produce a small but significant loss of dopamine neurons in the substantia nigra pars compacta (SNc). Injection of small ß-sheet fibril fragments, however, produced the most robust phenotypes, including reduction of striatal dopamine terminals, SNc loss of dopamine neurons, and motor-behavior defects. We conclude that although the ß-sheet oligomers cause some toxicity, the potent effects of the short fibrillar fragments can be attributed to their ability to recruit monomeric α-synuclein and spread in vivo and hence contribute to the development of PD-like phenotypes. These results suggest that strategies to reduce the formation and propagation of ß-sheet fibrillar species could be an important route for therapeutic intervention in PD and related disorders.

Behavioral defects associated with amygdala and cortical dysfunction in mice with seeded α-synuclein inclusions.

Parkinson's disease (PD) is defined by motor symptoms such as tremor at rest, bradykinesia, postural instability, and stiffness. In addition to the classical motor defects that define PD, up to 80% of patients experience cognitive changes and psychiatric disturbances, referred to as PD dementia (PDD). Pathologically, PD is characterized by loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and intracellular inclusions, called Lewy bodies and Lewy neurites, composed mostly of α-synuclein. Much of PD research has focused on the role of α-synuclein aggregates in degeneration of SNpc dopamine neurons because of the impact of loss of striatal dopamine on the classical motor phenotypes. However, abundant Lewy pathology is also found in other brain regions including the cortex and limbic brain regions such as the amygdala, which may contribute to non-motor phenotypes. Little is known about the consequences of α-synuclein inclusions in these brain regions, or in neuronal subtypes other than dopamine neurons. This project expands knowledge on how α-synuclein inclusions disrupt behavior, specifically non-motor symptoms of synucleinopathies. We show that bilateral injections of fibrils into the striatum results in robust bilateral α-synuclein inclusion formation in the cortex and amygdala. Inclusions in the amygdala and prefrontal cortex primarily localize to excitatory neurons, but unbiased stereology shows no significant loss of neurons in the amygdala or cortex. Fibril injected mice show defects in a social dominance behavioral task and fear conditioning, tasks that are associated with prefrontal cortex and amygdala function. Together, these observations suggest that seeded α-synuclein inclusion formation impairs behaviors associated with cortical and amygdala function, without causing cell loss, in brain areas that may play important roles in the complex cognitive features of PDD.

Templated α-Synuclein Inclusion Formation Is Independent of Endogenous Tau.

Synucleinopathies including Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are characterized by neuronal intracellular inclusions of α-synuclein. PD dementia (PDD) and DLB are collectively the second most common cause of neurodegenerative dementia. In addition to associated inclusions, Lewy body diseases (LBDs) have dopaminergic neurodegeneration, motor defects and cognitive changes. The microtubule-associated protein tau has been implicated in LBDs, but the exact role of the protein and how it influences formation of α-synuclein inclusions is unknown. Reducing endogenous tau levels is protective in multiple models of Alzheimer's disease (AD), tauopathies, and in some transgenic synucleinopathy mouse models. Recombinant α-synuclein and tau proteins interact in vitro Here, we show tau and α-synuclein colocalize at excitatory presynaptic terminals. However, tau heterozygous and tau knock-out mice do not show a reduction in fibril-induced α-synuclein inclusions formation in primary cortical neurons, or after intrastriatal injections of fibrils at 1.5 month or six months later. At six months following intrastriatal injections, wild-type, tau heterozygous and tau knock-out mice showed a 50% reduction in dopamine neurons in the substantia nigra pars compacta (SNc) compared with mice injected with α-synuclein monomer, but there were no statistically significant differences across genotypes. These data suggest the role of tau in the pathogenesis of LBDs is distinct from AD, and Lewy pathology formation may be independent of endogenous tau.