This Review provides an overview of the role of autophagy, a key lysosomal degradative process, in neurodegenerative diseases. The study of various neurodegenerative diseases has shown that defects in autophagy can arise at different points in the pathway, and this has implications for the successful modulation of autophagy for therapeutic purposes. The Review also discusses the latest developments in targeting alterations in autophagy as a therapeutic strategy for neurodegenerative diseases.

The role of autophagy in neurodegenerative disease

Misfolded protein aggregates represent a continuum with overlapping features in neurodegenerative diseases, but differences in protein components and affected brain regions. The molecular hallmark of synucleinopathies such as Parkinson's disease, dementia with Lewy bodies and multiple system atrophy are megadalton α-synuclein-rich deposits suggestive of one molecular event causing distinct disease phenotypes. Glial α-synuclein (α-SYN) filamentous deposits are prominent in multiple system atrophy and neuronal α-SYN inclusions are found in Parkinson's disease and dementia with Lewy bodies. The discovery of α-SYN assemblies with different structural characteristics or 'strains' has led to the hypothesis that strains could account for the different clinico-pathological traits within synucleinopathies. In this study we show that α-SYN strain conformation and seeding propensity lead to distinct histopathological and behavioural phenotypes. We assess the properties of structurally well-defined α-SYN assemblies (oligomers, ribbons and fibrils) after injection in rat brain. We prove that α-SYN strains amplify in vivo. Fibrils seem to be the major toxic strain, resulting in progressive motor impairment and cell death, whereas ribbons cause a distinct histopathological phenotype displaying Parkinson's disease and multiple system atrophy traits. Additionally, we show that α-SYN assemblies cross the blood-brain barrier and distribute to the central nervous system after intravenous injection. Our results demonstrate that distinct α-SYN strains display differential seeding capacities, inducing strain-specific pathology and neurotoxic phenotypes.

α-Synuclein strains cause distinct synucleinopathies after local and systemic administration

Distinct tau prion strains propagate in cells and mice and define different tauopathies.

Misfolded α-synuclein amyloid fibrils are the principal components of Lewy bodies and neurites, hallmarks of Parkinson's disease (PD). We present a high-resolution structure of an α-synuclein fibril, in a form that induces robust pathology in primary neuronal culture, determined by solid-state NMR spectroscopy and validated by EM and X-ray fiber diffraction. Over 200 unique long-range distance restraints define a consensus structure with common amyloid features including parallel, in-register β-sheets and hydrophobic-core residues, and with substantial complexity arising from diverse structural features including an intermolecular salt bridge, a glutamine ladder, close backbone interactions involving small residues, and several steric zippers stabilizing a new orthogonal Greek-key topology. These characteristics contribute to the robust propagation of this fibril form, as supported by the structural similarity of early-onset-PD mutants. The structure provides a framework for understanding the interactions of α-synuclein with other proteins and small molecules, to aid in PD diagnosis and treatment.

/pdf/solid-state-nmr-structure-of-a-pathogenic-fibril-of-full-33lvilvf6k.pdf

Solid-state NMR structure of a pathogenic fibril of full-length human α-synuclein

The pathological assembly of Aβ, tau, and α-synuclein is at the heart of Alzheimer's and Parkinson's diseases. Extracellular deposits of Aβ and intraneuronal tau inclusions define Alzheimer's disease, whereas intracellular inclusions of α-synuclein make up the Lewy pathology of Parkinson's disease. Most cases of disease are sporadic, but some are inherited in a dominant manner. Mutations frequently occur in the genes encoding Aβ, tau, and α-synuclein. Overexpression of these mutant proteins can give rise to disease-associated phenotypes. Protein assembly begins in specific regions of the brain during the process of Alzheimer's and Parkinson's diseases, from where it spreads to other areas.

http://science.sciencemag.org/content/sci/349/6248/1255555.full.pdf

Alzheimer’s and Parkinson’s diseases: The prion concept in relation to assembled Aβ, tau, and α-synuclein

α-Synuclein aggregation is implicated in a variety of diseases including Parkinson's disease, dementia with Lewy bodies, pure autonomic failure and multiple system atrophy. The association of protein aggregates made of a single protein with a variety of clinical phenotypes has been explained for prion diseases by the existence of different strains that propagate through the infection pathway. Here we structurally and functionally characterize two polymorphs of α-synuclein. We present evidence that the two forms indeed fulfil the molecular criteria to be identified as two strains of α-synuclein. Specifically, we show that the two strains have different structures, levels of toxicity, and in vitro and in vivo seeding and propagation properties. Such strain differences may account for differences in disease progression in different individuals/cell types and/or types of synucleinopathies.

/pdf/structural-and-functional-characterization-of-two-alpha-3qtl5vqbwf.pdf

Structural and functional characterization of two alpha-synuclein strains

To date, 3 rare missense mutations in the SNCA (α-synuclein) gene and the more frequent duplications or triplications of the wild-type gene are known to cause a broad array of clinical and pathological symptoms in familial Parkinson disease (PD). Here, we describe a French family with a parkinsonian-pyramidal syndrome harboring a novel heterozygous SNCA mutation. Whole exome sequencing of DNA from 3 patients in a 3-generation pedigree was used to identify a new PD-associated mutation in SNCA. Clinical and pathological features of the patients were analyzed. The cytotoxic effects of the mutant and wild-type proteins were assessed by analytical ultracentrifugation, thioflavin T binding, transmission electron microscopy, cell viability assay, and caspase-3 activation. We identified a novel SNCA G51D (c.152 G>A) mutation that cosegregated with the disease and was absent from controls. G51D was associated with an unusual PD phenotype characterized by early disease onset, moderate response to levodopa, rapid progression leading to loss of autonomy and death within a few years, marked pyramidal signs including bilateral extensor plantar reflexes, occasionally spasticity, and frequently psychiatric symptoms. Pathological lesions predominated in the basal ganglia and the pyramidal tracts and included fine, diffuse cytoplasmic inclusions containing phospho-α-synuclein in superficial layers of the cerebral cortex, including the entorhinal cortex. Functional studies showed that G51D α-synuclein oligomerizes more slowly and its fibrils are more toxic than those of the wild-type protein. We have identified a novel SNCA G51D mutation that causes a form of PD with unusual clinical, neuropathological, and biochemical features.

G51D α-synuclein mutation causes a novel parkinsonian-pyramidal syndrome.

Fibrillar α-Synuclein and Huntingtin Exon 1 Assemblies Are Toxic to the Cells

The deposition of fibrillar alpha-synuclein (α-syn) within inclusions (Lewy bodies and Lewy neurites) in neurons and glial cells is a hallmark of synucleinopathies. α-syn populates a variety of assemblies ranging from prefibrillar oligomeric species to fibrils whose specific contribution to neurodegeneration is still unclear. Here, we compare the specific structural and biological properties of distinct soluble prefibrillar α-syn oligomers formed either spontaneously or in the presence of dopamine and glutaraldehyde. We show that both on-fibrillar assembly pathway and distinct dopamine-mediated and glutaraldehyde-cross-linked α-syn oligomers are only slightly effective in perturbing cell membrane integrity and inducing cytotoxicity, while mature fibrils exhibit the highest toxicity. In contrast to low-molecular weight and unstable oligomers, large stable α-syn oligomers seed the aggregation of soluble α-syn within reporter cells although to a lesser extent than mature α-syn fibrils. These oligomers appear elongated in shape. Our findings suggest that α-syn oligomers represent a continuum of species ranging from unstable low molecular weight particles to mature fibrils via stable elongated oligomers composed of more than 15 α-syn monomers that possess seeding capacity.

/pdf/structural-and-functional-properties-of-prefibrillar-a-5dnfhlgeia.pdf

Karine Madiona

Papers

Structural and functional characterization of two alpha-synuclein strains

G51D α-synuclein mutation causes a novel parkinsonian-pyramidal syndrome.

Fibrillar α-Synuclein and Huntingtin Exon 1 Assemblies Are Toxic to the Cells

Structural and functional properties of prefibrillar α-synuclein oligomers.

Hsc70 Protein Interaction with Soluble and Fibrillar α-Synuclein