Targeting α-synuclein for treatment of Parkinson's disease: mechanistic and therapeutic considerations

Abstract

Progressive neuronal cell loss in a small subset of brainstem and mesencephalic nuclei and widespread aggregation of the α-synuclein protein in the form of Lewy bodies and Lewy neurites are neuropathological hallmarks of Parkinson's disease. Most cases occur sporadically, but mutations in several genes, including SNCA, which encodes α-synuclein, are associated with disease development. The discovery and development of therapeutic strategies to block cell death in Parkinson's disease has been limited by a lack of understanding of the mechanisms driving neurodegeneration. However, increasing evidence of multiple pivotal roles of α-synuclein in the pathogenesis of Parkinson's disease has led researchers to consider the therapeutic potential of several strategies aimed at reduction of α-synuclein toxicity. We critically assess the potential of experimental therapies targeting α-synuclein, and discuss steps that need to be taken for target validation and drug development.

Figure 1. Primary structure of human α-synuclein (UniProtKB/Swiss-Prot: P37840)

Clinical Mutations (A53T, A30P, E46K, H50Q, G51D, A53E) are indicated in red. Amphipathic N-terminal region contains six imperfect lysine-rich highly conserved motif repeats (KTKEGV), which involve in binding of lipids, marked in grey. Central hydrophobic region contains non-amyloid beta component (NAC) sequence from residue 61 to 95 is underlined. Two major phosphorylation sites (Ser87 and Ser129) are colored in yellow. CMA recognition sites are marked in green. Nitration sites (Y39, Y125, Y133, Y136) are colored in blue.

Figure 2. Schematic summary of α-synuclein aggregation pathway

The panel shows that α-syn exists under various conformational shapes. α-Syn exists as at least two structural isoforms: a natively unfolded monomer and a helix-rich membrane-bound form. Both isoforms may undergo dramatic structural changes resulting in the formation of β-sheet rich assemblies. From in vitro studies, it is clear that α-syn behaves in a dynamic equilibrium where monomer can aggregate first into several types of small oligomeric species that can be stabilized by β-sheet interactions and then into higher molecular weight insoluble protofibrils and may polymerize into amyloidogenic fibrils resembly those found in Lewy Body (LB). However, the mechanism governing the fundamental conformational change of normal monomeric α-syn to a pathological, disease-associated form, remains unknown. Photomicrograph illustrates one synuclein stained-mesencephalic LB (in red) in a neuromelanin-positive neuron from sporadic PD patient indicated by the white arrow. Scale bar: 5μm.

Figure 3. Schematic summary of established interactions between α-synuclein and cellular components

The figure highlights six different intracellular pathways affected by α-synuclein (α-syn). The protein α-syn is enriched at the pre-synaptic terminals of almost all types of neurons in the brain, where it participates in the vesicle recycling, thereby modulating synaptic function. α-syn can be degraded by the ubiquitin-proteasome system (UPS) and inside the lysosomes. α-syn interacts strongly with membranes, such as plasma membrane and mitochondrion. If misfolded, α-syn forms distinct structures that are prone to aggregation, first into oligomers, then into larger structures. It is now believed that α-syn oligomers are the toxic form that may impair basic neuronal processes, such as ER-Golgi trafficking, lysosome and UPS functions, reduced mitochondrial activity and alter the plasma membrane through the pore/perforations that can dysregulate calcium and cation homeostasis.

Figure 4. Analysis of α-synuclein-related publications, patents and press releases per year

(A) The emergence of the α-syn research field as measured by the number of scientific articles per year (in red) is accompanied by patents deposition (in blue). No gap between the two curves is observed. The patent (blue) line stops in 2011 since we used the priority date, which is closely equivalent to the deposition date (the patent will only become public 18 months after deposit). Literature search was conducted in PubMed, as well in Scopus^® (Elsevier) and Web of Science^® (Thomson Reuters) databases with MeSH terms and/or keywords in title, and abstract fields. Between 2127 and 5026 documents were retrieved on the subject depending on the database and the type of documents (article, review, notes, proceedings…) from 1997 to 2013. The patent analysis was run in the world wide collection of INPADOC family patents using Orbit^® (Questel) patent research platform through a boolean search combining keywords in different topic field (title, abstract & claims) and International Patent sub-class Code (IPC) A61P-025/16 for antiparkinsonian drugs. All subsequent analyses were performed on patent family, i.e. a set of patent applications with the same priority date in different countries related to the same invention. Since 1997, and up to December 31th, 2013, we identified 176 patent families filed worldwide (in blue). (B) Press release in large media per year. The curve tendency fits with scientific publications and patent deposits. For the analysis, we used a collection of 3576 documents retrieved from Scopus for which we extracted citations count and compared to trends of press releases in large media by searching Factiva^® (Dow Jones) database. Two peaks emerge, a first in 2005 with the post-genomic revolution and the interference RNA technology breakthrough, and a second one in 2010 with the support of the Michael J. Fox Foundation (MJFF) for candidate compounds and the launch of clinical trials (e.g. Affiris). (C) Of 1313 studies in the field of PD, only 13 open studies are associated with the “synuclein” keyword and 8 are actually biomarker studies. We divided clinical trials in two categories, observational or for diagnosis purpose (n=8) and disease modifying or neuroprotective strategies (n=5). Data were collected from the Food and Drug Administration’s clinical trials database (clinicaltrials.gov). Clinical trials, which were terminated, completed or of which the status was unknown were excluded. Abbreviations: NIH: national institute of health; α-syn: α-synuclein.