Parkinson's Disease Genes VPS35 and EIF4G1 Interact Genetically and Converge on α-Synuclein

Parkinson's disease (PD) is a common neurodegenerative disorder. Functional interactions between some PD genes, like PINK1 and parkin, have been identified, but whether other ones interact remains elusive. Here we report an unexpected genetic interaction between two PD genes, VPS35 and EIF4G1. We provide evidence that EIF4G1 upregulation causes defects associated with protein misfolding. Expression of a sortilin protein rescues these defects, downstream of VPS35, suggesting a potential role for sortilins in PD. We also show interactions between VPS35, EIF4G1, and α-synuclein, a protein with a key role in PD. We extend our findings from yeast to an animal model and show that these interactions are conserved in neurons and in transgenic mice. Our studies reveal unexpected genetic and functional interactions between two seemingly unrelated PD genes and functionally connect them to α-synuclein pathobiology in yeast, worms, and mouse. Finally, we provide a resource of candidate PD genes for future interrogation.

Exploring the role of post-translational modifications in regulating α-synuclein interactions by studying the effects of phosphorylation on nanobody binding

Anass Chiki, Farah El Turk, Bruno Claude Daniel Fauvet, Juan Kiwi, Hilal Lashuel

Intracellular deposits of α-synuclein in the form of Lewy bodies are major hallmarks of Parkinson’s disease (PD) and a range of related neurodegenerative disorders. Post-translational modifications (PTMs) of α-synuclein are increasingly thought to be major modulators of its structure, function, degradation and toxicity. Among these PTMs, phosphorylation near the C-terminus at S129 has emerged as a dominant pathogenic modification as it is consistently observed to occur within the brain and cerebrospinal fluid (CSF) of post-mortem PD patients, and its level appears to correlate with disease progression. Phosphorylation at the neighboring tyrosine residue Y125 has also been shown to protect against α-synuclein toxicity in a Drosophila model of PD. In the present study we address the potential roles of C-terminal phosphorylation in modulating the interaction of α-synuclein with other protein partners, using a single domain antibody fragment (NbSyn87) that binds to the C-terminal region of α-synuclein with nanomolar affinity. The results reveal that phosphorylation at S129 has negligible effect on the binding affinity of NbSyn87 to α-synuclein while phosphorylation at Y125, only four residues away, decreases the binding affinity by a factor of 400. These findings show that, despite the fact that α-synuclein is intrinsically disordered in solution, selective phosphorylation can modulate significantly its interactions with other molecules, and suggest how this particular form of modification could play a key role in regulating the normal and aberrant function of α-synuclein.

2018

Elucidating the role of post-translational modifications of alpha-synuclein using semisynthesis

Mirva Hejjaoui

Alpha-synuclein (α-syn) is a natively unfolded protein that is closely linked to Parkinson’s disease (PD) by genetic, neuropathologic and biochemical evidence. Aggregated and fibrillar forms of α-syn are the main components of intracellular protein inclusions found in PD patients’ brains, termed Lewy Bodies (LB). Both in animal models and in vitro, α-syn forms fibrillar aggregates that resemble those observed in PD brain tissues. Although disease-associated mutations have been shown to promote the fibrillization of α-syn, the exact mechanisms responsible for triggering α-syn aggregation and toxicity in sporadic PD remain unknown. Addressing this gap of knowledge is crucial for understanding the molecular basis of the disease and developing effective therapies for the treatment of PD and other synucleinopathies. This project was initiated on the basis of the working hypothesis that post-translational modifications (PTM) may play important roles in modulating α-syn function and/or regulating its aggregation and toxicity. More specifically, α-syn is ubiquitously N-terminally acetylated, and phosphorylated (serines 87 and S129), ubiquitinated (lysines 12, 21 and 23) and truncated forms of α-syn have been observed in association with wild-type α-syn in LB and in brain tissues from PD patients and transgenic animals. Other modifications, such as phosphorylation at tyrosine 125 (Y125), were significantly reduced in diseased brains. Despite the discovery of candidate enzymes that mediate α-syn phosphorylation, ubiquitination and truncation, little is known about how each of these modifications alters α-syn structure, function, aggregation and toxicity in vivo. This is primarily due to the lack of tools that allow site-specific introduction of these modifications and the lack of natural mutations that can mimic the effect of these modifications, including phosphorylation. The primary focus of this thesis was to develop strategies to overcome these limitations so as to elucidate the effect of phosphorylation at Y125 and monoubiquitination at lysine 6 (K6) on α-syn structure, fibril formation, membrane binding and subcellular localization. Towards this goal, we developed two semisynthetic strategies that allow site-specific introduction of PTM in α-syn based on the ligation of synthetic peptides containing the desired modified amino acid with recombinantly expressed proteins using Expressed Protein Ligation. This approach enables the introduction of single or multiple PTM in the N- or C-terminal regions of α-syn, and the preparation of modified α-syn in milligram quantities. Using these approaches, we were able to show for the first time that ubiquitination stabilizes the monomeric form of the protein and inhibits, rather than promotes, α-syn aggregation, while phosphorylation at Y125 does not significantly change the structure and aggregation propensity of α-syn. With the semisynthetic pY125 α-syn in our hands, we were also able to investigate for the first time the sub-cellular localization of pY125 α-syn through its microinjection into primary neurons. This was not previously possible due to technical limitations related to the absence of appropriate antibodies against pY125 α-syn for immunocytochemical studies and difficulties in generating sitespecifically modified α-syn. Furthermore, we were able to investigate the effect of α-syn phosphorylation on its interactions with other proteins, by probing the effect of pS129 and pY125 on the binding to a nanobody that was specifically selected by phage-display to tightly bind to the C-terminal domain of α-syn. Accordingly, we demonstrated that phosphorylation at a single residue is capable of disrupting the binding of full-length α-syn to another protein. These results have wide-ranging implications for the potential role of phosphorylation and other PTM in regulating α-syn’s function(s). We also exploited our ability to combine semisynthetic and enzymatic approaches to investigate potential cross-talk between different PTM, namely phosphorylation at Y125 and S129 and monoubiquitination at K6. These advances would eventually allow investigating the effect of cross-talk between other N and C-terminal modifications on α-syn’s properties and to investigate PTM-dependent protein-protein and protein-ligand interactions in vitro and in cellular models of synucleinopathies. Together, our semisynthetic approaches provide novel means for the introduction of site-specific modifications in the N- and C-termini of α-syn. Current efforts in our laboratory are focused on extending these approaches to investigate the dynamics of PTM through the use of photocaged modified amino acids and to prepare novel fluorescently labeled α-syn variants to investigate its folding at the single-molecule level in living cells. The results presented in this thesis and preliminary studies from our group demonstrate that semisynthetic α-syn can provide unique opportunities to investigate structure-function of α-syn and the role of PTM in the biology of α-syn in health and disease.

EPFL2012

New Insights Into Amyloid Formation and Structure by Innovative Atomic Force Microscopy Methods

Francesco Simone Ruggeri

Today, more than 40 million people worldwide are affected by neurodegenerative disorders. Onset of these diseases is associated with insoluble fibrillar protein aggregates, termed amyloids. The molecular origin and the link between amyloid formation and disease aetiology are unclear and there are not are available therapies for these disorders. Strong evidence links propensity of proteins to misfolding and aggregation to the pathological biology implicated in the onset of these diseases. Despite its importance, unraveling amyloids properties and formation is still a formidable experimental challenge, mainly because of their nanoscale dimensions and their heterogeneous and transient nature. Therefore, the investigation of the misfolding of monomers and oligomers into fibrils and their mechanical and structural properties is central to understand their stability and toxicity in the body and to design new therapeutic strategies to the amyloid diseases problem. The main objective of this PhD thesis was the biophysical investigation of amyloids structure and formation at the single scale aggregate. This objective was pursed mainly by the use of both conventional and innovative Atomic Force Microscopy (AFM) methodologies, such as peak force quantitative nanomechanical mapping (PF-QNM) and infrared nanospectroscopy (nanoIR). These methods were assessed to resolve the complex and heterogeneous energy landscape of proteins aggregation and to provide direct information on aggregates properties. Initially, we used AFM to compare the kinetics of aggregation of wild type and mutated forms of huntingtin and a-synuclein. In the first case, we focused on the effects of N-terminal post-translation modifications in the fibrillization. We demonstrated that a phosphorylation of the protein significantly slowed down the aggregation. In the latter case, we investigated wild type and H50Q mutated form of a-synuclein. We demonstrated the strict link between the disease and the mutation, which enhanced aggregation. Successively, we studied the early stages of a-synuclein fibrillization and we showed that the amyloid assembly proceed directly through the formation of single monomeric strands, which hierarchically assembly into the mature fibrils. Moreover, we performed force spectroscopy experiments, which confirmed the non-mature structure of this species and enabled studying their force of interaction with surfaces. Successively, PF-QNM was applied to investigate the mechanical properties of single aggregates forming during fibrillization of amyloid proteins. We demonstrated that Î²-sheet content is a major factor determining their intrinsic stiffness. Finally, nanoIR was applied to investigate at the nanoscale the misfolding process and the structure of the species present during the aggregation. The technique proved to be ideal to characterize individually the amyloid species formed by the Josephin domain of ataxin-3. For the first time, we were able to link their nanomechanical properties and secondary structure at the nanoscale. Innovative AFM-based techniques enabled correlating morphological and ultrastructural properties of amyloids at the single aggregate scale. Thus, they represent a future fruitful avenue to unravel protein misfolding process and the mechanisms of amyloid formation. The comprehension of these fundamental processes could allow the design of pharmacological approaches to contrast the onset of amyloid diseases.