Role of post-translational modifications in modulating the structure, function and toxicity of alpha-synuclein: implications for Parkinson's disease pathogenesis and therapies

A better understanding of the molecular and cellular determinants that influence the pathology of Parkinson's disease (PD) is essential for developing effective diagnostic, preventative and therapeutic strategies to treat this devastating disease. A number of post-translational modifications to alpha-syn are present within the Lewy bodies in the brains of affected patients and transgenic models of PD and related disorders. However, whether disease-associated alpha-syn post-translational modifications promote or inhibit alpha-syn aggregation and neurotoxicity in vivo remains unknown. Herein, we summarize and discuss the major disease-associated post-translational modifications (phosphorylation, truncation and ubiquitination) and present our current understanding of the effect of these modifications on alpha-syn aggregation and toxicity. Elucidating the molecular mechanisms underlying post-translation modifications of alpha-syn and the consequences of such modifications on the biochemical, structural, aggregation and toxic properties of the protein is essential for unravelling the molecular basis of its function(s) in health and disease. Furthermore, the identification of the natural enzymes involved in regulating the post-translational modifications of alpha-synuclein will yield novel and more tractable therapeutic targets to treat PD and related synucleinopathies.

Reversible attachment of Cell-Penetrating Peptides for the efficient delivery of α-synuclein to HeLa cells and primary cortical neurons

Carole Desobry

The discovery of alpha-synuclein (a-syn) as the main component of Lewy Bodies (LBs), the pathological hallmarks of Parkinson's disease (PD), and the identification of mutations in the gene coding for a-syn in familial form of PD have reinforced the central role of a-syn in the etiology of PD. Nevertheless, the molecular mechanisms by which a-syn is involved in neurodegeneration are still not fully understood. Many studies have raised the potential role of post-translational modifications (PTMs) in regulating a-syn physiological properties but also its pathogenicity in PD development. However, a-syn can be modified simultaneously by several PTMs making it extremely challenging to understand which of those PTMs is responsible in regulating the cellular properties of a-syn. Therefore, our laboratory developed a strategy to study the role of individual PTMs using semi-synthesis, enabling the generation of site-specifically modified a-syn. However, in the absence of an efficient system to internalize these semi-synthetic proteins in cells, those proteins have been limited to biophysical and structural studies. Therefore, we proposed to use cell penetrating peptides (CPPs) to deliver site-specifically modified a-syn inside cells. In a first step, we compared CPPs to deliver a-syn inside the cells. Strikingly, only N-terminal CPP fusions were efficient in delivering it to primary cortical neurons. In order to explain the differences, we performed biophysical assays and established that C-terminal CPPs formed a hairpin with the C-terminus of a-syn, preventing penetration of the plasma membrane. N-terminal fusions displayed a more transient interaction that allowed uptake. In addition, we showed that the CPPs were enhancing the aggregation of a-syn. Among the CPPs tested, TAT fused at the N-terminus was one of the most efficient. Therefore, we evaluated its capacity to deliver a-syn in different cellular models, its subcellular localization and its stability in the cells. We started by evaluating the uptake and molecular properties of TAT- a-syn in HeLa cells and in primary cortical neurons. We found that it was rapidly and efficiently internalized and distributed in the cytosol as punctuated structures. Once internalized, TAT-a-syn was cleared from the cells through autophagy and the proteasome without promoting any cellular toxicity. Finally, TAT-a-syn phosphorylated on Serine 129 or fibrillar TAT-a-syn were also successfully delivered to cells thereby showing that our tool could be used to deliver modified a-syn in cells. However, our data suggested that TAT could influence the properties of a-syn. Thus, we aimed at improving our delivery tool by developing two reversible approaches to relieve a-syn from the influence of TAT after its uptake: 1) a photocleavable linker to release a-syn from TAT upon UV irradiation or 2) a disulfide bond which can be cleaved once in the cell. Both approaches were successful in delivering a-syn to neurons. While the photocleavable version did not allow full release, TAT-S-S-a-syn was efficiently reduced and efficient in delivering pS129 a-syn. In summary, our study allowed developing a versatile tool capable of carrying a-syn inside the cells and allowing its study in the absence of constraints originating from the presence of the CPP sequence in fusion with a-syn. We believe that this tool will be useful for the study of a-syn's PTMs, interactions and cross-talk between modifications.

EPFL2014

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

Chemical strategies for controlling protein folding and elucidating the molecular mechanisms of amyloid formation and toxicity

Loay Awad, Bruno Claude Daniel Fauvet, Mirva Hejjaoui, Hilal Lashuel

It has been more than a century since the first evidence linking the process of amyloid formation to the pathogenesis of Alzheimer's disease. During the last three decades in particular, increasing evidence from various sources (pathology, genetics, cell culture studies, biochemistry, and biophysics) continues to point to a central role for the pathogenesis of several incurable neurodegenerative and systemic diseases. This is in part driven by our improved understanding of the molecular mechanisms of protein misfolding and aggregation and the structural properties of the different aggregates in the amyloid pathway and the emergence of new tools and experimental approaches that permit better characterization of amyloid formation in vivo. Despite these advances, detailed mechanistic understanding of protein aggregation and amyloid formation in vitro and in vivo presents several challenges that remain to be addressed and several fundamental questions about the molecular and structural determinants of amyloid formation and toxicity and the mechanisms of amyloid-induced toxicity remain unanswered. To address this knowledge gap and technical challenges, there is a critical need for developing novel tools and experimental approaches that will not only permit the detection and monitoring of molecular events that underlie this process but also allow for the manipulation of these events in a spatial and temporal fashion both in and out of the cell. This review is primarily dedicated in highlighting recent results that illustrate how advances in chemistry and chemical biology have been and can be used to address some of the questions and technical challenges mentioned above. We believe that combining recent advances in the development of new fluorescent probes, imaging tools that enabled the visualization and tracking of molecular events with advances in organic synthesis, and novel approaches for protein synthesis and engineering provide unique opportunities to gain a molecular-level understanding of the process of amyloid formation. We hope that this review will stimulate further research in this area and catalyze increased collaboration at the interface of chemistry and biology to decipher the mechanisms and roles of protein folding, misfolding, and aggregation in health and disease.