Toward the discovery of effective polycyclic inhibitors of alpha-synuclein amyloid assembly.

The fibrillation of amyloidogenic proteins is a critical step in the etiology of neurodegenerative disorders such as Alzheimer and Parkinson diseases. There is major interest in the therapeutic intervention on such aberrant aggregation phenomena, and the utilization of polyaromatic scaffolds has lately received considerable attention. In this regard, the molecular and structural basis of the anti-amyloidogenicity of polyaromatic compounds, required to evolve this molecular scaffold toward therapeutic drugs, is not known in detail. We present here biophysical and biochemical studies that have enabled us to characterize the interaction of metal-substituted, tetrasulfonated phthalocyanines (PcTS) with α-synuclein (AS), the major protein component of amyloid-like deposits in Parkinson disease. The inhibitory activity of the assayed compounds on AS amyloid fibril formation decreases in the order PcTS[Ni(II)] ~ PcTS > PcTS[Zn(II)] >> PcTS[Al(III)] ≈ 0. Using NMR and electronic absorption spectroscopies we demonstrated conclusively that the differences in binding capacity and anti-amyloid activity of phthalocyanines on AS are attributed to their relative ability to self-stack through π-π interactions, modulated by the nature of the metal ion bound at the molecule. Low order stacked aggregates of phthalocyanines were identified as the active amyloid inhibitory species, whose effects are mediated by residue specific interactions. Such sequence-specific anti-amyloid behavior of self-stacked phthalocyanines contrasts strongly with promiscuous amyloid inhibitors with self-association capabilities that act via nonspecific sequestration of AS molecules. The new findings reported here constitute an important contribution for future drug discovery efforts targeting amyloid formation.

Structural and mechanistic basis behind the inhibitory interaction of PcTS on alpha-synuclein amyloid fibril formation.

The identification of aggregation inhibitors and the investigation of their mechanism of action are fundamental in the quest to mitigate the pathological consequences of amyloid formation. Here, characterization of the structural and mechanistic basis for the antiamyloidogenic effect of phthalocyanine tetrasulfonate (PcTS) on alpha-synuclein (AS) allowed us to demonstrate that specific aromatic interactions are central for ligand-mediated inhibition of amyloid formation. We provide evidence indicating that the mechanism behind the antiamyloidogenic effect of PcTS is correlated with the trapping of prefibrillar AS species during the early stages of the assembly process. By using NMR spectroscopy, we have located the primary binding region for PcTS to a specific site in the N terminus of AS, involving the amino acid Tyr-39 as the anchoring residue. Moreover, the residue-specific structural characterization of the AS-PcTS complex provided the basis for the rational design of nonamyloidogenic species of AS, highlighting the role of aromatic interactions in driving AS amyloid assembly. A comparative analysis with other proteins involved in neurodegenerative disorders reveals that aromatic recognition interfaces might constitute a key structural element to target their aggregation pathways. These findings emphasize the use of aggregation inhibitors as molecular probes to assess structural and toxic mechanisms related to amyloid formation and the potential of small molecules as therapeutics for amyloid-related pathologies.

Phosphorylation at S87 is enhanced in synucleinopathies, inhibits alpha-synuclein oligomerization, and influences synuclein-membrane interactions.

Increasing evidence suggests that phosphorylation may play an important role in the oligomerization, fibrillogenesis, Lewy body (LB) formation, and neurotoxicity of alpha-synuclein (alpha-syn) in Parkinson disease. Herein we demonstrate that alpha-syn is phosphorylated at S87 in vivo and within LBs. The levels of S87-P are increased in brains of transgenic (TG) models of synucleinopathies and human brains from Alzheimer disease (AD), LB disease (LBD), and multiple system atrophy (MSA) patients. Using antibodies against phosphorylated alpha-syn (S129-P and S87-P), a significant amount of immunoreactivity was detected in the membrane in the LBD, MSA, and AD cases but not in normal controls. In brain homogenates from diseased human brains and TG animals, the majority of S87-P alpha-syn was detected in the membrane fractions. A battery of biophysical methods were used to dissect the effect of S87 phosphorylation on the structure, aggregation, and membrane-binding properties of monomeric alpha-syn. These studies demonstrated that phosphorylation at S87 expands the structure of alpha-syn, increases its conformational flexibility, and blocks its fibrillization in vitro. Furthermore, phosphorylation at S87, but not S129, results in significant reduction of alpha-syn binding to membranes. Together, our findings provide novel mechanistic insight into the role of phosphorylation at S87 and S129 in the pathogenesis of synucleinopathies and potential roles of phosphorylation in alpha-syn normal biology.

Site-specific interactions of Cu(II) with alpha and beta-synuclein: bridging the molecular gap between metal binding and aggregation.

The aggregation of alpha-synuclein (AS) is a critical step in the etiology of Parkinson's disease (PD) and other neurodegenerative synucleinopathies. Protein-metal interactions play a critical role in AS aggregation and might represent the link between the pathological processes of protein aggregation and oxidative damage. Our previous studies established a hierarchy in AS-metal ion interactions, where Cu(II) binds specifically to the protein and triggers its aggregation under conditions that might be relevant for the development of PD. In this work, we have addressed unresolved structural details related to the binding specificity of Cu(II) through the design of site-directed and domain-truncated mutants of AS and by the characterization of the metal-binding features of its natural homologue beta-synuclein (BS). The structural properties of the Cu(II) complexes were determined by the combined application of nuclear magnetic resonance, electron paramagnetic resonance, UV-vis, circular dichroism spectroscopy, and matrix-assisted laser desorption ionization mass spectrometry (MALDI MS). Two independent, noninteracting copper-binding sites with significantly different affinities for the metal ion were detected in the N-terminal regions of AS and BS. MALDI MS provided unique evidence for the direct involvement of Met1 as the primary anchoring residue for Cu(II) in both proteins. Comparative spectroscopic analysis of the two proteins allowed us to deconvolute the Cu(II) binding modes and unequivocally assign the higher-affinity site to the N-terminal amino group of Met1 and the lower-affinity site to the imidazol ring of the sole His residue. Through the use of competitive chelators, the affinity of the first equivalent of bound Cu(II) was accurately determined to be in the submicromolar range for both AS and BS. Our results prove that Cu(II) binding in the C-terminal region of synucleins represents a nonspecific, very low affinity process. These new insights into the bioinorganic chemistry of PD are central to an understanding of the role of Cu(II) in the fibrillization process of AS and have implications for the molecular mechanism by which BS might inhibit AS amyloid assembly.

Structural characterization of the intrinsically unfolded protein beta-synuclein, a natural negative regulator of alpha-synuclein aggregation.

The synuclein family of intrinsically unfolded proteins is composed of three highly homologous members, alpha-synuclein (alphaS), beta-synuclein (betaS) and gamma-synuclein (gammaS), which are linked to neurodegenerative disorders and cancer. alphaS has been studied intensively after its identification as the major protein component of amyloid-like deposits in Parkinson's disease and dementia with Lewy bodies. betaS, on the other hand, was found to act as a potent inhibitor of alphaS amyloid formation, and it is proposed as a natural regulator of its neurotoxicity. It is then of particular interest to elucidate the structural and dynamic features of the soluble state of betaS as a first step to understand the molecular basis of its anti-amyloidogenic effect on alphaS. We present here the characterization of natively unstructured betaS by high resolution heteronuclear NMR techniques. A combination of pulse-field gradient, three-dimensional heteronuclear correlation, residual dipolar couplings, paramagnetic relaxation enhancement and backbone relaxation experiments were employed to characterize the ensemble of conformations populated by the protein. The results indicate that betaS adopts extended conformations in its native state, characterized by the lack of the long-range contacts as previously reported for alphaS. Despite the lack of defined secondary structure, we found evidence for transient polyproline II conformations clustered at the C-terminal region. The structuring of the backbone at the C terminus is locally encoded, stabilized by the presence of eight proline residues embedded in a polypeptide stretch rich in hydrophilic and negatively charged amino acids. The structural and functional implications of these findings are analyzed via a thorough comparison with its neurotoxic homolog alphaS.

Phosphorylation at Ser-129 but not the phosphomimics S129E/D inhibits the fibrillation of alpha-synuclein.

alpha-Synuclein (alpha-syn) phosphorylation at serine 129 is characteristic of Parkinson disease (PD) and related alpha-synulceinopathies. However, whether phosphorylation promotes or inhibits alpha-syn aggregation and neurotoxicity in vivo remains unknown. This understanding is critical for elucidating the role of alpha-syn in the pathogenesis of PD and for development of therapeutic strategies for PD. To better understand the structural and molecular consequences of Ser-129 phosphorylation, we compared the biochemical, structural, and membrane binding properties of wild type alpha-syn to those of the phosphorylation mimics (S129E, S129D) as well as of in vitro phosphorylated alpha-syn using a battery of biophysical techniques. Our results demonstrate that phosphorylation at Ser-129 increases the conformational flexibility of alpha-syn and inhibits its fibrillogenesis in vitro but does not perturb its membrane-bound conformation. In addition, we show that the phosphorylation mimics (S129E/D) do not reproduce the effect of phosphorylation on the structural and aggregation properties of alpha-syn in vitro. Our findings have significant implications for current strategies to elucidate the role of phosphorylation in modulating protein structure and function in health and disease and provide novel insight into the underlying mechanisms that govern alpha-syn aggregation and toxicity in PD and related alpha-synulceinopathies.