A reduced dimensionality NMR pulse sequence and an efficient protocol for unambiguous assignment in intrinsically disordered proteins

SUMO1 hinders α-Synuclein fibrillation by inducing structural compaction

Small Ubiquitin-like Modifier 1 (SUMO1) is an essential protein for many cellular functions, including regulation, signaling, etc., achieved by a process known as SUMOylation, which involves covalent attachment of SUMO1 to target proteins. SUMO1 also regulates the function of several proteins via non-covalent interactions involving the hydrophobic patch in the target protein identified as SUMO Binding or Interacting Motif (SBM/SIM). Here, we demonstrate a crucial functional potential of SUMO1 mediated by its non-covalent interactions with α-Synuclein, a protein responsible for many neurodegenerative diseases called α-Synucleinopathies. SUMO1 hinders the fibrillation of α-Synuclein, an intrinsically disordered protein (IDP) that undergoes a transition to β-structures during the fibrillation process. Using a plethora of biophysical techniques, we show that SUMO1 transiently binds to the N-terminus region of α-Synuclein non-covalently and causes structural compaction, which hinders the self-association process and thereby delays the fibrillation process. On the one hand, this study demonstrates an essential functional role of SUMO1 protein concerning neurodegeneration; it also illustrates the commonly stated mechanism that IDPs carry out multiple functions by structural adaptation to suit specific target proteins, on the other. Residue-level details about the SUMO1-α-Synuclein interaction obtained here also serve as a reliable approach for investigating the detailed mechanisms of IDP functions.

Modulating α-Synuclein Liquid-Liquid Phase Separation

Liquid-liquid phase separation (LLPS) is a crucial phenomenon for the formation of functional membraneless organelles. However, LLPS is also responsible for protein aggregation in various neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's disease, and Parkinson's disease (PD). Recently, several reports, including ours, have shown that α-synuclein (α-Syn) undergoes LLPS and a subsequent liquid-to-solid phase transition, which leads to amyloid fibril formation. However, how the environmental (and experimental) parameters modulate the α-Syn LLPS remains elusive. Here, we show that in vitro α-Syn LLPS is strongly dependent on the presence of salts, which allows charge neutralization at both terminal segments of protein and therefore promotes hydrophobic interactions supportive for LLPS. Using various purification methods and experimental conditions, we showed, depending upon conditions, α-Syn undergoes either spontaneous (instantaneous) or delayed LLPS. Furthermore, we delineate that the kinetics of liquid droplet formation (i.e., the critical concentration and critical time) is relative and can be modulated by the salt/counterion concentration, pH, presence of surface, PD-associated multivalent cations, and N-terminal acetylation, which are all known to regulate α-Syn aggregation in vitro. Together, our observations suggest that α-Syn LLPS and subsequent liquid-to-solid phase transition could be pathological, which can be triggered only under disease-associated conditions (high critical concentration and/or conditions promoting α-Syn self-assembly). This study will significantly improve our understanding of the molecular mechanisms of α-Syn LLPS and the liquid-to-solid transition.

Benzimidazole-based fluorophores for the detection of amyloid fibrils with higher sensitivity than Thioflavin-T

Protein aggregation into amyloid fibrils is a key feature of a multitude of neurodegenerative diseases such as Alzheimer's, Parkinson's, and Prion disease. To detect amyloid fibrils, fluorophores with high sensitivity and better efficiency coupled with the low toxicity are in high demand even to date. In this pursuit, we have unveiled two benzimidazole-based fluorescence sensors ([C₁₅ H₁₅ N₃ ] (C1) and [C₁₆ H₁₆ N₃ O₂ ] (C2), which possess exceptional affinity toward different amyloid fibrils in its submicromolar concentration (8 × 10^-9  M), whereas under a similar concentration, the gold standard Thioflavin-T (ThT) fails to bind with amyloid fibrils. These fluorescent markers bind to α-Syn amyloid fibrils as well as amyloid fibrils forming other proteins/peptides including Aβ42 amyloid fibrils. The ¹ H-¹⁵ N heteronuclear quantum correlation spectroscopy nuclear magnetic resonance data collected on wild-type α-Syn monomer with and without the fluorophores (C1 and C2) reveal that there is weak or no interactions between C1 or C2 with residues in α-Syn monomer, which indirectly reflects the specific binding ability of C1 and C2 to the α-Syn amyloid fibrils. Detailed studies further suggest that C1 and C2 can detect/bind with the α-Syn amyloid fibril as low as 100 × 10^-9  M. Extremely low or no cytotoxicity is observed for C1 and C2 and they do not interfere with α-Syn fibrillation kinetics, unlike ThT. Both C1/C2 not only shows selective binding with amyloid fibrils forming various proteins/peptides but also displays excellent affinity and selectivity toward α-Syn amyloid aggregates in SH-SY5Y cells and Aβ42 amyloid plaques in animal brain tissues. Overall, our data show that the developed dyes could be used for the detection of amyloid fibrils including α-Syn and Aβ42 amyloids with higher sensitivity as compared to currently used ThT.

Triphala inhibits alpha-synuclein fibrillization and their interaction study by NMR provides insights into the self-association of the protein

The process of assembly and accumulation of the intrinsically disordered protein (IDP), alpha-synuclein (αSyn) into amyloid fibrils is a pathogenic process leading to several neurodegenerative disorders such as Parkinson's disease, multiple system atrophy and others. Although several molecules are known to inhibit αSyn fibrillization, the mechanism of inhibition is just beginning to emerge. Here, we report the inhibition of fibrillization of αSyn by Triphala, a herbal preparation in the traditional Indian medical system of Ayurveda. Triphala was found to be a rich source of polyphenols which are known to act as amyloid inhibitors. ThT fluorescence and TEM studies showed that Triphala inhibited the fibrillization of αSyn. However, it was observed that Triphala does not disaggregate preformed αSyn fibrils. Further, native-PAGE showed that Triphala reduces the propensity of αSyn to oligomerize during the lag phase of fibrillization. Our NMR results showed that certain stretches of residues in the N-terminal and NAC regions of αSyn play an anchor role in the self-association process of the protein, thereby providing mechanistic insights into the early events during αSyn fibrillization.

Triphala inhibits αSyn self-association by interacting with anchoring regions which are responsible for αSyn oligomerization.

Comparison of α-Synuclein Fibril Inhibition by Four Different Amyloid Inhibitors

Aggregation of α-synuclein (α-Syn) into toxic oligomers and fibrils leads to Parkinson's disease (PD) pathogenesis. Molecules that can inhibit the fibrillization and oligomerization of α-Syn have potential therapeutic value. Here, we studied four selective amyloid inhibitors: dopamine (Dopa), amphotericin-B (Amph), epigallocatechingallate (EGCG), and quinacrinedihydrochloride (Quin) for their effect on oligomerization, fibrillization, and preformed fibrils of α-Syn. The aggregation kinetics of α-Syn using ThT fluorescence and conformational transition by circular dichroism (CD) in the presence and absence of these four compounds suggest that, except Quin, the remaining three molecules inhibit α-Syn aggregation in a concentration dependent manner. Consistent with the aggregation kinetics data, the morphological study of aggregates formed in the presence of these compounds showed corresponding decrease in fibrillar size. The analysis of cell viability using MTT assay showed reduction in toxicity of α-Syn aggregates formed in the presence of these compounds, which also correlates with reduction of exposed hydrophobic surface as studied by ANS binding. Additionally, these inhibitors, except Quin, demonstrated reduction in size as well as the toxicity of oligomeric/fibrillar aggregates of α-Syn. The residue specific interaction to low molecular weight (LMW) species of α-Syn by 2D NMR study revealed that, the region and extent of binding are different for all these molecules. Furthermore, fibril-binding data using SPR suggested that there is no direct relationship between the binding affinity and fibril inhibition by these compounds. The present study suggests that sequence based interaction of small molecules with soluble α-Syn might dictate their inhibition or modulation capacity, which might be helpful in designing modulators of α-Syn aggregation.

Perturbation in Long-Range Contacts Modulates the Kinetics of Amyloid Formation in α-Synuclein Familial Mutants

The characteristic cross-β-sheet-rich amyloid fibril formation by intrinsically disordered α-synuclein proteins is one of the pathological hallmarks of Parkinson's disease. Although unstructured in solution, the presence of autoinhibitory long-range contacts in monomeric form prevents protein aggregation. Out of the various factors that affect the rate of amyloid formation, familial mutations play an important role in α-synuclein aggregation. Even though these mutations are believed to form an aggregation-prone intermediate by perturbing these contacts, the correlation between perturbation and rate of fibril formation is not very straightforward. A combination of solution and solid-state NMR in conjunction with other biophysical methods has been used to identify the underlying mechanism behind the anomaly in the rate of aggregation for the novel mutants H50Q (fast aggregating) and G51D (slow aggregating). Perturbation of long-range contacts at the mutation sites and C-termini in all of the six familial mutants of α-synuclein during the diseased condition (acidic pH) was observed. These contacts get rearranged at physiological pH resulting in the shielding of mutation sites. Additional contacts at the mutation site in a slow aggregating mutant could be the reason for slower aggregation. Indeed, these contacts provide more rigidity to the monomeric G51D. Nonetheless, these mutations did not alter the overall secondary structure. The differential pattern of the long-range contacts at the monomeric level resulted in the perturbation of the fibrillar-core region, which was evident in the solid-state NMR spectra. Our results provide valuable insights in understanding the effect of long-range contacts on the aggregation of α-synuclein and its mutants.

A Set of Efficient nD NMR Protocols for Resonance Assignments of Intrinsically Disordered Proteins

The RF pulse scheme RN[N-CA HEHAHA]NH, which provides a convenient approach to the acquisition of different multidimensional chemical shift correlation NMR spectra leading to backbone resonance assignments, including those of the proline residues of intrinsically disordered proteins (IDPs), is experimentally demonstrated. Depending on the type of correlation data required, the method involves the generation of in-phase ((15) N)(x) magnetisation via different magnetisation transfer pathways such as H→N→CO→N, HA→CA→CO→N, H→N→CA→N and H→CA→N, the subsequent application of (15) N-(13) C(α) heteronuclear Hartmann-Hahn mixing over a period of ≈100 ms, chemical-shift labelling of relevant nuclei before and after the heteronuclear mixing step and amide proton detection in the acquisition dimension. It makes use of the favourable relaxation properties of IDPs and the presence of (1) JCαN and (2) JCαN couplings to achieve efficient correlation of the backbone resonances of each amino acid residue "i" with the backbone amide resonances of residues "i-1" and "i+1". It can be implemented in a straightforward way through simple modifications of the RF pulse schemes commonly employed in protein NMR studies. The efficacy of the approach is demonstrated using a uniformly ((15) N,(13) C) labelled sample of α-synuclein. The different possibilities for obtaining the amino-acid-type information, simultaneously with the connectivity data between the backbone resonances of sequentially neighbouring residues, have also been outlined.

Six- and seven-dimensional experiments by combination of sparse random sampling and projection spectroscopy dedicated for backbone resonance assignment of intrinsically disordered proteins

Two novel six- and seven-dimensional NMR experiments are proposed. The new experiments employ non-uniform sampling that enables achieving high resolution in four indirectly detected dimensions and synchronous sampling in the additional dimensions using projection spectroscopy principle. The resulted data sets could be processed as five-dimensional data using existing software. The experiments facilitate resonance assignment of intrinsically disordered proteins. The novel experiments were successfully tested using 1 mM sample of α-synuclein on 600 and 800 MHz NMR spectrometers equipped with standard room temperature probes. The experiments allowed backbone assignment from a 1-day acquisition.

HN-NCA heteronuclear TOCSY-NH experiment for (1)H(N) and (15)N sequential correlations in ((13)C, (15)N) labelled intrinsically disordered proteins

A simple triple resonance NMR experiment that leads to the correlation of the backbone amide resonances of each amino acid residue 'i' with that of residues 'i-1' and 'i+1' in ((13)C, (15)N) labelled intrinsically disordered proteins (IDPs) is presented. The experimental scheme, {HN-NCA heteronuclear TOCSY-NH}, exploits the favourable relaxation properties of IDPs and the presence of (1) J CαN and (2) J CαN couplings to transfer the (15)N x magnetisation from amino acid residue 'i' to adjacent residues via the application of a band-selective (15)N-(13)C(α) heteronuclear cross-polarisation sequence of ~100 ms duration. Employing non-uniform sampling in the indirect dimensions, the efficacy of the approach has been demonstrated by the acquisition of 3D HNN chemical shift correlation spectra of α-synuclein. The experimental performance of the RF pulse sequence has been compared with that of the conventional INEPT-based HN(CA)NH pulse scheme. As the availability of data from both the HCCNH and HNN experiments will make it possible to use the information extracted from one experiment to simplify the analysis of the data of the other and lead to a robust approach for unambiguous backbone and side-chain resonance assignments, a time-saving strategy for the simultaneous collection of HCCNH and HNN data is also described.

(13)C-detected NMR experiments for automatic resonance assignment of IDPs and multiple-fixing SMFT processing

Intrinsically disordered proteins (IDPs) have recently attracted much interest, due to their role in many biological processes, including signaling and regulation mechanisms. High-dimensional (13)C direct-detected NMR experiments have proven exceptionally useful in case of IDPs, providing spectra with superior peak dispersion. Here, two such novel experiments recorded with non-uniform sampling are introduced, these are 5D HabCabCO(CA)NCO and 5D HNCO(CA)NCO. Together with the 4D (HACA)CON(CA)NCO, an extension of the previously published 3D experiments (Pantoja-Uceda and Santoro in J Biomol NMR 59:43-50, 2014. doi: 10.1007/s10858-014-9827-1), they form a set allowing for complete and reliable resonance assignment of difficult IDPs. The processing is performed with sparse multidimensional Fourier transform based on the concept of restricting (fixing) some of spectral dimensions to a priori known resonance frequencies. In our study, a multiple-fixing method was developed, that allows easy access to spectral data. The experiments were tested on a resolution-demanding alpha-synuclein sample. Due to superior peak dispersion in high-dimensional spectrum and availability of the sequential connectivities between four consecutive residues, the overwhelming majority of resonances could be assigned automatically using the TSAR program.