Efficient Detection of Early Events of α-Synuclein Aggregation Using a Cysteine Specific Hybrid Scaffold

A Zn-dependent structural transition of SOD1 modulates its ability to undergo phase separation

The misfolding and mutation of Cu/Zn superoxide dismutase (SOD1) is commonly associated with amyotrophic lateral sclerosis (ALS). SOD1 can accumulate within stress granules (SGs), a type of membraneless organelle, which is believed to form via liquid-liquid phase separation (LLPS). Using wild-type, metal-deficient, and different ALS disease mutants of SOD1 and computer simulations, we report here that the absence of Zn leads to structural disorder within two loop regions of SOD1, triggering SOD1 LLPS and amyloid formation. The addition of exogenous Zn to either metal-free SOD1 or to the severe ALS mutation I113T leads to the stabilization of the loops and impairs SOD1 LLPS and aggregation. Moreover, partial Zn-mediated inhibition of LLPS was observed for another severe ALS mutant, G85R, which shows perturbed Zn-binding. By contrast, the ALS mutant G37R, which shows reduced Cu-binding, does not undergo LLPS. In addition, SOD1 condensates induced by Zn-depletion exhibit greater cellular toxicity than aggregates formed by prolonged incubation under aggregating conditions. Overall, our work establishes a role for Zn-dependent modulation of SOD1 conformation and LLPS properties that may contribute to amyloid formation.

New strategies for fluorescently labeling proteins in the study of amyloids

Amyloid proteins are widely studied, both for their unusual biophysical properties and their association with disorders such as Alzheimer's and Parkinson's disease. Fluorescence-based methods using site-specifically labeled proteins can provide information on the details of their structural dynamics and their roles in specific biological processes. Here, we describe the application of different labeling methods and novel fluorescent probe strategies to the study of amyloid proteins, both for in vitro biophysical experiments and for in vivo imaging. These labeling tools can be elegantly used to answer important questions on the function and pathology of amyloid proteins.

Deciphering protein microenvironment by using a cysteine specific switch-ON fluorescent probe

Fluorescent probes provide an unparalleled opportunity to visualize and quantify dynamic events. Here, we employ a medium-size, cysteine specific coumarin based switch-ON fluorescent probe 'L' to track protein unfolding profiles and accessibility of cysteine residues in proteins. It was established that 'L' is highly selective and exhibits no artifact due to interaction with other bystander species. 'L' is able to gauge subtle changes in protein microenvironment and proved to be effective in delineating early unfolding events that are difficult to otherwise discern by classic techniques such as circular dichroism. By solving the X-ray structure of TadA and probing the temperature dependent fluorescence-ON response with native TadA and its cysteine mutants, it was revealed that unfolding occurs in a stage-wise manner and the regions that are functionally important form compact sub-domains and unfold at later stages. Our results assert that probe 'L' serves as an efficient tool to monitor subtle changes in protein structure and can be employed as a generic dye to study processes such as protein unfolding.

Fluorescent probes for bioimaging of potential biomarkers in Parkinson's disease

This review comprehensively summarizes various types of fluorescent probes for PD and their applications for detection of various PD biomarkers.

Evaluation of catacholase mimicking activity and apoptosis in human colorectal carcinoma cell line by activating mitochondrial pathway of copper(II) complex coupled with 2-(quinolin-8-yloxy)(methyl)benzonitrile and 8-hydroxyquinoline

To evaluate the cytotoxic potential of metal-based chemotherapeutic candidate towards the colorectal cancer, we have synthesized a new copper(II) complex [Cu(qmbn)(q)(Cl)] (1) (where, qmbn = 2-(quinolin-8-yloxy)(methyl)benzonitrile and q = 8-hydroxyquinoline) and structurally characterized by single crystal X-ray, Powder-XRD, FTIR and thermogravimetric analysis (TGA). The structural analysis reveals that copper(II) ions exist in a distorted square pyramidal (τ = ~0.1), with ligation of a chloride ion, oxygen atom and two nitrogen atoms at equatorial position and one oxygen atom at apical position. The cytotoxicity potential of complex 1 was executed against human colorectal cell lines (HCT116), which showed that 1 induces mitochondrion-mediated apoptotic cell death via activation of the Bax (pro-apoptotic protein) caspases-3 and 9 proteins. Interestingly, complex 1 was found to be a good candidate as electron-transfer catalyst which mimics catacholase with high turnover frequency (k_cat = 1.03 × 10² h^-1) for the conversion of the model substrate 3,5-di-tertbutylcatechol (3,5-DTBC) to 3,5-di-tertbutylquinone (3,5-DTBQ). Furthermore, molecular docking studies revealed that complex 1 was successfully localized inside the binding pocket of protein kinase (Akt), which validate the mechanism and mode of interaction of 1 that displayed cytotoxic activity experimentally. The obtained outcomes reveal that the complex 1 could be utilized as an encouraging perspective in the development of new therapeutic candidate for colon cancer.

A Novel Tool to Investigate the Early and Late Stages of α-Synuclein Aggregation

The accumulation of an inherently disordered protein α-synuclein (α-syn) aggregates in brain tissue play a pivotal role in the pathology and etiology of Parkinson's disease. Aggregation of α-syn has been found to be complex and heterogeneous, occurring through multitudes of early- and late-stage intermediates. Because of the inherent complexity and large dynamic range (between a few microseconds to several days under in vitro measurement conditions), it is difficult for the conventional biophysical and biochemical techniques to sample the entire time window of α-syn aggregation. Here, for the first time, we introduced the Z-scan technique as a novel tool to investigate different conformations formed in the early and late stage of temperature and mechanical stress-induced α-syn aggregation, in which different species showed its characteristic nonlinear characteristics. A power-dependent study was also performed to observe the changes in the protein nonlinearity. The perceived nonlinearity was accredited to the thermal-lensing effect. A switch in the sign of the refractive nonlinearity was observed for the first time as a signature of the late oligomeric conformation, a prime suspect that triggers cell death associated with neurodegeneration. We validate Z-scan results using a combination of different techniques, like thioflavin-T fluorescence assay, fluorescence correlation spectroscopy, Fourier-transform infrared spectroscopy, and atomic force microscopy. We believe that this simple, inexpensive, and sensitive method can have potential future applications in detecting/monitoring conformations in other essential peptides/proteins related to different neurodegenerative and other human diseases.

A Spectroscopic Marker for Structural Transitions Associated with Amyloid-β Aggregation

An amyloid aggregate evolves through a series of intermediates that have different secondary structures and intra- and intermolecular contacts. The structural parameters of these intermediates are important determinants of their toxicity. For example, the early oligomeric species of the amyloid-β (Aβ) peptide have been implicated as the most cytotoxic species in Alzheimer's disease but are difficult to identify because of their dynamic and transitory nature. Conventional aggregation monitors such as the fluorescent dye thioflavin T report on only the overall transition of the soluble species to the final amyloid fibrillar aggregated state. Here, we show that the fluorescent dye bis(triphenylphosphonium) tetraphenylethene (TPE-TPP) identifies at least three distinct aggregation intermediates of Aβ. Some atomic-level features of these intermediates are known from solid state nuclear magnetic resonance spectroscopy. Hence, the TPE-TPP fluorescence data may be interpreted in terms of these Aβ structural transitions. Steady state fluorescence and lifetime characteristics of TPE-TPP distinguish between the small oligomeric species (emission wavelength maximum, λ_max = 465 nm; average fluorescence lifetime, τ_Fl measured at 420 nm = 3.58 ± 0.04 ns), the intermediate species (λ_max = 452 nm; τ_Fl = 3.00 ± 0.03 ns), and the fibrils (λ_max = 406 nm; τ_Fl = 5.19 ± 0.08 ns). Thus, TPE-TPP provides a ready diagnostic for differentiating between the various, including the toxic, Aβ aggregates and potentially can be utilized to screen for amyloid aggregation inhibitors.