Role of conformational dynamics in pathogenic protein aggregation

Preferential Binding of Cations Modulates Electrostatically Driven Protein Aggregation and Disaggregation

Protein aggregation resulting in either ordered amyloids or amorphous aggregates is not only restricted to deadly human diseases but also associated with biotechnological challenges encountered in the therapeutic and food industries. Elucidating the key structural determinants of protein aggregation is important to devise targeted inhibitory strategies, but it still remains a formidable task owing to the underlying hierarchy, stochasticity, and complexity associated with the self-assembly processes. Additionally, alterations in solution pH, salt types, and ionic strength modulate various noncovalent interactions, thus affecting the protein aggregation propensity and the aggregation kinetics. However, the molecular origin and a detailed understanding of the effects of weakly and strongly hydrated salts on protein aggregation and their plausible roles in the dissolution of aggregates remain elusive. In this study, using fluorescence and circular dichroism spectroscopy in combination with electron microscopy and light scattering techniques, we show that the ionic size, valency, and extent of hydration of cations play a crucial role in regulating the protein aggregation and disaggregation processes, which may elicit unique methods for governing the balance between protein self-assembly and disassembly.

Selection of DNA aptamers that prevent the fibrillization of α-synuclein protein in cellular and mouse models

A neuropathological hallmark of Parkinson's disease (PD) is the aggregation and spreading of misfolded α-synuclein (αSyn) protein. In this study, a selection method was developed to identify aptamers that showed affinity for monomeric αSyn and inhibition of αSyn aggregation. Aptamer a-syn-1 exhibited strong inhibition of αSyn aggregation in vitro by transmission electron microscopy and Thioflavin T fluorescence. A-syn-1-treated SH-SY5Y cells incubated with pre-formed fibrils (PFFs) showed less intracellular aggregation of αSyn in comparison with a scrambled oligonucleotide control, as observed with fluorescent microscopy. Systemic delivery of a-syn-1 to the brain was achieved using a liposome vehicle and confirmed with fluorescence microscopy and qPCR. Transgenic mice overexpressing the human A53T variant of αSyn protein were injected with a-syn-1 loaded liposomes at 5 months of age both acutely (single intraperitoneal [i.p.] injection) and repeatedly (5 i.p. injections over 5 days). Western blot protein quantification revealed that both acute and repeated injections of a-syn-1 decreased levels of the aggregated form of αSyn in the transgenic mice in the prefrontal cortex, caudate, and substania nigra (SNc). These results provide in vitro and in vivo evidence that a-syn-1 can inhibit pathological αSyn aggregation and may have implications in treatment strategies to target dysregulation in PD.

Modulation of Primary and Secondary Processes in Tau Fibril Formation by Salt-Induced Dynamics

The initial stages of amyloid fibrilization begin with the monomers populating aggregation-prone conformers. Characterization of such aggregation-prone conformers is crucial in the study of neurodegenerative diseases. The current study characterizes the aggregation pathway of two tau protein constructs that have been recently demonstrated to form Alzheimer's (AD) fibril structures with divalent ions and chronic traumatic encephalopathy (CTE) fibril structures with monovalent ions. The results highlight the involvement of identical residues in both the primary and secondary processes of both AD and CTE fibril propagation. Nuclear magnetic resonance relaxation experiments reveal increased flexibility of the motifs 321KCGS within R3 and 364PGGGN within R4 in the presence of MgCl2/NaCl, correlating with faster aggregation kinetics and indicating efficient primary nucleation. Notably, the seeded aggregation kinetics of the tau monomers in the presence and absence of metal ions are strikingly different. This correlates with the overall sign of the 15N-ΔR2 profile specifying the dominant mechanism involved in the process of aggregation.

Flanking regions, amyloid cores, and polymorphism: the potential interplay underlying structural diversity

The β-sheet-rich amyloid core is the defining feature of protein aggregates associated with neurodegenerative disorders. Recent investigations have revealed that there exist multiple examples of the same protein, with the same sequence, forming a variety of amyloid cores with distinct structural characteristics. These structural variants, termed as polymorphs, are hypothesized to influence the pathological profile and the progression of different neurodegenerative diseases, giving rise to unique phenotypic differences. Thus, identifying the origin and properties of these structural variants remain a focus of studies, as a preliminary step in the development of therapeutic strategies. Here, we review the potential role of the flanking regions of amyloid cores in inducing polymorphism. These regions, adjacent to the amyloid cores, show a preponderance for being structurally disordered, imbuing them with functional promiscuity. The dynamic nature of the flanking regions can then manifest in the form of conformational polymorphism of the aggregates. We take a closer look at the sequences flanking the amyloid cores, followed by a review of the polymorphic aggregates of the well-characterized proteins amyloid-β, α-synuclein, Tau, and TDP-43. We also consider different factors that can potentially influence aggregate structure and how these regions can be viewed as novel targets for therapeutic strategies by utilizing their unique structural properties.

Hydroxytyrosol-Donepezil Hybrids Play a Protective Role in an In Vitro Induced Alzheimer's Disease Model and in Neuronal Differentiated Human SH-SY5Y Neuroblastoma Cells

Alzheimer's disease (AD) is the most common neurodegenerative pathology among progressive dementias, and it is characterized by the accumulation in the brain of extracellular aggregates of beta-amyloid proteins and neurofibrillary intracellular tangles consisting of τ-hyperphosphorylated proteins. Under normal conditions, beta-amyloid peptides exert important trophic and antioxidant roles, while their massive presence leads to a cascade of events culminating in the onset of AD. The fibrils of beta-amyloid proteins are formed by the process of fibrillogenesis that, starting from individual monomers of beta-amyloid, can generate polymers of this protein, constituting the hypothesis of the "amyloid cascade". To date, due to the lack of pharmacological treatment for AD without toxic side effects, chemical research is directed towards the realization of hybrid compounds that can act as an adjuvant in the treatment of this neurodegenerative pathology. The hybrid compounds used in this work include moieties of a hydroxytyrosol, a nitrohydroxytyrosol, a tyrosol, and a homovanillyl alcohol bound to the N-benzylpiperidine moiety of donepezil, the main drug used in AD. Previous experiments have shown different properties of these hybrids, including low toxicity and antioxidant and chelating activities. The purpose of this work was to test the effects of hybrid compounds mixed with Aβ1-40 to induce fibrillogenesis and mimic AD pathogenesis. This condition has been studied both in test tubes and by an in vitro model of neuronal differentiated human SH-SY5Y neuroblastoma cells. The results obtained from test tube experiments showed that some hybrids inhibit the activity of the enzymes AChE, BuChE, and BACE-1. Cell experiments suggested that hybrids could inhibit fibrillogenesis, negatively modulating caspase-3. They were also shown to exert antioxidant effects, and the acetylated hybrids were found to be more functional and efficient than nonacetylated forms.