Lysophospholipids induce fibrillation of the repeat domain of Pmel17 through intermediate core-shell structures

Tau Fibrillation Induced by Heparin or a Lysophospholipid Show Different Initial Oligomer Formation

The protein tau is involved in several neurogenerative diseases such as Alzheimer's Disease, where tau content and fibrillation have been linked to disease progression. Tau colocalizes with phospholipids and glycosaminoglycans in vivo. We investigated if and how tau fibrillation can be induced by two lysophospholipids, namely the zwitterionic 1-myristoyl-2-hydroxy-sn-glycero-3-phosphocholine (LPC) and the anionic 1-myristoyl-2-hydroxy-sn-glycero-3-phospho-(1'-rac-glycerol) (LPG) as well as the glycosaminoglycan heparin. We used a range of biophysical techniques including small-angle X-ray scattering, Thioflavin T fluorescence, and SDS-PAGE, collecting data at various time points to obtain structural information on each phase of the fibrillation. We find that LPC does not induce fibrillation or interact with tau. Low concentrations of LPG induce fibrillation by formation of small hydrophobic clusters with monomeric tau. At higher LPG concentrations, a core-shell complex is formed where tau wraps around LPG micelles with regions extending away from the micelles. For heparin, loosely associated oligomers are formed rapidly with around ten tau molecules. Fibrils formed with either LPG or heparin show similar final cross-section dimensions. Furthermore, SDS-resistant oligomers are observed for both LPG and heparin. Our study demonstrates that tau fibrillation can be induced by two different biologically relevant cofactors leading to structurally different initial states but similar cross-sectional dimensions for the fibrils. Structural information about initial states prior to fibril formation is important both to gain a better understanding of the onset of fibrillation in vivo, and for the development of targeted drugs that can reduce or abolish tau fibrillation.

Molecular insight into the modulation of ovalbumin fibrillation by allura red dye at acidic pH

The synthetic food additive dye induces amyloid fibrillation has many implications in the laboratory and industries. The effect of Allura red (AR), on the fibrillation of ovalbumin (Ova) at pH 2.0 was investigated. The influence of salt and pH was also seen on AR-induced Ova aggregation. We have used several spectroscopic and microscopy techniques to characterize the changes. The turbidity data suggest that concentrations above 0.05 mM of AR induce aggregation, and the size of aggregates increased in response to AR concentration. The kinetics data showed that the AR induces Ova aggregation quickly without lag time. The aggregates induced by AR have amyloid-like aggregates confirmed by far-UV CD and TEM. NaCl has very marginal effects in AR-induced aggregation. The turbidity results clearly state that Ova is not forming aggregates with pH above 4.0 due to electrostatic repulsion. However, Ova forms bigger aggregates in the presence of 0.5 mM AR at a pH below 4.0. These spectroscopic data suggest that the amyloid fibrillation that occurs in Ova is due to electrostatic and hydrophobic interaction. The amyloid fibrillation induced by AR dye in protein should be taken seriously for food safety purposes.

Monitoring Kinetics of pH-Dependent Aggregation and Disaggregation of the Pmel17 Repeat Domain

The premelanosomal protein (Pmel17) is a human functional amyloid that promotes pigmentation by serving as a scaffold for melanin polymerization. This occurs within the melanosome, where Pmel17 is first proteolyzed into smaller domain(s) that are responsible for fibril formation. Our work has shown that the Pmel17 repeat domain (RPT, residues 315-444) forms amyloid fibrils in vitro under acidic conditions similar to those found in melanosomes. Mechanistically, this is driven by the protonation of acidic residues, resulting in charge neutralization and subsequent aggregation. Interestingly, the deprotonation of acidic residues leads to rapid disaggregation, highlighting a reversible mechanism of fibril formation and dissolution thus far only observed for functional amyloid proteins. In this chapter, we describe how to monitor pH-dependent RPT aggregation and disaggregation using extrinsic thioflavin-T and intrinsic tryptophan fluorescence, respectively. These methods can also be adapted more broadly to investigate the reversibility of other amyloid systems, both functional and pathogenic.

Functional amyloid in a lipid-like environment: a merry dance of many steps

Functional amyloid, which unlike its pathological counterpart serves a biological purpose, is produced in a carefully orchestrated sequence of events. In bacteria, the major amyloid component is transported over the periplasm and through the outer membrane to assemble on the bacterial cell surface. During its life time, the amyloid protein may be exposed to both membrane lipids and extracellular surfactant, making it relevant to study its interactions with these components in vitro. Particularly for charged surfactants, the interaction is quite complex and highly dependent on the surfactant:protein molar ratio. Low ratios typically promote aggregation, likely by binding the proteins to micelles and thus increasing the local concentration of proteins, while higher concentrations see an inhibition of the same process as the protein is diluted out and immobilized on individual micelles. This is particularly pronounced for strongly anionic surfactants like SDS; the naturally occurring biosurfactant rhamnolipid interacts more weakly with the protein, which still not only allows aggregation but also leads to less detrimental effects at higher ratios. Similarly, anionic vesicle-forming lipids largely stimulate aggregation likely because of weaker interactions. Anionic lysolipids, thanks to their micelle-forming properties, resemble SDS in their impact on fibrillation. There are also examples of systems where membrane binding sequesters an otherwise amyloidogenic sequence and prevents fibrillation or-quite the opposite- liberates another part of the protein to engage in self-assembly. Thus, membranes and surfactants have very varied roles to play in the biogenesis and function of bacterial amyloid.

Induction, inhibition, and incorporation: Different roles for anionic and zwitterionic lysolipids in the fibrillation of the functional amyloid FapC

Amyloid proteins are widespread in nature both as pathological species involved in several diseases and as functional entities that can provide protection and storage for the organism. Lipids have been found in amyloid deposits from various amyloid diseases and have been shown to strongly affect the formation and structure of both pathological and functional amyloid proteins. Here, we investigate how fibrillation of the functional amyloid FapC from Pseudomonas is affected by two lysolipids, the zwitterionic lipid 1-myristoyl-2-hydroxy-sn-glycero-3-phosphocholine and the anionic lipid 1-myristoyl-2-hydroxy-sn-glycero-3-phospho-(1′-rac-glycerol) (LPG). Small-angle X-ray scattering, circular dichroism, dynamic light scattering, and thioflavin T fluorescence measurements were performed simultaneously on the same sample to ensure reproducibility and allow a multimethod integrated analysis. We found that LPG strongly induces fibrillation around its critical micelle concentration (cmc) by promoting formation of large structures, which mature via accumulation of intermediate fibril structures with a large cross section. At concentrations above its cmc, LPG strongly inhibits fibrillation by locking FapC in a core–shell complex. In contrast, lipid 1-myristoyl-2-hydroxy-sn-glycero-3-phosphocholine induces fibrillation at concentrations above its cmc, not via strong interactions with FapC but by being incorporated during fibrillation and likely stabilizing the fibrillation nucleus to reduce the lag phase. Finally, we show that LPG is not incorporated into the fibril during assembly but rather can coat the final fibril. We conclude that lipids affect both the mechanism and outcome of fibrillation of functional amyloid, highlighting a role for lipid concentration and composition in the onset and mechanism of fibrillation in vivo.

Interaction of membrane vesicles with the Pseudomonas functional amyloid protein FapC facilitates amyloid formation

Functional amyloids (FA) are proteins which are evolutionarily optimized to form highly stable fibrillar structures that strengthen the bacterial biofilm matrix. FA such as CsgA (E. coli) and FapC (Pseudomonas) are secreted to the bacterial surface where they integrate into growing fibril structures projecting from the outer membrane. FA are exposed to membrane surfaces in this process, but it remains unclear how membranes can interact with FA and potentially affect the self-assembly. Here we report the effect of different vesicles (DOPG, DMPG, DOPS, DOPC and DMPC) on the kinetics and structural endpoints of FapC fibrillation using various biophysical techniques. Particularly anionic lipids such as DMPG trigger FapC fibrillation, and the protein's second repeat sequence (R2) appears to be important for this interaction. Vesicles formed from phospholipids extracted from three different Pseudomonas strains (Δfap, ΔFapC and pfap) induce FapC fibrillation by accelerating nucleation. The general aggregation inhibitor epigallocatechin gallate (EGCG) inhibits FapC fibrillation by blocking interactions between FapC and vesicles and redirecting FapC monomers to oligomer structures. Our work indicates that biological membranes can contribute significantly to the fibrillation of functional amyloid.

Human Fibrinogen Inhibits Amyloid Assembly of Most Phenol-Soluble Modulins from Staphylococcus aureus

Functional amyloids are highly organized protein/peptide structures that inter alia promote biofilm formation in different bacteria. One such example is provided by a family of 20-45 residue-long peptides called phenol-soluble modulins (PSMs) from Staphylococcus aureus. External components such as eukaryotic host proteins, which alter self-assembly of bacterial amyloids, can affect the biofilm matrix. Here, we studied the effect of the highly prevalent human plasma protein fibrinogen (Fg) on fibrillation of PSMs. Fg inhibits or suppresses fibrillation of most PSMs tested (PSMα1, PSMβ1, and PSMβ2) except for PSMα3, whose already rapid aggregation is accelerated even further by Fg but leads to amorphous β-rich aggregates rather than fibrils. Fg also induces PSMβ2 to form amorphous aggregates and diverts PSMα1 into off-pathway oligomers which consist of both Fg and PSMα1 and cannot seed fibrillation. Peptide arrays showed that Fg bound to the N-terminus of PSMα1, while it bound to the entire length of PSMα3 (except the C terminus) and to the C-termini of PSMβ1 and PSMβ2. The latter peptides are all positively charged, while Fg is negatively charged at physiological pH. The positive charges complement Fg's net negative charge of -7.6 at pH 7.4. Fg's ability to inhibit PSM fibrillation reveals a potential host-defense mechanism to prevent bacterial biofilm growth and infections in the human body.

Purification and characterization of an amyloidogenic repeat domain from the functional amyloid Pmel17

The pre-melanosomal protein (Pmel17) is a human functional amyloid that supports melanin biosynthesis within melanocytes. This occurs in the melanosome, a membrane-bound organelle with an acidic intraluminal pH. The repeat region of Pmel17 (RPT, residues 315-444) has been previously shown to form amyloid aggregates under acidic melanosomal conditions, but not under neutral cytosolic conditions, when expressed and purified using a C-terminal hexa-histidine tag (RPT-His). Given the importance of protonation states in RPT-His aggregation, we questioned whether the histidine tag influenced the pH-dependent behavior. In this report, we generated a tagless RPT by inserting a tobacco etch virus (TEV) protease recognition sequence (ENLYGQ(G/S)) immediately upstream of a native glycine residue at position 312 in Pmel17. After purification of the fusion construct using a histidine tag, cleavage with TEV protease generated a fully native RPT (nRPT) spanning resides 312-444. We characterized the aggregation of nRPT, which formed amyloid fibrils under acidic conditions (pH ≤ 6) but not at neutral pH. Characterizing the morphologies of nRPT aggregates using transmission electron microscopy revealed a pH-dependent maturation from short, curved structures at pH 4 to paired, rod-like fibrils at pH 6. This was accompanied by a secondary structural transition from mixed random coil/β-sheet at pH 4 to canonical β-sheet at pH 6. We also show that pre-formed nRPT fibrils undergo disaggregation upon dilution into pH 7 buffer. More broadly, this strategy can be utilized to generate native amyloidogenic domains from larger proteins by utilizing intrinsic N-terminal glycine or serine residues.

Linking Parkinson's Disease and Melanoma: Interplay Between α-Synuclein and Pmel17 Amyloid Formation

Parkinson's disease (PD) is a neurodegenerative disorder associated with the death of dopaminergic neurons within the substantia nigra of the brain. Melanoma is a cancer of melanocytes, pigmented cells that give rise to skin tone, hair, and eye color. Although these two diseases fundamentally differ, with PD leading to cell degeneration and melanoma leading to cell proliferation, epidemiological evidence has revealed a reciprocal relationship where patients with PD are more susceptible to melanoma and patients with melanoma are more susceptible to PD. The hallmark pathology observed in PD brains is intracellular inclusions, of which the primary component is proteinaceous α-synuclein (α-syn) amyloid fibrils. α-Syn also has been detected in cultured melanoma cells and tissues derived from patients with melanoma, where an inverse correlation exists between α-syn expression and pigmentation. Although this has led to the prevailing hypothesis that α-syn inhibits enzymes involved in melanin biosynthesis, we recently reported an alternative hypothesis in which α-syn interacts with and modulates the aggregation of Pmel17, a functional amyloid that serves as a scaffold for melanin biosynthesis. In this perspective, we review the literature describing the epidemiological and molecular connections between PD and melanoma, presenting both the prevailing hypothesis and our amyloid-centric hypothesis. We offer our views of the essential questions that remain unanswered to motivate future investigations. Understanding the behavior of α-syn in melanoma could not only provide novel approaches for treating melanoma but also could reveal insights into the role of α-syn in PD. © 2021 International Parkinson and Movement Disorder Society.

Melanins as Sustainable Resources for Advanced Biotechnological Applications

Melanins are a class of biopolymers that are widespread in nature and have diverse origins, chemical compositions, and functions. Their chemical, electrical, optical, and paramagnetic properties offer opportunities for applications in materials science, particularly for medical and technical uses. This review focuses on the application of analytical techniques to study melanins in multidisciplinary contexts with a view to their use as sustainable resources for advanced biotechnological applications, and how these may facilitate the achievement of the United Nations Sustainable Development Goals.

A high-flux automated laboratory small-angle X-ray scattering instrument optimized for solution scattering

A commercially available small-angle X-ray scattering (SAXS) NanoSTAR instrument (Bruker AXS) with a liquid-metal-jet source (Excillum) has been optimized for solution scattering and installed at iNANO at Aarhus University. The instrument (named HyperSAXS) employs long high-quality parabolic Montel multilayer optics (Incoatec) and a novel compact scatterless pinhole slit with Ge edges, which was designed and built at Aarhus University. The combination of the powerful source and optimized geometry gives an integrated X-ray intensity close to 109 photons s−1 for a standard range of scattering vector moduli q = 0.0098–0.425 Å−1, where q = (4πsinθ)/λ and λ is the Ga Kα wavelength of 1.34 Å. The high intensity of the instrument makes it possible to measure dilute samples of, for example, protein or surfactant with concentrations of 1 mg ml−1 in a few minutes. A flow-through cell, built at Aarhus University, in combination with an automated sample handler has been installed on the instrument. The sample handler is based on the commercial Gilson GX-271 injection system (Biolab), which also allows samples to be stored under thermostatted conditions. The sample handler inserts and removes samples, and also cleans and dries the sample cell between measurements. The minimum volume of the flow-through capillary is about 20 µl. The high intensity additionally allows time-resolved measurements to be performed with a temporal resolution of seconds. For this purpose a stopped-flow apparatus, (SFM-3000, Bio-Logic) was connected to the flow-through cell by high-performance liquid chromatography tubing. This configuration was chosen as it allows vacuum around the sample cell and thus maintains a low background. The instrument can readily be converted into a low-q setup with a q range of 0.0049–0.34 Å−1 and an X-ray intensity of about 5 × 107 photons s−1.

Modulating functional amyloid formation via alternative splicing of the premelanosomal protein PMEL17

The premelanosomal protein (PMEL17) forms functional amyloid fibrils involved in melanin biosynthesis. Multiple PMEL17 isoforms are produced, two of which arise from excision of a cryptic intron within the amyloid-forming repeat (RPT) domain, leading to long (lRPT) and short (sRPT) isoforms with 10 and 7 imperfect repeats, respectively. Both lRPT and sRPT isoforms undergo similar pH-dependent mechanisms of amyloid formation and fibril dissolution. Here, using human PMEL17, we tested the hypothesis that the minor, but more aggregation-prone, sRPT facilitates amyloid formation of lRPT. We observed that cross-seeding by sRPT fibrils accelerates the rate of lRPT aggregation, resulting in propagation of an sRPT-like twisted fibril morphology, unlike the rodlike structure that lRPT normally adopts. This templating was specific, as the reversed reaction inhibited sRPT fibril formation. Despite displaying ultrastructural differences, self- and cross-seeded lRPT fibrils had a similar β-sheet structured core, revealed by Raman spectroscopy, limited-proteolysis, and fibril disaggregation experiments, suggesting the fibril twist is modulated by N-terminal residues outside the amyloid core. Interestingly, bioinformatics analysis of PMEL17 homologs from other mammals uncovered that long and short RPT isoforms are conserved among members of this phylogenetic group. Collectively, our results indicate that the short isoform of RPT serves as a "nucleator" of PMEL17 functional amyloid formation, mirroring how bacterial functional amyloids assemble during biofilm formation. Whereas bacteria regulate amyloid assembly by using individual genes within the same operon, we propose that the modulation of functional amyloid formation in higher organisms can be accomplished through alternative splicing.

Bacterial amphiphiles as amyloid inducers: Effect of Rhamnolipid and Lipopolysaccharide on FapC fibrillation

Pseudomonas species export the amyloid-forming protein FapC to strengthen bacterial biofilm. P. species also produce the biosurfactant rhamnolipid (Rhl) and its outer membrane contains lipopolysaccharide (LPS). Given the possible contacts between FapC, Rhl and LPS, we here investigate how Rhl and LPS affect FapC fibrillation compared with SDS, known to promote fibrillation of proteins at sub-micellar concentrations. Micelles of all three surfactants help FapC bypass the nucleation lag phase, leading to rapid fibrillation, which persists even at high concentrations of micelles and incorporates almost all available FapC monomers. Fibrils formed at high micellar concentrations of Rhl and SDS seed fibrillation at low surfactant concentrations while retaining the original fibril structure. FapC interacts strongly with SDS to form a dense network of narrow fibrils. Small angle X-ray scattering (SAXS) analyses reveal that surfactants reduce the population of intermediates in the fibrillation process and detect a fast aggregation step over the first 2-4 h which precedes the main fibrillation monitored by Thioflavin T. An additional SAXS-detected rearrangement of early aggregates occurs after 4-10 h. At high Rhl concentrations, the micelles are decorated with protein fibrils. SDS induces FapC fibrillation so efficiently that epigallocatechin-3-gallate (EGCG) is unable to inhibit this process. However, EGCG stimulates FapC oligomer formation and inhibits fibrillation both on its own and in the presence of Rhl and LPS. This oligomer could be modelled as a compact core with a flexible shell. This suggests that EGCG can override the natural amyloid-stimulatory properties of these biosurfactants and thus target biofilm.