Formation of amyloid fibrils by bovine carbonic anhydrase

Investigation of the Differences in Amyloid-Like Fibrils Derived from Wheat Gluten with Varying Structures under Typical Food Processing Conditions

This study investigates the differences in physicochemical properties, structural characteristics, and fibril morphology among three wheat gluten with distinct secondary structure contents (A protein: high α-helices, low β-sheets, low random coils; C protein: low α-helices, high β-sheets, high random coils; B protein: intermediate structure) when amyloid-like fibrils (AFs) are formed under boiling and steaming conditions. Congo red absorption, polarized light microscopy, and X-ray diffraction confirmed the formation of AFs in proteins A, B, and C under boiling and steaming conditions. Thioflavin T fluorescence revealed that C-protein-derived fibrils (CPF) exhibited the highest intensity, indicating the strongest fibril-forming ability. SE-HPLC analysis showed a gradual increase in molecular weight and AFs contents with prolonged heating. Increased heating time led to larger particle sizes, higher β-sheet content, and involvement of aromatic amino acids in β-sheet formation via π-π stacking, promoting fibril growth. These changes were more pronounced under steaming conditions. AFM revealed that under steaming, the C protein formed longer and taller fibril structures than under boiling. This work establishes a theoretical foundation for understanding the growth mechanism of AFs formed by gluten proteins with different structures during food processing.

Acceleration of carbonic anhydrase amyloid aggregation leads to a decrease in the fibrils toxicity

Cells damage by protein aggregates is one of the causes of amyloid diseases. This study aimed to explore the structural features of cytotoxic amyloid fibrils and to find strategies to reduce their damaging effect. Bovine carbonic anhydrase B (BCAB) was chosen for this work due to high toxicity of its amyloid fibrils. The kinetics of amyloid formation, structural features and cytotoxicity of mature fibrils and early globular aggregates formed by wild type protein and six mutant variants have been investigated. The results showed that an increase in residue hydrophobicity accelerates amyloid aggregation, but the formed fibrils have a reduced content of cross-β-structure and are non-toxic to cells. On the contrary, a decrease in hydrophobicity due to the L139A substitution and slowing the initiation of aggregation leads to the formation of highly toxic oligomers during the lag-period. Thus, the data obtained conclude that accelerating amyloid formation can alter the structure of amyloid aggregates and decrease their cytotoxicity.

Development of Antioxidant and Stable Conjugated Linoleic Acid Pickering Emulsion with Protein Nanofibers by Microwave-Assisted Self-Assembly

Whey protein isolate nanofibrils (WPNFs) can be used as a novel stabilizer in the Pickering emulsion system to improve the water solubility, stability and bioavailability of lipophilic bioactive ingredients. In this study, conjugated linoleic acid (CLA) and WPNFs were used to prepare a stable Pickering emulsion. We used a transmission electron microscope, low-temperature scanning electron micrographs and other methods to evaluate the micromorphology, surface hydrophobicity and structural units of the obtained WPNFs. Compared with whey protein isolate/CLA Pickering emulsion, the WPNFs/CLA Pickering emulsion has greater ability to remove 2,2-Diphenyl-1-picrylhydrazyl and 2,2′-amino-di(2-ethyl-benzothiazoline sulphonic acid-6) ammonium salt free radicals. Furthermore, the WPNFs/CLA Pickering emulsion has a more stable effect in terms of droplet size and zeta potential over a wider range of ionic strength and temperature conditions. These findings indicate that Pickering emulsion stabilized by WPNFs is more suitable as a carrier of CLA, as it increases the solubility of CLA and has better active applications in biology and food.

Brain Metabolite, N-Acetylaspartate Is a Potent Protein Aggregation Inhibitor

Deposition of toxic protein inclusions is a common hallmark of many neurodegenerative disorders including Alzheimer's disease, Parkinson disease etc. N-acetylaspartate (NAA) is an important brain metabolite whose levels got altered under various neurodegenerative conditions. Indeed, NAA has been a widely accepted biological marker for various neurological disorders. We have also reported that NAA is a protein stabilizer. In the present communication, we investigated the role of NAA in modulating the aggregation propensity on two model proteins (carbonic anhydrase and catalase). We discovered that NAA suppresses protein aggregation and could solubilize preformed aggregates.

The multiple roles of carbonic anhydrase in calcium carbonate mineralization

Carbonic anhydrase (CA) accelerates, templates and arrests calcium carbonate mineralization by playing both enzymatic and structural protein roles.

Amyloid Aggregation of Bacillus circulans Xylanase under Native Conditions and its Modulation by β-Amyloid-Derived Peptide Fragments

The aggregation of intrinsically disordered proteins into fibrils is implicated in many neurodegenerative diseases. Amyloid aggregation is a generic property of proteins as evidenced by globular proteins that often form amyloid aggregates under partially denaturing conditions. Recently, multiple lines of evidence have suggested that the amyloid aggregation of globular proteins can also occur under native conditions. Unfortunately, amyloid aggregation under native conditions has been demonstrated in only a handful of cases. Engineering a globular protein's amyloid aggregation might benefit from its fusion to an amyloid-derived fragment with reduced aggregation propensity. Unfortunately, the impacts of such fragments on the amyloid aggregation under native conditions have yet to be examined. In this study, we show that a globular protein, Bacillus circulans xylanase (BCX), can aggregate to form amyloid fibrils under native conditions. When BCX was mixed with or fused to the non-self-aggregating fragments, KLVFWAK and ELVFWAE-which were derived from β-amyloid (Aβ)-they modulated the BCX amyloid aggregation to differing extents. This study also provides insight into a correlation between the kinetic stability and amyloid aggregation of BCX, and supports a view that Aβ-derived fragments can be useful for the modulating amyloid aggregation of some, though not all, proteins.

Sequestration of Ribosome during Protein Aggregate Formation: Contribution of ribosomal RNA

An understanding of the mechanisms underlying protein aggregation and cytotoxicity of the protein aggregates is crucial in the prevention of several diseases in humans. Ribosome, the cellular protein synthesis machine is capable of acting as a protein folding modulator. The peptidyltransferase center residing in the domain V of large ribosomal subunit 23S rRNA is the centre for the protein folding ability of the ribosome and is also the cellular target of several antiprion compounds. Our in vitro studies unexpectedly reveal that the partial unfolding or aggregation of lysozyme under reducing conditions in presence of the ribosome can induce aggregation of ribosomal components. Electrostatic interactions complemented by specific rRNA-protein interaction drive the ribosome-protein aggregation process. Under similar conditions the rRNA, especially the large subunit rRNA and in vitro transcribed RNA corresponding to domain V of 23S rRNA (bDV RNA) stimulates lysozyme aggregation leading to RNA-protein aggregate formation. Protein aggregation during the refolding of non-disulfide containing protein BCAII at high concentrations also induces ribosome aggregation. BCAII aggregation was also stimulated in presence of the large subunit rRNA. Our observations imply that the specific sequestration of the translation machine by aggregating proteins might contribute to their cytotoxicity.

Proteins that accumulate with age in human skeletal-muscle aggregates contribute to declines in muscle mass and function in Caenorhabditis elegans

Protein aggregation increases with age in normal tissues, and with pathology and age in Alzheimer's hippocampus and mouse cardiac muscle. We now ask whether human skeletal muscle accumulates aggregates with age. Detergent-insoluble protein aggregates were isolated from vastus lateralis biopsies from 5 young (23–27 years of age) and 5 older (64-80 years) adults. Aggregates, quantified after gel electrophoresis, contain 2.1-fold more protein (P<0.0001) when isolated from older subjects relative to young. Of 515 proteins identified by liquid chromatography coupled to tandem mass spectrometry, 56 (11%) were significantly more abundant in older muscle, while 21 (4%) were depleted with age (each P<0.05). Orthologs to seven of these proteins were then targeted in C. elegans by RNA interference. Six of the seven knockdown treatments decreased protein aggregation (range 6-45%, P<0.01 to <0.0001) and increased muscle mass (range 1.5- to 1.85-fold, P<0.01 to <0.0001) in aged nematodes, and rescued mobility (range 1.4 to 1.65-fold, P≤0.0005 each) in a nematode amyloidopathy model. We conclude that specific aggregate proteins, discovered as differentially abundant in aging human muscle, have orthologs that contribute functionally to aggregation and age-associated muscle loss in nematodes, and thus can be considered potential drug targets for sarcopenia in humans.

Clues for divergent, polymorphic amyloidogenesis through dissection of amyloid forming steps of bovine carbonic anhydrase and its critical amyloid forming stretch

Certain amino acid stretches are considered 'critical' to trigger amyloidogenesis in a protein. Synthetic peptides corresponding to these stretches are often used as experimental mimics for studying the amyloidogenesis of their parent protein. Here we provide evidence that such simple extrapolation is misleading. We scrutinized each step of amyloid progression of full length bovine carbonic anhydrase (BCA) and compared it with the amyloidogenic process of its critical peptide stretch 201-227 (PepB). We found that under similar solution conditions amyloidogenesis of BCA followed surface-catalyzed secondary nucleation, whereas, that of PepB followed classical nucleation-dependent pathway. AFM images showed that while BCA formed short, thick and branched fibrils, PepB formed thin, long and unbranched fibrils. Structural information obtained by ATR-FTIR spectroscopy suggested parallel arrangement of intermolecular β-sheet in BCA amyloids in contrast to PepB amyloids which arranged into antiparallel β sheets. Amyloids formed by BCA were unable to seed the fibrillation of PepB and vice versa. Even the intermediates formed during lag phase revealed contrasting FTIR and far UV CD signature, hydrophobicity, morphology and cell cytotoxicity. Thus, we propose that sequences other than critical amyloidogenic stretches may significantly influence the initiation, polymerization and final fibrillar morphology of amyloid forming protein. The results have been discussed in light of primary sequence mediated amyloid polymorphism and its importance in the rational design of amyloid nanomaterials possessing desired physico-chemical properties.

Selective interception of gelsolin amyloidogenic stretch results in conformationally distinct aggregates with reduced toxicity

The pathogenesis of protein misfolding diseases is attributed to the cytotoxicity caused by amyloidogenic prefibrillar aggregates, rather than mature fibrils. The presence of one or more amyloidogenic stretches in different proteins has been proven critical for initiating fibril formation. In the present study, we show that two natural compounds, curcumin and emetine, bind tightly (Kd < 1.6 μM) to the core amyloidogenic stretch (182-192) of gelsolin (AGel). Binding happens in different structural orientations, distinctly modulating the amyloidogenic pathway of AGel. While AGel alone undergoes sigmoidal transition to thioflavin T (ThT)-responsive fibrillar aggregates with clear lag phase, the presence of curcumin or emetine abolishes the lag phase and produces starkly different, noncytotoxic end products. Atomic force microscopy revealed that while curcumin augments fibril formation, emetine arrests it at an intermediate aggregated stage with no fibrillar morphology. FTIR spectroscopy, dynamic light scattering, and ANS fluorescence experiments also suggest that these two species are distinct. Curcumin and emetine also differentially affect the preformed amyloids with the former thickening the fibrils and the latter releasing reclusive oligomers. MD simulations further provided mechanistic insights of differential interaction by the two compounds modulating amyloid formation. The results were also confirmed on the disease-associated amyloidogenic fragment of gelsolin (fAGel). Thus, our findings suggest that targeting amyloidogenic stretches in proteins could be useful in designing novel molecules against protein misfolding diseases.

Amyloid fibrillation in native and chemically-modified forms of carbonic anhydrase II: role of surface hydrophobicity

Chemical modification or mutation of proteins may bring about significant changes in the net charge or surface hydrophobicity of a protein structure. Such events may be of major physiological significance and may provide important insights into the genetics of amyloid diseases. In the present study, fibrillation potential of native and chemically-modified forms of bovine carbonic anhydrase II (BCA II) were investigated. Initially, various denaturing conditions including low pH and high temperatures were tested to induce fibrillation. At a low pH of around 2.4, where the protein is totally dissociated, the apo form was found to take up a pre-molten globular (PMG) conformation with the capacity for fibril formation. Upon increasing the pH to around 3.6, a molten globular (MG) form became abundant, forming amorphous aggregates. Charge neutralization and enhancement of hydrophobicity by methylation, acetylation and propionylation of lysine residues appeared very effective in promoting fibrillation of both the apo and holo forms under native conditions, the rates and extents of which were directly proportional to surface hydrophobicity, and influenced by salt concentration and temperature. These modified structures underwent more pronounced fibrillation under native conditions, than the PMG intermediate form, observed under denaturing conditions. The nature of the fibrillation products obtained from intermediate and modified structures were characterized and compared and their possible cytotoxicity determined. Results are discussed in terms of the importance of surface net charge and hydrophobicity in controlling protein aggregation. A discussion on the physiological significance of the observations is also presented.

Coumarin 6 and 1,6-diphenyl-1,3,5-hexatriene (DPH) as fluorescent probes to monitor protein aggregation

We report the use of Coumarin 6 and 1,6-diphenyl-1,3,5-hexatriene (DPH) for the identification of protein aggregates for the first time. The two dyes can be used at very low (nanomolar) concentrations and do not interfere with the aggregation process, as is reported for other commonly used fluorescent protein probes. In the presence of protein aggregates, their quantum yields are significantly high. DPH is able to recognize both amorphous and fibrillar aggregates but cannot distinguish between them. Coumarin 6 can distinguish between both types of aggregates. It also exhibits the characteristic sigmoidal curve of amyloid formation, with higher sensitivity for detection of fibrillation than the conventionally used Thioflavin T.