Distinct Amyloid Research Articles

Distinct amyloid-\u03b2 and tau-associated microglia profiles in Alzheimer\u2019s disease

Alzheimer’s disease (AD) is the most prevalent form of dementia and is characterized by abnormal extracellular aggregates of amyloid-β and intraneuronal hyperphosphorylated tau tangles and neuropil threads. Microglia, the tissue-resident macrophages of the central nervous system (CNS), are important for CNS homeostasis and implicated in AD pathology. In amyloid mouse models, a phagocytic/activated microglia phenotype has been identified. How increasing levels of amyloid-β and tau pathology affect human microglia transcriptional profiles is unknown. Here, we performed snRNAseq on 482,472 nuclei from non-demented control brains and AD brains containing only amyloid-β plaques or both amyloid-β plaques and tau pathology. Within the microglia population, distinct expression profiles were identified of which two were AD pathology-associated. The phagocytic/activated AD1-microglia population abundance strongly correlated with tissue amyloid-β load and localized to amyloid-β plaques. The AD2-microglia abundance strongly correlated with tissue phospho-tau load and these microglia were more abundant in samples with overt tau pathology. This full characterization of human disease-associated microglia phenotypes provides new insights in the pathophysiological role of microglia in AD and offers new targets for microglia-state-specific therapeutic strategies.

The role of water in the primary nucleation of protein amyloid aggregation

The understanding of the complex conformational landscape of amyloid aggregation and its modulation by relevant physicochemical and cellular factors is a prerequisite for elucidating some of the molecular basis of pathology in amyloid related diseases, and for developing and evaluating effective disease-specific therapeutics to reduce or eliminate the underlying sources of toxicity in these diseases. Interactions of proteins with solvating water have been long considered to be fundamental in mediating their function and folding; however, the relevance of water in the process of protein amyloid aggregation has been largely overlooked. Here, we provide a perspective on the role water plays in triggering primary amyloid nucleation of intrinsically disordered proteins (IDPs) based on recent experimental evidences. The initiation of amyloid aggregation likely results from the synergistic effect between both protein intermolecular interactions and the properties of the water hydration layer of the protein surface. While the self-assembly of both hydrophobic and hydrophilic IDPs would be thermodynamically favoured due to large water entropy contributions, large desolvation energy barriers are expected, particularly for the nucleation of hydrophilic IDPs. Under highly hydrating conditions, primary nucleation is slow, being facilitated by the presence of nucleation-active surfaces (heterogeneous nucleation). Under conditions of poor water activity, such as those found in the interior of protein droplets generated by liquid-liquid phase separation, however, the desolvation energy barrier is significantly reduced, and nucleation can occur very rapidly in the bulk of the solution (homogeneous nucleation), giving rise to structurally distinct amyloid polymorphs. Water, therefore, plays a key role in modulating the transition free energy of amyloid nucleation, thus governing the initiation of the process, and dictating the type of preferred primary nucleation and the type of amyloid polymorph generated, which could vary depending on the particular microenvironment that the protein molecules encounter in the cell.

Impact of the APOE gene on amyloid deposition in participants with abnormal soluble amyloid levels

AbstractBackgroundAmyloid pathology can be quantified both in the cerebro‐spinal fluid (CSF) and in positron emission tomography (PET) images, even though they measure two distinct amyloid pools. While CSF Aβ reflects the clearance of soluble amyloid species from the brain, PET imaging informs on the presence of insoluble fibrillary plaques. Many studies have shown the concordance of the two measures in individuals along the Alzheimer’s disease (AD) continuum. Here, we assessed whether APOE‐ε4, the major genetic risk factor for AD, affects the pattern of cerebral Aβ‐load as a function of CSF Aβ levels.MethodThe first consecutive 276 cognitively unimpaired (CDR=0) individuals of the ALFA+ study, aged between 48 and 74, with valid structural MRI and [18F]Flutemetamol (FTM) PET, APOE genotyping and CSF samples were included. Aβ‐40 and Aβ‐42 concentrations were determined with the NeuroToolKit (Roche Diagnostics International Ltd.) on a Elecsys cobas e‐411 instrument and parametric standardized uptake value ratio images (SUVr) were calculated with the whole cerebellum as reference region. A voxelwise regression to predict FTM uptake was performed, with the following independent variables: Age, Sex, APOE‐ε4, and CSF Aβ‐42/40. Statistical threshold was set to p<0.001 with a cluster extent correction of 100 voxels.ResultAs expected, CSF Aβ‐42/40 was strongly and negatively associated to cerebral FTM uptake (Fig. 1a‐1b). Age and APOE‐ε4 gene dose mapped onto distinct spatial patterns of FTM binding across the whole sample (Fig. 2b‐2c). Importantly, we found a significant interaction between Aβ‐42/40 and APOE‐ε4 indicating that carriers had a negative association between fluid and imaging Aβ measures in the bilateral hippocampus, as well as the bilateral middle temporal and temporo‐parietal junction (Fig. 3).ConclusionCompared to non‐carriers, APOE‐ε4 carriers show higher cerebral amyloid load for any given level of CSF Aβ 42/40 in areas which are not typically vulnerable to Aβ deposition yet undergoing neurodegeneration early in the AD continuum, such as the hippocampus. This suggests that APOE‐ε4 may influence the biological mechanisms regulating the equilibrium between soluble and deposited amyloid, which may transcend the hindering effect caused by Aβ fibrillary plaques and render medial temporal lobe structures particularly vulnerable when Aβ metabolism becomes deficient.

Distinct Animal Food Allergens Form IgE-Binding Amyloids

Several animal food allergens assemble into amyloids under gastric-like environments. These aggregated structures provide Gad m 1 with an enhanced immunoglobulin E (IgE) interaction due to the fibrillation of the epitope regions. However, whether these properties are unique to Gad m 1 or shared by other food allergens has not yet been addressed. Using Bos d 5, Bos d 12 and Gal d 2 as allergen models and Gad m 1 as the control, aggregation reactions and the sera of milk, egg and fish allergic patients have been analyzed, assessing the IgE interactions of their amyloids. We found that amyloids formed by Bos d 12 and Gal d 2 full-length and truncated chains are recognized by the IgEs of milk and egg allergic patient sera. As with Gad m 1, in most cases amyloid recognition is higher than that of the native structure. Bos d 5 was not recognized under any fold by the IgE of the sera studied. These results suggest that the formation of IgE-binding amyloids could be a common feature to animal food allergens.

Cross-Species and Cross-Polymorph Seeding of Lysozyme Amyloid Reveals a Dominant Polymorph.

The ability to self-propagate is one of the most intriguing characteristics of amyloid fibrils, and is a feature of great interest both to stopping unwanted pathological amyloid, and for engineering functional amyloid as a useful nanomaterial. The sequence and structural tolerances for amyloid seeding are not well understood, particularly concerning the propagation of distinct fibril morphologies (polymorphs) across species. This study examined the seeding and cross-seeding reactions between two unique fibril polymorphs, one long and flexible (formed at pH 2) and the other short and rigid (formed at pH 6.3), of human lysozyme and hen egg-white lysozyme. Both polymorphs could cross-seed aggregation across species, but this reaction was markedly reduced under physiological conditions. For both species, the pH 6.3 fibril polymorph was dominant, seeding fibril growth with a faster growth rate constant at pH 2 than the pH 2 polymorph. Based on fibrillation kinetics and fibril morphology, we found that the pH 2 polymorph was not able to faithfully replicate itself at pH 6.3. These results show that two distinct amyloid polymorphs are both capable of heterologous seeding across two species (human and hen) of lysozyme, but that the pH 6.3 polymorph is favored, regardless of the species, likely due to a lower energy barrier, or faster configurational diffusion, to accessing this particular misfolded form. These findings contribute to our better understanding of amyloid strain propagation across species barriers.

Crystal structure of a conformational antibody that binds tau oligomers and inhibits pathological seeding by extracts from donors with Alzheimer's disease

Soluble oligomers of aggregated tau accompany the accumulation of insoluble amyloid fibrils, a histological hallmark of Alzheimer disease (AD) and two dozen related neurodegenerative diseases. Both oligomers and fibrils seed the spread of Tau pathology, and by virtue of their low molecular weight and relative solubility, oligomers may be particularly pernicious seeds. Here, we report the formation of in vitro tau oligomers formed by an ionic liquid (IL15). Using IL15-induced recombinant tau oligomers and a dot blot assay, we discovered a mAb (M204) that binds oligomeric tau, but not tau monomers or fibrils. M204 and an engineered single-chain variable fragment (scFv) inhibited seeding by IL15-induced tau oligomers and pathological extracts from donors with AD and chronic traumatic encephalopathy. This finding suggests that M204-scFv targets pathological structures that are formed by tau in neurodegenerative diseases. We found that M204-scFv itself partitions into oligomeric forms that inhibit seeding differently, and crystal structures of the M204-scFv monomer, dimer, and trimer revealed conformational differences that explain differences among these forms in binding and inhibition. The efficiency of M204-scFv antibodies to inhibit the seeding by brain tissue extracts from different donors with tauopathies varied among individuals, indicating the possible existence of distinct amyloid polymorphs. We propose that by binding to oligomers, which are hypothesized to be the earliest seeding-competent species, M204-scFv may have potential as an early-stage diagnostic for AD and tauopathies, and also could guide the development of promising therapeutic antibodies.

Infrared Laser-Induced Amyloid Fibril Dissociation: A Joint Experimental/Theoretical Study on the GNNQQNY Peptide.

Neurodegenerative diseases are usually characterized by plaques made of well-ordered aggregates of distinct amyloid proteins. Dissociating these very stable amyloid plaques is a critical clinical issue. In this study, we present a joint mid-infrared free electron laser experiment/nonequilibrium molecular dynamics simulation to understand the dissociation process of a representative example GNNQQNY fibril. By tuning the laser frequency to the amide I band of the fibril, the resonance takes place and dissociation is occurred. With the calculated and observed wide-angle X-ray scattering profiles and secondary structures before and after laser irradiation being identical, we can propose a dissociation mechanism with high confidence from our simulations. We find that dissociation starts in the core of the fibrils by fragmenting the intermolecular hydrogen bonds and separating the peptides and then propagates to the fibril extremities leading to the formation of unstructured expanded oligomers. We suggest that this should be a generic mechanism of the laser-induced dissociation of amyloid fibrils.

Sequence- and seed-structure-dependent polymorphic fibrils of alpha-synuclein

Synucleinopathies comprise a diverse group of neurodegenerative diseases including Parkinson's disease (PD), dementia with Lewy bodies, and multiple system atrophy. These share a common pathological feature, the deposition of alpha-synuclein (a-syn) in neurons or oligodendroglia. A-syn is highly conserved in vertebrates, but the primary sequence of mouse a-syn differs from that of human at seven positions. However, structural differences of their aggregates remain to be fully characterized. In this study, we found that human and mouse a-syn aggregated in vitro formed morphologically distinct amyloid fibrils exhibiting twisted and straight structures, respectively. Furthermore, we identified different protease-resistant core regions, long and short, in human and mouse a-syn aggregates. Interestingly, among the seven unconserved amino acids, only A53T substitution, one of the familial PD mutations, was responsible for structural conversion to the straight-type. Finally, we checked whether the structural differences are transmissible by seeding and found that human a-syn seeded with A53T aggregates formed straight-type fibrils with short protease-resistant cores. These results suggest that a-syn aggregates form sequence-dependent polymorphic fibrils upon spontaneous aggregation but become seed structure-dependent upon seeding.

Distinct amyloid distribution patterns in amyloid positive subcortical vascular cognitive impairment

Amyloid-β (Aβ) and cerebral small vessel disease (CSVD) commonly coexist. They can occur independently by chance, or may interact with each other. We aimed to determine whether the distribution of Aβ in subcortical vascular cognitive impairments (SVCI) patients can be classified by the underlying pathobiologies. A total of 45 11C-Pittsburgh compound B PET positive (PiB(+)) SVCI patients were included in this study. They were classified using a new cluster analysis method which adopted the Louvain method, which finds optimal decomposition of the participants based on similarity of relative Aβ deposition pattern. We measured atherosclerotic cerebral small vessel disease (CSVD) markers and cerebral amyloid angiopathy (CAA) markers. Forty-five PiB(+) SVCI patients were classified into two groups: 17 patients with the characteristic Alzheimer’s disease like Aβ uptake with sparing of occipital region (OccSp) and 28 patients with occipital predominant Aβ uptake (OccP). Compared to OccSp group, OccP group had more postive association of atherosclerotic CSVD score (p for interaction = 0.044), but not CAA score with occipital/global ratio of PiB uptake. Our findings suggested that Aβ positive SVCI patients might consist of heterogeneous groups with combined CSVD and Aβ resulting from various pathobiologies. Furthermore, atherosclerotic CSVD might explain increased occipital Aβ uptakes.

SynAggreg: A Multifunctional High-Throughput Technology for Precision Study of Amyloid Aggregation and Systematic Discovery of Synergistic Inhibitor Compounds

Numerous proteins can coalesce into amyloid self-assemblies, which are responsible for a class of diseases called amyloidoses, but which can also fulfill important biological functions and are of great interest for biotechnology. Amyloid aggregation is a complex multi-step process, poorly prone to detailed structural studies. Therefore, small molecules interacting with amyloids are often used as tools to probe the amyloid aggregation pathway and in some cases to treat amyloidoses as they prevent pathogenic protein aggregation. Here, we report on SynAggreg, an in vitro high-throughput (HT) platform dedicated to the precision study of amyloid aggregation and the effect of modulator compounds. SynAggreg relies on an accurate bi-fluorescent amyloid-tracer readout that overcomes some limitations of existing HT methods. It allows addressing diverse aspects of aggregation modulation that are critical for pathomechanistic studies, such as the specificity of compounds toward various amyloids and their effects on aggregation kinetics, as well as the co-assembly propensity of distinct amyloids and the influence of prion-like seeding on self-assembly. Furthermore, SynAggreg is the first HT technology that integrates tailored methodology to systematically identify synergistic compound combinations—an emerging strategy to improve fatal amyloidoses by targeting multiple steps of the aggregation pathway. To this end, we apply analytical combinatorial scores to rank the inhibition efficiency of couples of compounds and to readily detect synergism. Finally, the SynAggreg platform should be suited for the characterization of a broad class of amyloids, whether of interest for drug development purposes, for fundamental research on amyloid functions, or for biotechnological applications.

Amyloid assembly and disassembly.

ABSTRACTAmyloid fibrils are protein homopolymers that adopt diverse cross-β conformations. Some amyloid fibrils are associated with the pathogenesis of devastating neurodegenerative disorders, including Alzheimer's disease and Parkinson's disease. Conversely, functional amyloids play beneficial roles in melanosome biogenesis, long-term memory formation and release of peptide hormones. Here, we showcase advances in our understanding of amyloid assembly and structure, and how distinct amyloid strains formed by the same protein can cause distinct neurodegenerative diseases. We discuss how mutant steric zippers promote deleterious amyloidogenesis and aberrant liquid-to-gel phase transitions. We also highlight effective strategies to combat amyloidogenesis and related toxicity, including: (1) small-molecule drugs (e.g. tafamidis) to inhibit amyloid formation or (2) stimulate amyloid degradation by the proteasome and autophagy, and (3) protein disaggregases that disassemble toxic amyloid and soluble oligomers. We anticipate that these advances will inspire therapeutics for several fatal neurodegenerative diseases.

Decoding the Prion‐Chaperone Interaction Network in Yeast: Prion‐specific Chaperone Functions Promote Amyloid Diversity

Yeast prions are heritable protein‐based elements, most of which are amyloid aggregates of functional proteins. Their propagation in cell populations is dependent upon the fragmentation of these aggregates by a core set of chaperone proteins including the J‐protein/Hsp40 Sis1, Hsp70s, and Hsp104. Prions form distinct amyloid structural polymorphisms, called ‘strains' in mammalian systems and ‘variants' in yeast. Recently we and others have uncovered significant complexity in the chaperone requirements of various yeast prions and prion variants, demonstrating that, in contrast to Hsp104 and Hsp70, which have general roles, J‐proteins represent a prion‐specific component of the prion‐propagation machinery. Most notable has been a direct demonstration that the persistence of alternative conformations of prions is dependent on the action of different molecular chaperones. Because J‐proteins often act as targeting factors for Hsp70s, they may constitute the first chaperone response to the presence of amyloid, but a critical barrier to advancing the understanding of chaperone function in prion biology is that the fundamental chaperone requirements for most yeast prions remain unidentified, precluding a comprehensive understanding of how protein sequences give rise to amyloids with distinct patterns of chaperone interaction. Here we will present our latest undergraduate‐generated data regarding: 1) the unusual role of the J‐protein Swa2 in the propagation of the [URE3] prion and our proposal of a novel chaperone complex involving Hsp90, and 2) the conservation of the prion‐maintaining J‐protein functions of both Sis1 and Swa2 in plants. These findings suggest that amyloid structures of alternative prion conformations have unique features that are differentiable by chaperone proteins, revealing a previously unappreciated level of additional complexity that may be exploitable for therapeutic intervention.Support or Funding InformationThis work was supported by the National Institute of General Medical Sciences of the National Institutes of Health under Award Number R15GM110606. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Development of a cell‐based assay for Tau strain discrimination

Alzheimer's Disease and other related tauopathies are characterized by the accumulation of the microtubule‐associated protein Tau into fibrillary deposits. Interestingly, there is evidence supporting amyloid structures, or strains, formed as a consequence of Tau aggregation are unique to each pathology, suggesting different conformations play an important role in the development of specific tauopathies. The potential critical role of Tau in disease progression, together with the isolation of 19 different Tau strains, led us to work in developing a panel of cell‐based fluorescence resonance energy transfer (FRET) biosensors. Each biosensor carried a different mutation on Tau's aggregation‐prone repeat domain, based on the hypothesis that each mutant would have unique interactions with Tau strains and, thus, alter Tau aggregation kinetics within the cell. We hoped that by studying these differences in FRET signal we would be able to discriminate between distinct amyloid conformations. In order to do this, we first created the biosensors by transducing HEK 293 cells with lentivirus coding Tau's repeat domain with specific mutations, fused to both, cyan florescent protein (CFP) or yellow florescent protein (YFP). These biosensors were later treated with different sources of Tau seeds and FRET flow cytometry was used to describe their seeding activity. Experiments with the 19 strains in our Tau library and the engineered biosensors revealed noticeable differences in percent FRET positivity, such that discrimination was achieved for most of the strains. Interestingly, the highest seeding activity among all the biosensors was obtained from strain 15 (DS15), even at low total protein concentrations. Additionally, biosensors expressing the wild‐type four‐repeat domain of Tau had higher percent FRET positivity only when seeded with strains derived from Corticobasal Degeneration (CBD) patients. Furthermore, treatment of our panel of biosensors with brain homogenates of multiple tauopathy patients correlated with our previous results and revealed more distinct patterns of FRET behavior. Together, these results suggest that measuring proteopathic seeding activity while using our panel of biosensors is a sensitive and reliable tool to discriminate between different amyloid conformations, which could potentially lead the development of improved diagnostic methods for neurodegenerative disorders.Support or Funding InformationThis research project was supported by the NIH‐Maximizing Access to Research Careers (Grant # 5T34GM0072821‐38), the Summer Undergraduate Research Fellowship at UT Southwestern Medical Center and the Center for Alzheimer's and Neurodegenerative Diseases at UT Southwestern Medical Center.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Surface patterns of insulin fibrils revealed by time-resolved spectroscopy measurements of fluorescent probes

Amyloid fibrils are a hallmark of neurodegeneration. The structural diversity of amyloids necessitates sensitive methods and probes that can be reliably used to characterize them. Here, we study insulin fibrils and its polymorphs seeded with LVEALYL peptide in context of probing the surface patterns using Thioflavin T (ThT) and polythiophene derivative – Poly[2-(3-thienyl)ethoxy-4-butylsulfonate] (PTEBS) polymer. We investigated the dynamics and lifetimes of these two probes using time-resolved absorption and fluorescence spectroscopy. The photoluminescence emission lifetimes of the probes showed different relaxation times in the presence of structurally different amyloid fibrils. However, only PTEBS revealed sensitivity to the surface patterns of the fibrils that was explained by uneven charge distribution and exposure of different amino acids to the fibril surface where probe is interacting with the fibril. We find that PTEBS binding to distinct amyloid surfaces causes perturbation of the main chain of the polymer creating new conditions for energy distribution in the excited states that favors formation of various emitting species. The results indicate that PTEBS and ThT are comparable in terms of recognition of fibril polymorphs whereas polymer probe provides additional dimension to study fibrils surface patterns that can be revealed using time-resolved spectroscopy.

Molecular basis for diversification of yeast prion strain conformation

Self-propagating β-sheet-rich fibrillar protein aggregates, amyloid fibers, are often associated with cellular dysfunction and disease. Distinct amyloid conformations dictate different physiological consequences, such as cellular toxicity. However, the origin of the diversity of amyloid conformation remains unknown. Here, we suggest that altered conformational equilibrium in natively disordered monomeric proteins leads to the adaptation of alternate amyloid conformations that have different phenotypic effects. We performed a comprehensive high-resolution structural analysis of Sup35NM, an N-terminal fragment of the Sup35 yeast prion protein, and found that monomeric Sup35NM harbored latent local compact structures despite its overall disordered conformation. When the hidden local microstructures were relaxed by genetic mutations or solvent conditions, Sup35NM adopted a strikingly different amyloid conformation, which redirected chaperone-mediated fiber fragmentation and modulated prion strain phenotypes. Thus, dynamic conformational fluctuations in natively disordered monomeric proteins represent a posttranslational mechanism for diversification of aggregate structures and cellular phenotypes.

Distinct amyloid precursor protein processing machineries of the olfactory system

Processing of amyloid precursor protein (APP) occurs through sequential cleavages first by β-secretase and then by the γ-secretase complex. However, abnormal processing of APP leads to excessive production of β-amyloid (Aβ) in the central nervous system (CNS), an event which is regarded as a primary cause of Alzheimer's disease (AD). In particular, gene mutations of the γ-secretase complex—which contains presenilin 1 or 2 as the catalytic core—could trigger marked Aβ accumulation.Olfactory dysfunction usually occurs before the onset of typical AD-related symptoms (eg, memory loss or muscle retardation), suggesting that the olfactory system may be one of the most vulnerable regions to AD. To date however, little is known about why the olfactory system is affected so early by AD prior to other regions. Thus, we examined the distribution of secretases and levels of APP processing in the olfactory system under either healthy or pathological conditions.Here, we show that the olfactory system has distinct APP processing machineries. In particular, we identified higher expressions levels and activity of γ-secretase in the olfactory epithelium (OE) than other regions of the brain. Moreover, APP c-terminal fragments (CTF) are markedly detected. During AD progression, we note increased expression of presenilin2 of γ-secretases in the OE, not in the OB, and show that neurotoxic Aβ*56 accumulates more quickly in the OE.Taken together, these results suggest that the olfactory system has distinct APP processing machineries under healthy and pathological conditions. This finding may provide a crucial understanding of the unique APP-processing mechanisms in the olfactory system, and further highlights the correlation between olfactory deficits and AD symptoms.

Species-dependent structural polymorphism of Y145Stop prion protein amyloid revealed by solid-state NMR spectroscopy

One of the most puzzling aspects of the prion diseases is the intricate relationship between prion strains and interspecies transmissibility barriers. Previously we have shown that certain fundamental aspects of mammalian prion propagation, including the strain phenomenon and species barriers, can be reproduced in vitro in seeded fibrillization of the Y145Stop prion protein variant. Here, we use solid-state nuclear magnetic resonance spectroscopy to gain atomic level insight into the structural differences between Y145Stop prion protein amyloids from three species: human, mouse, and Syrian hamster. Remarkably, we find that these structural differences are largely controlled by only two amino acids at positions 112 and 139, and that the same residues appear to be key to the emergence of structurally distinct amyloid strains within the same protein sequence. The role of these residues as conformational switches can be rationalized based on a model for human Y145Stop prion protein amyloid, providing a foundation for understanding cross-seeding specificity.

Site-Specific Fluorescence Depolarization Kinetics Distinguishes the Amyloid Folds Responsible for Distinct Yeast Prion Strains.

The prion determinant of a yeast prion protein, Sup35NM, assembles into β-rich amyloid fibrils that switch the nonprion [psi-] state to the prion [PSI+] state of yeast. Previous studies showed that two distinct forms of amyloids (Sc4 and Sc37), generated in vitro at two different temperatures (4 and 37 °C), recapitulate the strain phenomenon in Saccharomyces cerevisiae. Sc4 demonstrates a strong [PSI+] phenotype, whereas Sc37 shows a weak phenotype. To discern the residue-specific structural and dynamical attributes associated with the amyloids that display strain diversity, we took advantage of the nonoccurrence of tryptophan (Trp) in the NM-domain and created 18 single-Trp variants spanning the entire polypeptide length. The fluorescence readouts from these locations reported the site-specific structural details in Sc4 and Sc37 fibrils. Highly sensitive picosecond fluorescence depolarization measurements at these positions allowed a conformational mobility map to be constructed. Nearly all of the residue positions demonstrated higher local flexibility in Sc4 amyloid, which exhibits a strong phenotype. The differences in the amplitude of local mobility were more pronounced at the two end segments of the N-domain than in the central region. The M-domain is partially exposed and exhibits a higher amplitude of local mobility, indicating a lower degree of chain packing in the amyloid state, as well as a higher mobility in the Sc4 state compared to the Sc37 state. The altered local conformational dynamics in these two distinct amyloid states provide molecular insights into the varied fragility and severing efficiency that govern the inheritance patterns of strong and weak prion strains.

Key aromatic/hydrophobic amino acids controlling a cross-amyloid peptide interaction versus amyloid self-assembly

The interaction of the intrinsically disordered polypeptide islet amyloid polypeptide (IAPP), which is associated with type 2 diabetes (T2D), with the Alzheimer's disease amyloid-β (Aβ) peptide modulates their self-assembly into amyloid fibrils and may link the pathogeneses of these two cell-degenerative diseases. However, the molecular determinants of this interaction remain elusive. Using a systematic alanine scan approach, fluorescence spectroscopy, and other biophysical methods, including heterocomplex pulldown assays, far-UV CD spectroscopy, the thioflavin T binding assay, transmission EM, and molecular dynamics simulations, here we identified single aromatic/hydrophobic residues within the amyloid core IAPP region as hot spots or key residues of its cross-interaction with Aβ40(42) peptide. Importantly, we also find that none of these residues in isolation plays a key role in IAPP self-assembly, whereas simultaneous substitution of four aromatic/hydrophobic residues with Ala dramatically impairs both IAPP self-assembly and hetero-assembly with Aβ40(42). Furthermore, our experiments yielded several novel IAPP analogs, whose sequences are highly similar to that of IAPP but have distinct amyloid self- or cross-interaction potentials. The identified similarities and major differences controlling IAPP cross-peptide interaction with Aβ40(42) versus its amyloid self-assembly offer a molecular basis for understanding the underlying mechanisms. We propose that these insights will aid in designing intervention strategies and novel IAPP analogs for the management of type 2 diabetes, Alzheimer's disease, or other diseases related to IAPP dysfunction or cross-amyloid interactions.

Critical Influence of Cosolutes and Surfaces on the Assembly of Serpin-Derived Amyloid Fibrils

Many proteins and peptides self-associate into highly ordered and structurally similar amyloid cross-β aggregates. This fibrillation is critically dependent on properties of the protein and the surrounding environment that alter kinetic and thermodynamic equilibria. Here, we report on dominating surface and solution effects on the fibrillogenic behavior and amyloid assembly of the C-36 peptide, a circulating bioactive peptide from the α1-antitrypsin serine protease inhibitor. C-36 converts from an unstructured peptide to mature amyloid twisted-ribbon fibrils over a few hours when incubated on polystyrene plates under physiological conditions through a pathway dominated by surface-enhanced nucleation. In contrast, in plates with nonbinding surfaces, slow bulk nucleation takes precedence over surface catalysis and leads to fibrillar polymorphism. Fibrillation is strongly ion-sensitive, underlining the interplay between hydrophilic and hydrophobic forces in molecular self-assembly. The addition of exogenous surfaces in the form of silica glass beads and polyanionic heparin molecules potently seeds the amyloid conversion process. In particular, heparin acts as an interacting template that rapidly forces β-sheet aggregation of C-36 to distinct amyloid species within minutes and leads to a more homogeneous fibril population according to solid-state NMR analysis. Heparin’s template effect highlights its role in amyloid seeding and homogeneous self-assembly, which applies both in vitro and in vivo, where glycosaminoglycans are strongly associated with amyloid deposits. Our study illustrates the versatile thermodynamic landscape of amyloid formation and highlights how different experimental conditions direct C-36 into distinct macromolecular structures.