Backbone Engineering within a Latent β-Hairpin Structure to Design Inhibitors of Polyglutamine Amyloid Formation

Solid-state nuclear magnetic resonance in the structural study of polyglutamine aggregation

The aggregation of proteins into amyloid-like fibrils is seen in many neurodegenerative diseases. Recent years have seen much progress in our understanding of these misfolded protein inclusions, thanks to advances in techniques such as solid-state nuclear magnetic resonance (ssNMR) spectroscopy and cryogenic electron microscopy (cryo-EM). However, multiple repeat-expansion-related disorders have presented special challenges to structural elucidation. This review discusses the special role of ssNMR analysis in the study of protein aggregates associated with CAG repeat expansion disorders. In these diseases, the misfolding and aggregation affect mutant proteins with expanded polyglutamine segments. The most common disorder, Huntington's disease (HD), is connected to the mutation of the huntingtin protein. Since the discovery of the genetic causes for HD in the 1990s, steady progress in our understanding of the role of protein aggregation has depended on the integrative and interdisciplinary use of multiple types of structural techniques. The heterogeneous and dynamic features of polyQ protein fibrils, and in particular those formed by huntingtin N-terminal fragments, have made these aggregates into challenging targets for structural analysis. ssNMR has offered unique insights into many aspects of these amyloid-like aggregates. These include the atomic-level structure of the polyglutamine core, but also measurements of dynamics and solvent accessibility of the non-core flanking domains of these fibrils' fuzzy coats. The obtained structural insights shed new light on pathogenic mechanisms behind this and other protein misfolding diseases.

In situ captured antibacterial action of membrane-incising peptide lamellae

Developing unique mechanisms of action are essential to combat the growing issue of antimicrobial resistance. Supramolecular assemblies combining the improved biostability of non-natural compounds with the complex membrane-attacking mechanisms of natural peptides are promising alternatives to conventional antibiotics. However, for such compounds the direct visual insight on antibacterial action is still lacking. Here we employ a design strategy focusing on an inducible assembly mechanism and utilized electron microscopy (EM) to follow the formation of supramolecular structures of lysine-rich heterochiral β3-peptides, termed lamellin-2K and lamellin-3K, triggered by bacterial cell surface lipopolysaccharides. Combined molecular dynamics simulations, EM and bacterial assays confirmed that the phosphate-induced conformational change on these lamellins led to the formation of striped lamellae capable of incising the cell envelope of Gram-negative bacteria thereby exerting antibacterial activity. Our findings also provide a mechanistic link for membrane-targeting agents depicting the antibiotic mechanism derived from the in-situ formation of active supramolecules.

Photocontrol of the β-Hairpin Polypeptide Structure through an Optimized Azobenzene-Based Amino Acid Analogue

A family of neurodegenerative diseases, including Huntington's disease (HD) and spinocerebellar ataxias, are associated with an abnormal polyglutamine (polyQ) expansion in mutant proteins that become prone to form amyloid-like aggregates. Prior studies have suggested a key role for β-hairpin formation as a driver of nucleation and aggregation, but direct experimental studies have been challenging. Toward such research, we set out to enable spatiotemporal control over β-hairpin formation by the introduction of a photosensitive β-turn mimic in the polypeptide backbone, consisting of a newly designed azobenzene derivative. The reported derivative overcomes the limitations of prior approaches associated with poor photochemical properties and imperfect structural compatibility with the desired β-turn structure. A new azobenzene-based β-turn mimic was designed, synthesized, and found to display improved photochemical properties, both prior and after incorporation into the backbone of a polyQ polypeptide. The two isomers of the azobenzene-polyQ peptide showed different aggregate structures of the polyQ peptide fibrils, as demonstrated by electron microscopy and solid-state NMR (ssNMR). Notably, only peptides in which the β-turn structure was stabilized (azobenzene in the cis configuration) closely reproduced the spectral fingerprints of toxic, β-hairpin-containing fibrils formed by mutant huntingtin protein fragments implicated in HD. These approaches and findings will enable better deciphering of the roles of β-hairpin structures in protein aggregation processes in HD and other amyloid-related neurodegenerative diseases.

Mechanistic Insight into the Suppression of Polyglutamine Aggregation by SRCP1

Protein aggregation is a hallmark of the polyglutamine diseases. One potential treatment for these diseases is suppression of polyglutamine aggregation. Previous work identified the cellular slime mold Dictyostelium discoideum as being naturally resistant to polyglutamine aggregation. Further work identified serine-rich chaperone protein 1 (SRCP1) as a protein that is both necessary in Dictyostelium and sufficient in human cells to suppress polyglutamine aggregation. Therefore, understanding how SRCP1 suppresses aggregation may be useful for developing therapeutics for the polyglutamine diseases. Here we utilized a de novo protein modeling approach to generate predictions of SRCP1's structure. Using our best-fit model, we generated mutants that were predicted to alter the stability of SRCP1 and tested these mutants' stability in cells. Using these data, we identified top models of SRCP1's structure that are consistent with the C-terminal region of SRCP1 forming a β-hairpin with a highly dynamic N-terminal region. We next generated a series of peptides that mimic the predicted β-hairpin and validated that they inhibit aggregation of a polyglutamine-expanded mutant huntingtin exon 1 fragment in vitro. To further assess mechanistic details of how SRCP1 inhibits polyglutamine aggregation, we utilized biochemical assays to determine that SRCP1 inhibits secondary nucleation in a manner dependent upon the regions flanking the polyglutamine tract. Finally, to determine if SRCP1 more could generally suppress protein aggregation, we confirmed that it was sufficient to inhibit aggregation of polyglutamine-expanded ataxin-3. Together these studies provide details into the structural and mechanistic basis of the inhibition of protein aggregation by SRCP1.

van der Wel P. Selective observation of semi-rigid non-core residues in dynamically complex mutant huntingtin protein fibrils

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Mutant huntingtin exon 1 fibrils feature a broad range of molecular dynamics.

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Molecular motion is coupled to water dynamics outside the fiber core.

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Dynamics-based spectral editing ssNMR reveals mobile non-core residues.

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Intermediate-motion selection via dipolar dephasing of rigid sites.

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Semi-mobile glutamines outside the fiber core observed and identified.

Many amyloid-forming proteins, which are normally intrinsically disordered, undergo a disorder-to-order transition to form fibrils with a rigid β-sheet core flanked by disordered domains. Solid-state NMR (ssNMR) and cryogenic electron microscopy (cryoEM) excel at resolving the rigid structures within amyloid cores but studying the dynamically disordered domains remains challenging. This challenge is exemplified by mutant huntingtin exon 1 (HttEx1), which self-assembles into pathogenic neuronal inclusions in Huntington disease (HD). The mutant protein’s expanded polyglutamine (polyQ) segment forms a fibril core that is rigid and sequestered from the solvent. Beyond the core, solvent-exposed surface residues mediate biological interactions and other properties of fibril polymorphs. Here we deploy magic angle spinning ssNMR experiments to probe for semi-rigid residues proximal to the fibril core and examine how solvent dynamics impact the fibrils’ segmental dynamics. Dynamic spectral editing (DYSE) 2D ssNMR based on a combination of cross-polarization (CP) ssNMR with selective dipolar dephasing reveals the weak signals of solvent-mobilized glutamine residues, while suppressing the normally strong background of rigid core signals. This type of ‘intermediate motion selection’ (IMS) experiment based on cross-polarization (CP) ssNMR, is complementary to INEPT- and CP-based measurements that highlight highly flexible or highly rigid protein segments, respectively. Integration of the IMS-DYSE element in standard CP-based ssNMR experiments permits the observation of semi-rigid residues in a variety of contexts, including in membrane proteins and protein complexes. We discuss the relevance of semi-rigid solvent-facing residues outside the fibril core to the latter’s detection with specific dyes and positron emission tomography tracers.

An Improved Turn Structure for Inducing β-Hairpin Formation in Peptides

Although β-hairpins are widespread in proteins, there is no tool to coax any small peptide to adopt a β-hairpin conformation, regardless of sequence. Here, we report that δ-linked γ(R)-methyl-ornithine (^δ MeOrn) provides an improved β-turn template for inducing a β-hairpin conformation in peptides. We developed a synthesis of protected ^δ MeOrn as a building block suitable for use in Fmoc-based solid-phase peptide synthesis. The synthesis begins with l-leucine and affords gram quantities of the N^α -Boc-N^δ -Fmoc-γ(R)-methyl-ornithine building block. X-ray crystallography confirms that the ^δ MeOrn turn unit adopts a folded structure in a macrocyclic β-hairpin peptide. CD and NMR spectroscopy allow comparison of the ^δ MeOrn turn template to the δ-linked ornithine (^δ Orn) turn template that we previously introduced and to the popular d-Pro-Gly turn template. These studies show that the folding of the ^δ MeOrn turn template is substantially better than that of ^δ Orn and is comparable to d-Pro-Gly.

Evidence of Anti-amyloid Characteristics of Plumbagin via Inhibition of Protein Aggregation and Disassembly of Protein Fibrils

The biological consequences associated with the conversion of soluble proteins into insoluble toxic amyloids are not only limited to the onset of neurodegenerative diseases but also to the potential health risks associated with supplements of protein therapeutic agents as well. Hence, finding inhibitors against amyloid formation is important, and natural product-based anti-amyloid compounds have gained much interest because of their higher efficacy and biocompatibility. Plumbagin has been identified as a potential natural product with multiple medical benefits; however, it remains largely unclear whether plumbagin can act against amyloid formation of proteins. Here, we show that plumbagin can effectively inhibit the temperature-induced amyloid aggregation of important proteins (insulin and serum albumin). Both experimental and computational data revealed that the presence of plumbagin in protein solutions, under aggregating conditions, promotes a direct protein-plumbagin interaction, which is predominantly stabilized by stronger H-bonds and hydrophobic interactions. Plumbagin-mediated retention of the native structures of proteins appears to play a crucial role in preventing their conversion into insoluble β-sheet-rich amyloid aggregates. More importantly, the addition of plumbagin into a suspension of protein fibrils triggered their spontaneous disassembly, promoting the release of soluble proteins. The results highlight that a possible synergistic effect via both the stabilization of protein structures and the restriction of the monomer recruitment at the fibril growth sites could be important for the mechanism of plumbagin's anti-aggregation effect. These findings may inspire the development of plumbagin-based formulations to benefit both the prevention and treatment of amyloid-related health complications.

Exploding the Repeat Length Paradigm while Exploring Amyloid Toxicity in Huntington's Disease

Huntington's disease (HD) is a progressive, familial neurodegenerative disease triggered by the expansion of a polyglutamine (polyQ) track in the protein huntingtin (htt). PolyQ sequences up to Q₃₆ in htt are not known to be toxic, while polyQ lengths above Q₃₆ almost invariably lead to increased disease risk and decreased ages of onset. The large number of physical states (monomers, dimers, tetramers, non-β oligomers, nanofibrils, and clustered amyloid fibrils) on the self-association landscape, with their overlapping kinetics of formation, have greatly complicated identification of the molecular species responsible for HD toxicity, drawing attention to the need for innovative approaches.After reports of HD-associated intraneuronal htt inclusions in 1997, we elucidated aggregation mechanisms of both simple polyQ sequences and the more complex polyQ-containing "exon1" fragment of htt (htt-ex1). Grounded in this work, the more recent results described here were made possible by breakthroughs in the molecular design of diagnostic polyQ derivatives and in fluorescence applications for characterizing amyloid assembly intermediates. Thus, insertion of β-turn-promoting mutations into relatively short, disordered polyQ sequences created "pro-β-hairpin" polyQs (βHPs) that exhibit amyloid formation rates comparable to the enhanced rates seen with expanded polyQ peptides. Introduction of "β-breaker" mutations into these βHP polyQ sequences created molecules that are blocked from aggregating into amyloid and also can inhibit amyloid formation by other polyQ proteins. These mutational effects were then successfully transferred into more complex htt-ex1 sequence backgrounds. Insights into the aggregation properties of htt-ex1 derivatives-as well as into the nucleation process itself-were obtained using fluorescence correlation spectroscopy (FCS) and a novel thioflavin-T (ThT) protocol that allows quantitation of htt-ex1 assembly intermediates.Using these tools, we quantified physical states of htt-ex1 at different growth times in mammalian PC12 cells engineered for inducible expression of both normal and expanded polyQ repeat length versions of htt-ex1. For expanded polyQ versions, we found tetramers, oligomers, and fibrils (but no monomers) all populated in these cells at a time when the first indication of toxicity (nuclear DNA damage) was observed. These experiments provided a strong hint that monomeric forms of htt-ex1 are not involved in toxicity, but we were otherwise unable to implicate a specific toxic self-assembled state because of the overlapping kinetics of formation. To gain a more intimate focus and control over the timelines of htt-ex1 self-assembly and the resulting toxic response, we engineered various htt-ex1-βHP molecules-with and without added β-breaker mutations-that could be expressed in rat neuronal and Drosophila models of HD. In both models, novel htt-ex1-βHP analogues exhibiting strong aggregation in spite of their very short polyQ repeat lengths proved to be toxic, dramatically breaking the "repeat length paradigm" and strongly suggesting that the toxic species must be some kind of aggregate. In both models, β-breaker analogues of htt-ex1-βHP that are slow to make amyloid-instead favoring accumulation of non-β oligomers-were nontoxic. In contrast, htt-ex1-βHP analogues that rapidly progress to amyloid states were toxic, suggesting that an aggregate possessing the fundamental amyloid folding motif is very likely the major toxic species in HD.

Membrane active Janus-oligomers of β3-peptides

Self-assembly of an acyclic β³-hexapeptide with alternating side chain chirality, into nanometer size oligomeric bundles showing membrane activity and hosting capacity for hydrophobic small molecules.

Self-assembling peptides offer a versatile set of tools for bottom-up construction of supramolecular biomaterials. Among these compounds, non-natural peptidic foldamers experience increased focus due to their structural variability and lower sensitivity to enzymatic degradation. However, very little is known about their membrane properties and complex oligomeric assemblies – key areas for biomedical and technological applications. Here we designed short, acyclic β³-peptide sequences with alternating amino acid stereoisomers to obtain non-helical molecules having hydrophilic charged residues on one side, and hydrophobic residues on the other side, with the N-terminus preventing formation of infinite fibrils. Our results indicate that these β-peptides form small oligomers both in water and in lipid bilayers and are stabilized by intermolecular hydrogen bonds. In the presence of model membranes, they either prefer the headgroup regions or they insert between the lipid chains. Molecular dynamics (MD) simulations suggest the formation of two-layered bundles with their side chains facing opposite directions when compared in water and in model membranes. Analysis of the MD calculations showed hydrogen bonds inside each layer, however, not between the layers, indicating a dynamic assembly. Moreover, the aqueous form of these oligomers can host fluorescent probes as well as a hydrophobic molecule similarly to e.g. lipid transfer proteins. For the tested, peptides the mixed chirality pattern resulted in similar assemblies despite sequential differences. Based on this, it is hoped that the presented molecular framework will inspire similar oligomers with diverse functionality.

Peptide-Based Molecular Strategies To Interfere with Protein Misfolding, Aggregation, and Cell Degeneration

Protein misfolding into amyloid fibrils is linked to more than 40 as yet incurable cell- and neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and type 2 diabetes. So far, however, only one of the numerous anti-amyloid molecules has reached patients. This Minireview gives an overview of molecular strategies and peptide chemistry "tools" to design, develop, and discover peptide-based molecules as anti-amyloid drug candidates. We focus on two major inhibitor rational design strategies: 1) the oldest and most common strategy, based on molecular recognition elements of amyloid self-assembly, and 2) a more recent approach, based on cross-amyloid interactions. We discuss why peptide-based amyloid inhibitors, in particular their advanced generations, can be promising leads or candidates for anti-amyloid drugs as well as valuable tools for deciphering amyloid-mediated cell damage and its link to disease pathogenesis.

Proteomimetics as protein-inspired scaffolds with defined tertiary folding patterns

Proteins have evolved as a variable platform that provides access to molecules with diverse shapes, sizes and functions. These features have inspired chemists for decades to seek artificial mimetics of proteins with improved or novel properties. Such work has focused primarily on small protein fragments, often isolated secondary structures; however, there has lately been a growing interest in the design of artificial molecules that mimic larger, more complex tertiary folds. In this Perspective, we define these agents as 'proteomimetics' and discuss the recent advances in the field. Proteomimetics can be divided into three categories: protein domains with side-chain functionality that alters the native linear-chain topology; protein domains in which the chemical composition of the polypeptide backbone has been partially altered; and protein-like folded architectures that are composed entirely of non-natural monomer units. We give an overview of these proteomimetic approaches and outline remaining challenges facing the field.

Biocompatible Inhibitor Based on Chitosan and Amphiphilic Peptide against Mutant Huntingtin Toxicity

Huntington's disease (HD) is classified as a protein-misfolding disease correlated with the mutant Huntingtin (mHtt) protein with abnormally expanded polyglutamine (polyQ) domains. Because no effective drugs have yet been reported, attempts to develop better therapy to delay the age of onset are in urgent demand. In this study, an amphiphilic peptide consisting of negatively charged hexaglutamic acid and a stretch of decaglutamine (E₆ Q₁₀ ) was chemically synthesized as an inhibitor against polyQ and mHtt toxicity. It is found that E₆ Q₁₀ selfassembles into spherical vesicles, as shown by means of TEM, cryoelectron microscopy, and dynamic light scattering. Assembled E₆ Q₁₀ prevented the polyQ-rich peptide (KKWQ₂₀ AKK) from forming amyloid fibrils. To enable the cell-penetration ability of E₆ Q₁₀ , the E₆ Q₁₀ ⋅chitosan complex was generated. It is demonstrated that the complex penetrates cells, interferes with the mHtt oligomerization and aggregation process, and prevents mHtt cytotoxicity. By combining positively charged chitosan and amphiphilic peptides with a negatively charge moiety, a new strategy is provided to develop biocompatible and biodegradable inhibitors against mHtt toxicity.

Self-Assembly of Artificial Sweetener Aspartame Yields Amyloid-like Cytotoxic Nanostructures

Recent reports have revealed the intrinsic propensity of single aromatic metabolites to undergo self-assembly and form nanostructures of amyloid nature. Hence, identifying whether aspartame, a universally consumed artificial sweetener, is inherently aggregation prone becomes an important area of investigation. Although the reports on aspartame-linked side effects describe a multitude of metabolic disorders, the mechanistic understanding of such destructive effects is largely mysterious. Since aromaticity, an aggregation-promoting factor, is intrinsic to aspartame's chemistry, it is important to know whether aspartame can undergo self-association and if such a property can predispose any cytotoxicity to biological systems. Our study finds that aspartame molecules, under mimicked physiological conditions, undergo a spontaneous self-assembly process yielding regular β-sheet-like cytotoxic nanofibrils of amyloid nature. The resultant aspartame fibrils were found to trigger amyloid cross-seeding and become a toxic aggregation trap for globular proteins, Aβ peptides, and aromatic metabolites that convert native structures to β-sheet-like fibrils. Aspartame fibrils were also found to induce hemolysis, causing DNA damage resulting in both apoptosis and necrosis-mediated cell death. Specific spatial arrangement between aspartame molecules is predicted to form a regular amyloid-like architecture with a sticky exterior that is capable of promoting viable H-bonds, electrostatic interactions, and hydrophobic contacts with biomolecules, leading to the onset of protein aggregation and cell death. Results reveal that the aspartame molecule is inherently amyloidogenic, and the self-assembly of aspartame becomes a toxic trap for proteins and cells, exposing the bitter side of such a ubiquitously used artificial sweetener.

The N terminus of the small heat shock protein HSPB7 drives its polyQ aggregation-suppressing activity

Heat shock protein family B (small) member 7 (HSPB7) is a unique, relatively unexplored member within the family of human small heat shock proteins (HSPBs). Unlike most HSPB family members, HSPB7 does not oligomerize and so far has not been shown to associate with any other member of the HSPB family. Intriguingly, it was found to be the most potent member within the HSPB family to prevent aggregation of proteins with expanded polyglutamine (polyQ) stretches. How HSPB7 suppresses polyQ aggregation has remained elusive so far. Here, using several experimental strategies, including in vitro aggregation assay, immunoblotting and fluorescence approaches, we show that the polyQ aggregation-inhibiting activity of HSPB7 is fully dependent on its flexible N-terminal domain (NTD). We observed that the NTD of HSPB7 is both required for association with and inhibition of polyQ aggregation. Remarkably, replacing the NTD of HSPB1, which itself cannot suppress polyQ aggregation, with the NTD of HSPB7 resulted in a hybrid protein that gained anti-polyQ aggregation activity. The hybrid NTD_HSPB7-HSPB1 protein displayed a reduction in oligomer size and, unlike WT HSPB1, associated with polyQ. However, experiments with phospho-mimicking HSPB1 mutants revealed that de-oligomerization of HSPB1 alone does not suffice to gain polyQ aggregation-inhibiting activity. Together, our results reveal that the NTD of HSPB7 is both necessary and sufficient to bind to and suppress the aggregation of polyQ-containing proteins.

Mutational analysis implicates the amyloid fibril as the toxic entity in Huntington's disease

In Huntington disease (HD), an expanded polyglutamine (polyQ > 37) sequence within huntingtin (htt) exon1 leads to enhanced disease risk. It has proved difficult, however, to determine whether the toxic form generated by polyQ expansion is a misfolded or avid-binding monomer, an α-helix-rich oligomer, or a β-sheet-rich amyloid fibril. Here we describe an engineered htt exon1 analog featuring a short polyQ sequence that nonetheless quickly forms amyloid fibrils and causes HD-like toxicity in rat neurons and Drosophila. Additional modifications within the polyQ segment produce htt exon1 analogs that populate only spherical oligomers and are non-toxic in cells and flies. Furthermore, in mixture with expanded-polyQ htt exon1, the latter analogs in vitro suppress amyloid formation and promote oligomer formation, and in vivo rescue neurons and flies expressing mhtt exon1 from dysfunction and death. Thus, in our experiments, while htt exon1 toxicity tracks with aggregation propensity, it does so in spite of the toxic construct's possessing polyQ tracts well below those normally considered to be disease-associated. That is, aggregation propensity proves to be a more accurate surrogate for toxicity than is polyQ repeat length itself, strongly supporting a major toxic role for htt exon1 aggregation in HD. In addition, the results suggest that the aggregates that are most toxic in these model systems are amyloid-related. These engineered analogs are novel tools for mapping properties of polyQ self-assembly intermediates and products that should similarly be useful in the analysis of other expanded polyQ diseases. Small molecules with similar amyloid inhibitory properties might be developed into effective therapeutic agents.

Foldamer Tertiary Structure through Sequence-Guided Protein Backbone Alteration

The prospect of recreating the complex structural hierarchy of protein folding in synthetic oligomers with backbones that are artificial in covalent structure ("foldamers") has long fascinated chemists. Foldamers offer complex functions from biostable scaffolds and have found widespread applications in fields from biomedical to materials science. Most precedent has focused on isolated secondary structures or their assemblies. In considering the goal of complex protein-like tertiary folding patterns, a key barrier became apparent. How does one design a backbone with covalent connectivity and a sequence of side-chain functional groups that will support defined intramolecular packing of multiple artificial secondary structures? Two developments were key to overcoming this challenge. First was the recognition of the power of blending α-amino acid residues with monomers differing in backbone connectivity to create "heterogeneous-backbone" foldamers. Second was the finding that replacing some of the natural α-residues in a biological sequence with artificial-backbone variants can result in a mimic that retains both the fold and function of the native sequence and, in some cases, gains advantageous characteristics. Taken together, these precedents lead to a view of a protein as chemical entity having two orthogonal sequences: a sequence of side-chain functional groups and a separate sequence of backbone units displaying those functional groups. In this Account, we describe our lab's work over the last ∼10 years to leverage the above concept of protein sequence duality in order to develop design principles for constructing heterogeneous-backbone foldamers that adopt complex protein-like tertiary folds. Fundamental to the approach is the utilization of a variety of artificial building blocks (e.g., d-α-residues, Cα-Me-α-residues, N-Me-α-residues, β-residues, γ-residues, δ-residues, polymer segments) in concert, replacing a fraction of α-residues in a given prototype sequence. We provide an overview of the state-of-the-art in terms of design principles for choosing substitutions based on consideration of local secondary structure and retention of key side-chain functional groups. We survey high-resolution structures of backbone-modified proteins to illustrate how diverse artificial moieties are accommodated in tertiary fold contexts. We detail efforts to elucidate how backbone alteration impacts folding thermodynamics and describe how such data informs the development of improved design rules. Collectively, a growing body of results by our lab and others spanning multiple protein systems suggests there is a great deal of plasticity with respect to the backbone chemical structures upon which sequence-encoded tertiary folds can manifest. Moreover, these efforts suggest sequence-guided backbone alteration as a broadly applicable strategy for generating foldamers with complex tertiary folding patterns. We conclude by offering some perspective regarding the near future of this field, in terms of unanswered questions, technological needs, and opportunities for new areas of inquiry.

Insights into protein misfolding and aggregation enabled by solid-state NMR spectroscopy

The aggregation of proteins and peptides into a variety of insoluble, and often non-native, aggregated states plays a central role in many devastating diseases. Analogous processes undermine the efficacy of polypeptide-based biological pharmaceuticals, but are also being leveraged in the design of biologically inspired self-assembling materials. This Trends article surveys the essential contributions made by recent solid-state NMR (ssNMR) studies to our understanding of the structural features of polypeptide aggregates, and how such findings are informing our thinking about the molecular mechanisms of misfolding and aggregation. A central focus is on disease-related amyloid fibrils and oligomers involved in neurodegenerative diseases such as Alzheimer's, Parkinson's and Huntington's disease. SSNMR-enabled structural and dynamics-based findings are surveyed, along with a number of resulting emerging themes that appear common to different amyloidogenic proteins, such as their compact alternating short-β-strand/β-arc amyloid core architecture. Concepts, methods, future prospects and challenges are discussed.

Formation and Structure of Wild Type Huntingtin Exon-1 Fibrils

The fact that the heritable neurodegenerative disorder Huntington's disease (HD) is autosomal dominant means that there is one wild type and one mutant allele in most HD patients. The CAG repeat expansion in the exon 1 of the protein huntingtin (HTT_ex1) that causes the disease leads to the formation of HTT fibrils in vitro and vivo. An important question for understanding the molecular mechanism of HD is which role wild type HTT plays for the formation, propagation, and structure of these HTT fibrils. Here we report that fibrils of mutant HTT_ex1 are able to seed the aggregation of wild type HTT_ex1 into amyloid fibrils, which in turn can seed the fibril formation of mutant HTT_ex1. Solid-state NMR and electron paramagnetic resonance data showed that wild type HTT_ex1 fibrils closely resemble the structure of mutant fibrils, with small differences indicating a less extended fibril core. These data suggest that wild type fibrils can faithfully perpetuate the structure of mutant fibrils in HD. However, wild type HTT_ex1 monomers have a much higher equilibrium solubility compared to mutant HTT_ex1, and only a small fraction incorporates into fibrils.