Structural polymorphism of amyloid fibrils in ATTR amyloidosis revealed by cryo-electron microscopy

Surfactant-like peptide gels are based on cross-β amyloid fibrils

Surfactant-like peptides, in which hydrophilic and hydrophobic residues are encoded within different domains in the peptide sequence, undergo facile self-assembly in aqueous solution to form supramolecular hydrogels. These peptides have been explored extensively as substrates for the creation of functional materials since a wide variety of amphipathic sequences can be prepared from commonly available amino acid precursors. The self-assembly behavior of surfactant-like peptides has been compared to that observed for small molecule amphiphiles in which nanoscale phase separation of the hydrophobic domains drives the self-assembly of supramolecular structures. Here, we investigate the relationship between sequence and supramolecular structure for a pair of bola-amphiphilic peptides, Ac-KLIIIK-NH2 (L2) and Ac-KIIILK-NH2 (L5). Despite similar length, composition, and polar sequence pattern, L2 and L5 form morphologically distinct assemblies, nanosheets and nanotubes, respectively. Cryo-EM helical reconstruction was employed to determine the structure of the L5 nanotube at near-atomic resolution. Rather than displaying self-assembly behavior analogous to conventional amphiphiles, the packing arrangement of peptides in the L5 nanotube displayed steric zipper interfaces that resembled those observed in the structures of β-amyloid fibrils. Like amyloids, the supramolecular structures of the L2 and L5 assemblies were sensitive to conservative amino acid substitutions within an otherwise identical amphipathic sequence pattern. This study highlights the need to better understand the relationship between sequence and supramolecular structure to facilitate the development of functional peptide-based materials for biomaterials applications.

Green tea polyphenol EGCG acts differentially on end-stage amyloid polymorphs of α-synuclein formed in different polyol osmolytes

Synucleinopathies are heterogenous group of disorders characterized by α-synuclein amyloid aggregates in the nervous system. Different synucleinopathy clinical subtypes are encoded by structurally diverse α-synuclein amyloid polymorphs referred to as 'strains'. The underlying structural differences between polymorphs can potentially hamper the drug design against synucleinopathies. Polyphenolic compounds like EGCG have shown promise in inhibiting and remodeling of α-synuclein amyloid aggregates, but their effects on different polymorphs are not well-studied. The cellular environment is one factor contributing to the heterogeneity in the amyloid landscape. Herein, we generated diverse polymorphs of α-synuclein by fine-tuning its aggregation using different polyol osmolytes, varying in their physicochemical properties. These osmolytes act as globular protein stabilizers and conformational modulators of intrinsically disordered proteins. While the buffer control α-synuclein aggregates were evenly dispersed, the polyol-induced aggregate solutions contained a heterogeneous mixture of co-existing polymorphs, as evidenced by AFM and TEM measurements. The polyol-induced aggregated solutions consisted of a mixture of both fibrillar and nonfibrillar cross-β-rich species. Using various spectroscopic tools, we observed differences in the structures of osmolyte-induced polymorphic aggregates. We incubated these aggregates with EGCG and observed its disparate action over polymorphs wherein the treated species were either disintegrated or structurally altered. Contrary to previous reports, all EGCG-treated polymorphs were β-sheet-rich and seeding-competent. Our findings are relevant in assessing the efficacy of polyphenolic compounds on diverse aggregate strains encoding different proteinopathy variants. The formation of β-sheet-rich species in our study also engenders a more critical examination of EGCG's mode of action on diverse classes of amyloids.

Initiation of transthyretin aggregation at neutral pH by fluid agitation

The transthyretin (TTR) tetramer, assembled as a dimer of dimers, transports thyroxine and retinol binding protein in blood plasma and cerebrospinal fluid. Aggregation of wild type (WT) or pathogenic variant TTR leads to transthyretin amyloidosis, which is associated with neurodegenerative and cardiac disease. The trigger for TTR aggregation under physiological conditions is unknown. The tetramer is extremely stable at neutral pH, but aggregation via tetramer dissociation and monomer misfolding can be induced in vitro by lowering the pH. To elucidate factors that may cause TTR aggregation at neutral pH, we examined the effect of shear forces such as those that arise from fluid flow in the vascular system. Fluid shear forces were generated by rapidly stirring TTR solutions in conical microcentrifuge tubes. Under agitation, TTR formed β-rich aggregates and fibrils at a rate that was dependent upon protein concentration. The lag time before the onset of agitation-induced aggregation increases as the total TTR concentration is increased, consistent with a mechanism in which the tetramer first dissociates to form monomer that either partially unfolds to enter the aggregation pathway or reassociates to form tetramer. NMR spectra recorded at various time points during the lag phase revealed growth of an aggregation-prone intermediate trapped as a dynamically perturbed tetramer. Enhanced conformational fluctuations in the weak dimer-dimer interface suggest loosening of critical intersubunit contacts which likely destabilizes the agitated tetramer and predisposes it toward dissociation. These studies provide insights into the mechanism of aggregation of WT human TTR under near-physiological conditions.

Hereditary Transthyretin Cardiac Amyloidosis With the p.V142I Variant: Mechanistic Insights and Diagnostic Challenges

The most common form of hereditary transthyretin cardiac amyloidosis (hATTR-CA) in the United States and the United Kingdom is the p.V142I variant. About 3% to 4% of patients with African ancestry carry this genetic predisposition to develop signs and symptoms of hATTR-CA. Nevertheless, clinical manifestations of hATTR-CA appear only late in the fifth and sixth decades of life, despite its clear genetic background. Imbalances in native protein-stabilizing and elementary breakdown cellular mechanisms are postulated as potential causes for affecting transthyretin structural integrity and myocardial fibril deposition. Noncoding variants, epigenetic and environmental factors, as well as gut microbiome derangements may serve as disease-modifying factors that feature detrimental amyloidogenic organ involvement and impact disease severity. Organ amyloid deposition varies widely among different carriers of a genetic transthyretin variant. The genotype-phenotype interdependence causes unpredictable phenotypic penetrance that results in a variety of signs and symptoms and patient outcomes. Cardiovascular biomarkers and multimodality imaging may identify initial amyloidogenic organ involvement. These early clinical clues through the course of hATTR-CA offer a window of opportunity for early treatment onset to cease disease progression and alter prognosis. Identifying at-risk patients requires information on the genetic background of probands and their relatives. Initiatives to reveal asymptomatic gene carriers early in the disease should be encouraged, as it necessitates stringent patient follow-up and immediate treatment onset to reduce the burden of heart failure hospitalization and mortality in hATTR-CA.

A computational approach to predict the effects of missense mutations on protein amyloidogenicity: A case study in hereditary transthyretin cardiomyopathy

With many amyloidosis-associated missense mutations still unidentified and early diagnostic methods largely unavailable, there is an urgent need for a reliable computational approach to predict hereditary amyloidoses from gene sequencing data. Progress has been made in predicting amyloidosis-triggering sequences within intrinsically disordered regions. However, some diseases are caused by mutations in amyloidogenic regions within structured domains that must unfold for amyloid formation. Accurate prediction of amyloidogenic regions requires tools for detecting amyloidogenicity and assessing mutation effects on protein stability. We developed datasets of mutations linked to hereditary ATTR cardiomyopathy and others likely unrelated, evaluating TTR mutants with amyloidogenicity and stability predictors. Notably, the stability predictors consistently indicated that ATTR-related mutations tend to destabilize the TTR structure more than non-ATTR-associated mutations. Using these datasets and newly generated mutation features, we developed a machine learning model SDAM-TTR to predict mutations leading to ATTR cardiomyopathy.

Biomarkers in Subclinical Transthyretin Cardiac Amyloidosis

PURPOSE OF REVIEW

The most common type of cardiac amyloidosis is transthyretin amyloidosis (ATTR-CM). Early forms of the disease can often go undetected. Effective pharmacological treatments are available for ATTR-CM. However, current treatment options may be more effective when used earlier in the disease, making early detection paramount. Below, we discuss updates with regards to the role that blood-based biomarkers play in detecting subclinical cardiac amyloidosis.

RECENT FINDINGS

Carriers of amyloidogenic mutations in the TTR gene are at a heightened risk of developing heart failure and have higher mortality rates compared with noncarrier counterparts. Conventional biomarkers, such as the cardiac troponins and natriuretic peptides, may be useful to monitor subclinical cardiac amyloidosis. In addition, recent studies have demonstrated links between amyloidogenic TTR carrier status and low levels of circulating transthyretin (TTR) and retinol-binding protein 4 (RBP4). Laboratory advances have also allowed for the development of peptide-based detection methods. Probes targeting transthyretin aggregates and nonnative TTR peptides have shown promise in differentiating ATTR from non-ATTR amyloidosis populations. Finally, recent studies have identified neurofilament light chains as potential biomarkers for detecting polyneuropathy-predominant amyloidosis. Conventional biomarkers, such as cardiac troponin and natriuretic peptides may indicate evolving amyloid deposition in early ATTR-CM. However, they are non-specific and emerging biomarkers such as serum transthyretin levels, retinol-binding protein 4, transthyretin aggregates, nonnative TTR, and neurofilament light chains may hold promise in characterizing subclinical ATTR.

How is the Amyloid Fold Built? Polymorphism and the Microscopic Mechanisms of Fibril Assembly

For a given protein sequence, many, up to sometimes hundreds of different amyloid fibril folds, can be formed in vitro. Yet, fibrils extracted from, or found in, human tissue, usually at the end of a long disease process, are often structurally homogeneous. Through monitoring of amyloid assembly reactions in vitro, the scientific community has gained a detailed understanding of the kinetic mechanisms of fibril assembly and the rates at which the different processes involved occur. However, how this kinetic information relates to the structural changes as a protein transforms from its initial, native structure to the canonical cross-β structure of amyloid remain obscure. While cryoEM has yielded a plethora of high-resolution information that portray a vast variety of fibril structures, there remains little knowledge of how and why each particular structure resulted. Recent work has demonstrated that fibril structures can change over an assembly time course, despite the different fibril types having similar thermodynamic stability. This points to kinetic control of the fibrils formed, with structures that initiate or elongate faster becoming the dominant products of assembly. Annotating kinetic assembly mechanisms alongside structural analysis of the fibrils formed is required to truly understand the molecular mechanisms of amyloid formation. However, this is a complicated task. In this review, we discuss how embracing this challenge could open new frontiers in amyloid research and new opportunities for therapy.

Does the structure of transthyretin amyloid fibrils vary depending on the organ of accumulation?

In this issue of Structure, Nguyen et al.1 reveal that amyloid fibrils of the transthyretin (TTR) V30M variant from the heart and nerves of the same patient exhibit structural homogeneity. This finding is crucial for advancing our understanding of V30M-TTR amyloid deposition, which leads to fatal ATTRv amyloidosis.

ATTRv-V30M amyloid fibrils from heart and nerves exhibit structural homogeneity

Amyloidogenic transthyretin (ATTR) amyloidosis is a systemic disease characterized by the deposition of amyloid fibrils made of transthyretin. Transthyretin is primarily produced in tetrameric form by the liver, but also by retinal epithelium and choroid plexus. The deposition of these fibrils in the myocardium and peripheral nerves causes cardiomyopathies and neuropathies, respectively. Using cryoelectron microscopy (cryo-EM), we investigated fibrils extracted from cardiac and nerve tissues of an ATTRv-V30M patient. We found consistent fibril structures from both tissues, similar to cardiac fibrils previously described, but different from vitreous humor fibrils of the same genotype. Our findings, along with previous ATTR fibrils structural studies, suggest a uniform fibrillar architecture across different tissues when transthyretin originates from the liver. This study advances our understanding of how deposition and production sites influence fibril structure in ATTRv-V30M amyloidosis.

Aggregation of Transthyretin by Fluid Agitation

The transthyretin (TTR) tetramer, assembled as a dimer of dimers, transports thyroxine and retinol binding protein in blood plasma and cerebrospinal fluid. Aggregation of wild type or pathogenic variant TTR leads to transthyretin amyloidosis (ATTR), which is associated with neurodegenerative and cardiac disease. The trigger for TTR aggregation under physiological conditions is unknown. The tetramer is extremely stable at neutral pH, but aggregation via tetramer dissociation and monomer misfolding can be induced in vitro by lowering the pH. To elucidate factors that may cause TTR aggregation at neutral pH, we examined the effect of shear forces such as arise from fluid flow in the vascular system. Fluid shear forces were generated by rapidly stirring TTR solutions in conical microcentrifuge tubes. Under agitation, TTR formed β-rich aggregates and fibrils at a rate that was dependent upon protein concentration. The lag time before the onset of agitation-induced aggregation increases as the total TTR concentration is increased, consistent with a mechanism in which the tetramer first dissociates to form monomer that either partially unfolds to enter the aggregation pathway or reassociates to form tetramer. NMR spectra recorded at various time points during the lag phase revealed growth of an aggregation-prone intermediate trapped as a dynamically perturbed tetramer. Enhanced conformational fluctuations in the weak dimer-dimer interface suggests loosening of critical inter-subunit contacts which likely destabilizes the agitated tetramer and predisposes it towards dissociation. These studies provide new insights into the mechanism of aggregation of wild type human TTR under near physiological conditions.

Amyloid and collagen templates in aortic valve calcification

Calcific aortic valve disease (CAVD) is a widely prevalent heart disorder in need of pharmacological interventions. Calcified areas in aortic valves often contain amyloid fibrils that promote calcification in vitro. This opinion paper suggests that amyloid contributes to CAVD development; amyloid-assisted nucleation can accelerate hydroxyapatite deposition onto collagen matrix. Notably, acidic arrays in amyloid match calcium-calcium spacing in the amorphous hydroxyapatite precursor, while oscillating hemodynamic perturbations promote amyloid deposition in the valve. Lipoprotein(a), a genetic risk factor for CAVD, augments calcification via several mechanisms, wherein hydrolysis of oxidized phospholipids (oxPLs) by Lp(a)-associated enzymes helps generate orthophosphate, and apolipoprotein(a) blocks plasmin-induced fibril degradation. Current studies of amyloid-calcium-collagen interactions in solution and in fibrillar complexes allow deeper insight into the role of amyloid in calcification.

Cryo-EM vs. Disease X

In this issue of Cell, Penzes et al. describe the use of cryo-EM to identify the cause of a mysterious disease affecting farmed superworms across the US. The study illustrates the power of ex vivo cryo-EM, which uses amplification-free samples to advance at once diagnostic, DNA packaging mechanism, and preventative measures.

Structural Basis for Monoclonal Antibody Therapy for Transthyretin Amyloidosis

The disease of transthyretin (TTR) amyloidosis (ATTR) has been known since the 1960s, and during the past 60 or so years, there has been a sustained period of steady discoveries that have led to the current model of ATTR pathogenesis. More recent research has achieved major advances in both diagnostics and therapeutics for ATTR, which are having a significant impact on ATTR patients today. Aiding these recent achievements has been the remarkable ability of cryo-electron microscopy (EM) to determine high-resolution structures of amyloid fibrils obtained from individual patients. Here, we will examine the cryo-EM structures of transthyretin amyloid fibrils to explore the structural basis of the two monoclonal antibody therapies for ATTR that are in clinical trials, ALXN-2220 and Coramitug, as well as to point out potential applications of this approach to other systemic amyloid diseases.

Structure-Based Probe Reveals the Presence of Large Transthyretin Aggregates in Plasma of ATTR Amyloidosis Patients

Amyloidogenic transthyretin (ATTR) amyloidosis is a relentlessly progressive disease caused by the misfolding and systemic accumulation of amyloidogenic transthyretin into amyloid fibrils. These fibrils cause diverse clinical phenotypes, mainly cardiomyopathy and/or polyneuropathy. Little is known about the aggregation of transthyretin during disease development and whether this has implications for diagnosis and treatment. Using the cryogenic electron microscopy structures of mature ATTR fibrils, we developed a peptide probe for fibril detection. With this probe, we have identified previously unknown aggregated transthyretin species in plasma of patients with ATTR amyloidosis. These species are large, non-native, and distinct from monomeric and tetrameric transthyretin. Observations from our study open many questions about the biology of ATTR amyloidosis and reveal a potential diagnostic and therapeutic target.

Molecular basis for non-invasive diagnostics of cardiac amyloids using bone tracers

Amyloid diseases including Alzheimer's, Parkinson's and over 30 others are incurable life-threatening disorders caused by abnormal protein deposition as fibrils in various organs. Cardiac amyloidosis is particularly challenging to diagnose and treat. Identification of the fibril-forming protein, which in the heart is usually amyloid transthyretin (ATTR) or amyloid immunoglobulin light chain (AL), is paramount to treatment. A transformative non-invasive diagnostic modality is imaging using technetium-labeled pyrophosphate or diphosphonate bone tracers, 99mTc-PYP/DPD/HMDP. For unknown reasons, these tracers show preferential uptake by ATTR deposits. The tracer-binding moiety is unknown and potentially involves amyloid fibrils and/or amyloid-associated calcific deposits. We propose that, like in the bone, the tracers chelate to surface-bound Ca2+ in amyloid. In high-affinity protein sites, Ca2+ is coordinated by pairs of acidic residues. To identify such residues on amyloids, we harnessed atomic structures of patient-derived cardiac amyloids determined using cryogenic electron microscopy since 2019. These structures help explain why most but not all ATTR deposits uptake 99mTc-PYP/DPD/HMDP radiotracers, while in AL the opposite is true. Moreover, fibril structures help explain greater microcalcification observed in ATTR vs. AL deposits. These findings may aid the diagnostics and therapeutic targeting of cardiac amyloidosis and are relevant to other amyloids.

Conformational Dynamics of an Amyloidogenic Intermediate of Transthyretin: Implications for Structural Remodeling and Amyloid Formation

The aggregation pathway of transthyretin (TTR) proceeds through rate-limiting dissociation of the tetramer (a dimer of dimers) and partial misfolding of the resulting monomer, which assembles into amyloid structures through a downhill polymerization mechanism. The structural features of the aggregation-prone monomeric intermediate are poorly understood. NMR relaxation dispersion offers a unique opportunity to characterize amyloidogenic intermediates when they exchange on favorable timescales with NMR-visible ground states. Here we use NMR to characterize the structure and conformational dynamics of the monomeric F87E mutant of human TTR. Chemical shifts derived from analysis of multinuclear relaxation dispersion data provide insights into the structure of a low-lying excited state that exchanges with the ground state of the F87E monomer at a rate of 3800 s-1. Disruption of the subunit interfaces of the TTR tetramer leads to destabilization of edge strands in both β-sheets of the F87E monomer. Conformational fluctuations are propagated through the entire hydrogen bonding network of the DAGH β-sheet, from the inner β-strand H, which forms the strong dimer-dimer interface in the TTR tetramer, to outer strand D which is unfolded in TTR fibrils. Fluctuations are also propagated from the AB loop in the weak dimer-dimer interface to the EF helix, which undergoes structural remodeling in fibrils. The conformational fluctuations in both regions are enhanced at acidic pH where amyloid formation is most favorable. The relaxation dispersion data provide insights into the conformational dynamics of the amyloidogenic state of monomeric TTR that predispose it for structural remodeling and progression to amyloid fibrils.

Cryo-EM confirms a common fibril fold in the heart of four patients with ATTRwt amyloidosis

ATTR amyloidosis results from the conversion of transthyretin into amyloid fibrils that deposit in tissues causing organ failure and death. This conversion is facilitated by mutations in ATTRv amyloidosis, or aging in ATTRwt amyloidosis. ATTRv amyloidosis exhibits extreme phenotypic variability, whereas ATTRwt amyloidosis presentation is consistent and predictable. Previously, we found unique structural variabilities in cardiac amyloid fibrils from polyneuropathic ATTRv-I84S patients. In contrast, cardiac fibrils from five genotypically different patients with cardiomyopathy or mixed phenotypes are structurally homogeneous. To understand fibril structure's impact on phenotype, it is necessary to study the fibrils from multiple patients sharing genotype and phenotype. Here we show the cryo-electron microscopy structures of fibrils extracted from four cardiomyopathic ATTRwt amyloidosis patients. Our study confirms that they share identical conformations with minimal structural variability, consistent with their homogenous clinical presentation. Our study contributes to the understanding of ATTR amyloidosis biopathology and calls for further studies.

Effects of Pathogenic Mutants of the Neuroprotective RNase 5-Angiogenin in Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease that affects the motoneurons. More than 40 genes are related with ALS, and amyloidogenic proteins like SOD1 and/or TDP-43 mutants are directly involved in the onset of ALS through the formation of polymorphic fibrillogenic aggregates. However, efficacious therapeutic approaches are still lacking. Notably, heterozygous missense mutations affecting the gene coding for RNase 5, an enzyme also called angiogenin (ANG), were found to favor ALS onset. This is also true for the less-studied but angiogenic RNase 4. This review reports the substrate targets and illustrates the neuroprotective role of native ANG in the neo-vascularization of motoneurons. Then, it discusses the molecular determinants of many pathogenic ANG mutants, which almost always cause loss of function related to ALS, resulting in failures in angiogenesis and motoneuron protection. In addition, ANG mutations are sometimes combined with variants of other factors, thereby potentiating ALS effects. However, the activity of the native ANG enzyme should be finely balanced, and not excessive, to avoid possible harmful effects. Considering the interplay of these angiogenic RNases in many cellular processes, this review aims to stimulate further investigations to better elucidate the consequences of mutations in ANG and/or RNase 4 genes, in order to achieve early diagnosis and, possibly, successful therapies against ALS.

Multi-organ structural homogeneity of amyloid fibrils in ATTRv-T60A amyloidosis patients, revealed by Cryo-EM

ATTR amyloidosis is a degenerative disorder characterized by the systemic deposition of the protein transthyretin. These amyloid aggregates of transthyretin (ATTR) can deposit in different parts of the body causing diverse clinical manifestations. Our laboratory aims to investigate a potential relationship between the different genotypes, organ of deposition, clinical phenotypes, and the structure of ATTR fibrils. Using cryo-electron microscopy, we have recently described how the neuropathic related mutations ATTRv-I84S and ATTRv-V122∆ can drive structural polymorphism in ex vivo fibrils. Here we question whether the mutation ATTRv-T60A, that commonly triggers cardiac and neuropathic symptoms, has a similar effect. To address this question, we extracted and determined the structure of ATTR-T60A fibrils from multiple organs (heart, thyroid, kidney, and liver) from the same patient and from the heart of two additional patients. We have found a consistent conformation among all the fibril structures, acquiring the "closed-gate morphology" previously found in ATTRwt and others ATTRv related to cardiac or mixed manifestations. The closed-gate morphology is composed by two segments of the protein that interact together forming a polar channel, where the residues glycine 57 to isoleucine 68 act as a gate of the polar cavity. Our study indicates that ATTR-T60A fibrils present in peripheral organs adopt the same structural conformation in all patients, regardless of the organ of deposition.

ATTRv-V30M Type A amyloid fibrils from heart and nerves exhibit structural homogeneity

ATTR amyloidosis is a systemic disease characterized by the deposition of amyloid fibrils made of transthyretin, a protein integral to transporting retinol and thyroid hormones. Transthyretin is primarily produced by the liver and circulates in blood as a tetramer. The retinal epithelium also secretes transthyretin, which is secreted to the vitreous humor of the eye. Because of mutations or aging, transthyretin can dissociate into amyloidogenic monomers triggering amyloid fibril formation. The deposition of transthyretin amyloid fibrils in the myocardium and peripheral nerves causes cardiomyopathies and neuropathies, respectively. Using cryo-electron microscopy, here we determined the structures of amyloid fibrils extracted from cardiac and nerve tissues of an ATTRv-V30M patient. We found that fibrils from both tissues share a consistent structural conformation, similar to the previously described structure of cardiac fibrils from an individual with the same genotype, but different from the fibril structure obtained from the vitreous humor. Our study hints to a uniform fibrillar architecture across different tissues within the same individual, only when the source of transthyretin is the liver. Moreover, this study provides the first description of ATTR fibrils from the nerves of a patient and enhances our understanding of the role of deposition site and protein production site in shaping the fibril structure in ATTRv-V30M amyloidosis.

Amyloid fibril polymorphism in the heart of an ATTR amyloidosis patient with polyneuropathy attributed to the V122Δ variant

ATTR amyloidosis is a phenotypically heterogeneous disease characterized by the pathological deposition of transthyretin in the form of amyloid fibrils into various organs. ATTR amyloidosis may stem from mutations in variant (ATTRv) amyloidosis, or aging in wild-type (ATTRwt) amyloidosis. ATTRwt generally manifests as a cardiomyopathy phenotype, whereas ATTRv may present as polyneuropathy, cardiomyopathy, or mixed, in combination with many other symptoms deriving from secondary organ involvement. Over 130 different mutational variants of transthyretin have been identified, many of them being linked to specific disease symptoms. Yet, the role of these mutations in the differential disease manifestation remains elusive. Using cryo-electron microscopy, here we structurally characterized fibrils from the heart of an ATTRv patient carrying the V122Δ mutation, predominantly associated with polyneuropathy. Our results show that these fibrils are polymorphic, presenting as both single and double filaments. Our study alludes to a structural connection contributing to phenotypic variation in ATTR amyloidosis, as polymorphism in ATTR fibrils may manifest in patients with predominantly polyneuropathic phenotypes.

O-GlcNAc forces an α-synuclein amyloid strain with notably diminished seeding and pathology

Amyloid-forming proteins such α-synuclein and tau, which are implicated in Alzheimer's and Parkinson's disease, can form different fibril structures or strains with distinct toxic properties, seeding activities and pathology. Understanding the determinants contributing to the formation of different amyloid features could open new avenues for developing disease-specific diagnostics and therapies. Here we report that O-GlcNAc modification of α-synuclein monomers results in the formation of amyloid fibril with distinct core structure, as revealed by cryogenic electron microscopy, and diminished seeding activity in seeding-based neuronal and rodent models of Parkinson's disease. Although the mechanisms underpinning the seeding neutralization activity of the O-GlcNAc-modified fibrils remain unclear, our in vitro mechanistic studies indicate that heat shock proteins interactions with O-GlcNAc fibril inhibit their seeding activity, suggesting that the O-GlcNAc modification may alter the interactome of the α-synuclein fibrils in ways that lead to reduce seeding activity in vivo. Our results show that posttranslational modifications, such as O-GlcNAc modification, of α-synuclein are key determinants of α-synuclein amyloid strains and pathogenicity.

Cryo-EM confirms a common fibril fold in the heart of four patients with ATTRwt amyloidosis

ATTR amyloidosis results from the conversion of transthyretin into amyloid fibrils that deposit in tissues causing organ failure and death. This conversion is facilitated by mutations in ATTRv amyloidosis, or aging in ATTRwt amyloidosis. ATTRv amyloidosis exhibits extreme phenotypic variability, whereas ATTRwt amyloidosis presentation is consistent and predictable. Previously, we found an unprecedented structural variability in cardiac amyloid fibrils from polyneuropathic ATTRv-I84S patients. In contrast, cardiac fibrils from five genotypically-different patients with cardiomyopathy or mixed phenotypes are structurally homogeneous. To understand fibril structure's impact on phenotype, it is necessary to study the fibrils from multiple patients sharing genotype and phenotype. Here we show the cryo-electron microscopy structures of fibrils extracted from four cardiomyopathic ATTRwt amyloidosis patients. Our study confirms that they share identical conformations with minimal structural variability, consistent with their homogenous clinical presentation. Our study contributes to the understanding of ATTR amyloidosis biopathology and calls for further studies.