Transthyretin variants with improved inhibition of β-amyloid aggregation

An in vitro model for vitamin A transport across the human blood-brain barrier

Vitamin A, supplied by the diet, is critical for brain health, but little is known about its delivery across the blood-brain barrier (BBB). Brain microvascular endothelial-like cells (BMECs) differentiated from human-derived induced pluripotent stem cells (iPSCs) form a tight barrier that recapitulates many of the properties of the human BBB. We paired iPSC-derived BMECs with recombinant vitamin A serum transport proteins, retinol-binding protein (RBP), and transthyretin (TTR), to create an in vitro model for the study of vitamin A (retinol) delivery across the human BBB. iPSC-derived BMECs display a strong barrier phenotype, express key vitamin A metabolism markers, and can be used for quantitative modeling of retinol accumulation and permeation. Manipulation of retinol, RBP, and TTR concentrations, and the use of mutant RBP and TTR, yielded novel insights into the patterns of retinol accumulation in, and permeation across, the BBB. The results described herein provide a platform for deeper exploration of the regulatory mechanisms of retinol trafficking to the human brain.

An in vitro model for vitamin A transport across the human blood-brain barrier

Vitamin A, supplied by the diet, is critical for brain health, but little is known about its delivery across the blood-brain barrier (BBB). Brain microvascular endothelial-like cells (BMECs) differentiated from human-derived induced pluripotent stem cells (iPSC) form a tight barrier that recapitulates many of the properties of the human BBB. We paired iPSC-derived BMECs with recombinant vitamin A serum transport proteins, retinol binding protein (RBP) and transthyretin (TTR), to create an in vitro model for the study of vitamin A (retinol) delivery across the human BBB. iPSC-derived BMECs display a strong barrier phenotype, express key vitamin A metabolism markers and can be used for quantitative modeling of retinol accumulation and permeation. Manipulation of retinol, RBP and TTR concentrations, and the use of mutant RBP and TTR, yielded novel insights into the patterns of retinol accumulation in, and permeation across, the BBB. The results described herein provide a platform for deeper exploration of the regulatory mechanisms of retinol trafficking to the human brain.

The Regulatory Mechanism of Transthyretin Irreversible Aggregation through Liquid-to-Solid Phase Transition

Transthyretin (TTR) aggregation and amyloid formation are associated with several ATTR diseases, such as senile systemic amyloidosis (SSA) and familial amyloid polyneuropathy (FAP). However, the mechanism that triggers the initial pathologic aggregation process of TTR remains largely elusive. Lately, increasing evidence has suggested that many proteins associated with neurodegenerative diseases undergo liquid-liquid phase separation (LLPS) and subsequent liquid-to-solid phase transition before the formation of amyloid fibrils. Here, we demonstrate that electrostatic interactions mediate LLPS of TTR, followed by a liquid-solid phase transition, and eventually the formation of amyloid fibrils under a mildly acidic pH in vitro. Furthermore, pathogenic mutations (V30M, R34T, and K35T) of TTR and heparin promote the process of phase transition and facilitate the formation of fibrillar aggregates. In addition, S-cysteinylation, which is a kind of post-translational modification of TTR, reduces the kinetic stability of TTR and increases the propensity for aggregation, while another modification, S-sulfonation, stabilizes the TTR tetramer and reduces the aggregation rate. Once TTR was S-cysteinylated or S-sulfonated, they dramatically underwent the process of phase transition, providing a foundation for post-translational modifications that could modulate TTR LLPS in the context of pathological interactions. These novel findings reveal molecular insights into the mechanism of TTR from initial LLPS and subsequent liquid-to-solid phase transition to amyloid fibrils, providing a new dimension for ATTR therapy.

Endogenous Human Proteins Interfering with Amyloid Formation

Amyloid formation is a pathological process associated with a wide range of degenerative disorders, including Alzheimer’s disease, Parkinson’s disease, and diabetes mellitus type 2. During disease progression, abnormal accumulation and deposition of proteinaceous material are accompanied by tissue degradation, inflammation, and dysfunction. Agents that can interfere with the process of amyloid formation or target already formed amyloid assemblies are consequently of therapeutic interest. In this context, a few endogenous proteins have been associated with an anti-amyloidogenic activity. Here, we review the properties of transthyretin, apolipoprotein E, clusterin, and BRICHOS protein domain which all effectively interfere with amyloid in vitro, as well as displaying a clinical impact in humans or animal models. Their involvement in the amyloid formation process is discussed, which may aid and inspire new strategies for therapeutic interventions.

Neuroserpin and transthyretin are extracellular chaperones that preferentially inhibit amyloid formation

Neuroserpin is a secreted protease inhibitor known to inhibit amyloid formation by the Alzheimer’s beta peptide (Aβ). To test whether this effect was constrained to Aβ, we used a range of in vitro assays to demonstrate that neuroserpin inhibits amyloid formation by several different proteins and protects against the associated cytotoxicity but, unlike other known chaperones, has a poor ability to inhibit amorphous protein aggregation. Collectively, these results suggest that neuroserpin has an unusual chaperone selectivity for intermediates on the amyloid-forming pathway. Bioinformatics analyses identified a highly conserved 14-residue region containing an α helix shared between neuroserpin and the thyroxine-transport protein transthyretin, and we subsequently demonstrated that transthyretin also preferentially inhibits amyloid formation. Last, we used rationally designed neuroserpin mutants to demonstrate a direct involvement of the conserved 14-mer region in its chaperone activity. Identification of this conserved region may prove useful in the future design of anti-amyloid reagents.

Mechanisms of Transthyretin Inhibition of IAPP Amyloid Formation

Amyloid-formation by the islet amyloid polypeptide (IAPP), produced by the β-cells in the human pancreas, has been associated with the development of type II diabetes mellitus (T2DM). The human plasma-protein transthyretin (TTR), a well-known amyloid-inhibiting protein, is interestingly also expressed within the IAPP producing β-cells. In the present study, we have characterized the ability of TTR to interfere with IAPP amyloid-formation, both in terms of its intrinsic stability as well as with regard to the effect of TTR-stabilizing drugs. The results show that TTR can prolong the lag-phase as well as impair elongation in the course of IAPP-amyloid formation. We also show that the interfering ability correlates inversely with the thermodynamic stability of TTR, while no such correlation was observed as a function of kinetic stability. Furthermore, we demonstrate that the ability of TTR to interfere is maintained also at the low pH environment within the IAPP-containing granules of the pancreatic β-cells. However, at both neutral and low pH, the addition of TTR-stabilizing drugs partly impaired its efficacy. Taken together, these results expose mechanisms of TTR-mediated inhibition of IAPP amyloid-formation and highlights a potential therapeutic target to prevent the onset of T2DM.

The interaction of zinc with the multi-functional plasma thyroid hormone distributor protein, transthyretin: evolutionary and cross-species comparative aspects

A considerable body of evidence has been accumulated showing the interrelationship between zinc and the plasma thyroid hormone (TH) distributor protein, transthyretin (TTR). TTR is a multi-functional protein, which emerged from 5-hydroxyisourate hydrolase (HIUHase) by neo-functionalization after gene duplication during early chordate evolution. HIUHase is also a zinc-binding protein. Most biochemical and molecular biological findings have been obtained from mammalian studies. However, in the past two decades, it has become clear that fish TTR displays zinc-dependent TH binding. After a brief introduction on plasma zinc, THs and their binding proteins, this review will focus on the role of zinc in TTR functions of various vertebrates. In particular primitive fish TTR has an extremely high zinc content, with an increased number of histidine residues which are involved in TH binding. However, zinc-dependent TH binding may have been gradually lost from TTRs during higher vertebrate evolution. Although human TTR has a low zinc content, zinc plays an essential role in TTR functions other than TH binding: the stability of TTR-holo retinol binding protein 4 (holoRBP4) complex, TTR amyloidogenesis, the sequestration of amyloid β (Aβ) fibrils and cryptic proteolytic activity. The interaction of TTR with metallothioneins may be a critical step in the exertion of some of these functions. Evolutionary and physiological insights on zinc-dependent functions of TTRs are also discussed.

Apolipoprotein E Interferes with IAPP Aggregation and Protects Pericytes from IAPP-Induced Toxicity

Apolipoprotein E (ApoE) has become a primary focus of research after the discovery of its strong linkage to Alzheimer’s disease (AD), where the ApoE4 variant is the highest genetic risk factor for this disease. ApoE is commonly found in amyloid deposits of different origins, and its interaction with amyloid-β peptide (Aβ), the hallmark of AD, is well known. However, studies on the interaction of ApoEs with other amyloid-forming proteins are limited. Islet amyloid polypeptide (IAPP) is an amyloid-forming peptide linked to the development of type-2 diabetes and has also been shown to be involved in AD pathology and vascular dementia. Here we studied the impact of ApoE on IAPP aggregation and IAPP-induced toxicity on blood vessel pericytes. Using both in vitro and cell-based assays, we show that ApoE efficiently inhibits the amyloid formation of IAPP at highly substoichiometric ratios and that it interferes with both nucleation and elongation. We also show that ApoE protects the pericytes against IAPP-induced toxicity, however, the ApoE4 variant displays the weakest protective potential. Taken together, our results suggest that ApoE has a generic amyloid-interfering property and can be protective against amyloid-induced cytotoxicity, but there is a loss of function for the ApoE4 variant.

Morphological analysis of Apolipoprotein E binding to Aβ Amyloid using a combination of Surface Plasmon Resonance, Immunogold Labeling and Scanning Electron Microscopy

Background

Immunogold labeling in combination with transmission electron microscopy analysis is a technique frequently used to correlate high-resolution morphology studies with detailed information regarding localization of specific antigens. Although powerful, the methodology has limitations and it is frequently difficult to acquire a stringent system where unspecific low-affinity interactions are removed prior to analysis.

Results

We here describe a combinatorial strategy where surface plasmon resonance and immunogold labeling are used followed by a direct analysis of the sensor-chip surface by scanning electron microscopy. Using this approach, we have probed the interaction between amyloid-β fibrils, associated to Alzheimer’s disease, and apolipoprotein E, a well-known ligand frequently found co-deposited to the fibrillar form of Aβ in vivo. The results display a lateral binding of ApoE along the amyloid fibrils and illustrates how the gold-beads represent a good reporter of the binding.

Conclusions

This approach exposes a technique with generic features which enables both a quantitative and a morphological evaluation of a ligand-receptor based system. The methodology mediates an advantage compared to traditional immunogold labeling since all washing steps can be monitored and where a high stringency can be maintained throughout the experiment.

Plasma Transthyretin as a Predictor of Amnestic Mild Cognitive Impairment Conversion to Dementia

Amnestic mild cognitive impairment (MCI) is a prodromal stage of dementia, with a higher incidence of these patients progressing to Alzheimer’s disease (AD) than normal aging people. A biomarker for the early detection and prediction for this progression is important. We recruited MCI subjects in three teaching hospitals and conducted longitudinal follow-up for 5 years at one-year intervals. Cognitively healthy controls were recruited for comparisom at baseline. Plasma transthyretin (TTR) levels were measured by ELISA. Survival analysis with time to AD conversion as an outcome variable was calculated with the multivariable Cox proportional hazards models using TTR as a continuous variable with adjustment for other covariates and bootstrapping resampling analysis. In total, 184 MCI subjects and 40 sex- and age-matched controls were recruited at baseline. At baseline, MCI patients had higher TTR levels compared with the control group. During the longitudinal follow-ups, 135 MCI patients (73.4%) completed follow-up at least once. The TTR level was an independent predictor for MCI conversion to AD when using TTR as a continuous variable (p = 0.023, 95% CI 1.001–1.007). In addition, in MCI converters, the TTR level at the point when they converted to AD was significantly lower than that at baseline (328.6 ± 66.5 vs. 381.9 ± 77.6 ug/ml, p < 0.001). Our study demonstrates the temporal relationship between the plasma TTR level and the conversion from MCI to AD.

Apolipoprotein E impairs amyloid-β fibril elongation and maturation

Alzheimer's disease (AD) is strongly linked to amyloid depositions of the Aβ peptide (Aβ). The lipid-binding protein apolipoprotein E (ApoE) has been found to interfere with Aβ amyloid formation and to exert a strong clinical impact to the pathology of AD. The APOE gene exists in three allelic isoforms represented by APOE ε2, APOE ε3, and APOE ε4. Carriers of the APOE ε4 variant display a gene dose-dependent increased risk of developing the disease. Aβ amyloids are formed via a nucleation-dependent mechanism where free monomers are added onto a nucleus in a template-dependent manner. Using a combination of surface plasmon resonance and thioflavin-T assays, we here show that ApoE can target the process of fibril elongation and that its interference effectively prevents amyloid maturation. We expose a complex equilibrium where the concentration of ApoE, Aβ monomers, and the amount of already formed Aβ fibrils will affect the relative proportion and formation rate of mature amyloids versus alternative assemblies. The result illustrates a mechanism which may affect both the clearance rate of Aβ assemblies in vivo and the population of cytotoxic Aβ assemblies.

Self-Assembled Curcumin-Poly(carboxybetaine methacrylate) Conjugates: Potent Nano-Inhibitors against Amyloid β-Protein Fibrillogenesis and Cytotoxicity

Fibrillogenesis of amyloid β-protein (Aβ) is a pathological hallmark of Alzheimer's disease, so inhibition of Aβ aggregation is considered as an important strategy for the precaution and treatment of AD. Curcumin (Cur) has been recognized as an effective inhibitor of Aβ fibrillogenesis, but its potential application is limited by its poor bioavailability. Herein, we proposed to conjugate Cur to a zwitterionic polymer, poly(carboxybetaine methacrylate) (pCB), and synthesized three Cur@pCB conjugates of different degrees of substitution (DS, 1.9-2.9). Cur@pCB conjugates self-assembled into nanogels of 120-190 nm. The inhibition effects of Cur@pCB conjugates on the fibrillation and cytotoxicity of Aβ₄₂ was investigated by extensive biophysical and biological analyses. Thioflavin T fluorescence assays and atomic force microscopic observations revealed that the Cur@pCB conjugates were much more efficient than molecular curcumin on inhibiting Aβ₄₂ fibrillation, and cytotoxicity assays also indicated the same tendency. Of the three conjugates, Cur1@pCB of the lowest DS (1.97) exhibited the best performance; 5 μM Cur1@pCB functioned similarly with 25 μM free curcumin. Moreover, 5 μM Cur1@pCB increased the cell viability by 43% but free curcumin at the same concentration showed little effect. It is considered that the highly hydrated state of the zwitterionic polymers resulted in the superiority of Cur@pCB over free curcumin. Namely, the dense hydration layer on the conjugates strongly stabilized the bound Aβ on curcumin anchored on the polymer, suppressing the conformational transition of the protein to β-sheet-rich structures. This was demonstrated by circular dichroism spectroscopy, in which Cur1@pCB was proven to be the strongest in the three conjugates. The research has thus revealed a new function of zwitterionic polymer pCBMA and provided new insights into the development of more potent nanoinhibitors for suppressing Aβ fibrillogenesis and cytotoxicity.

Copper mediated amyloid-β binding to Transthyretin

Transthyretin (TTR), a homotetrameric protein that transports thyroxine and retinol both in plasma and in cerebrospinal (CSF) fluid provides a natural protective response against Alzheimer's disease (AD), modulates amyloid-β (Aβ) deposition by direct interaction and co-localizes with Aβ in plaques. TTR levels are lower in the CSF of AD patients. Zn²⁺, Mn²⁺ and Fe²⁺ transform TTR into a protease able to cleave Aβ. To explain these activities, monomer dissociation or conformational changes have been suggested. Here, we report that when TTR crystals are exposed to copper or iron salts, the tetramer undergoes a significant conformational change that alters the dimer-dimer interface and rearranges residues implicated in TTR's ability to neutralize Aβ. We also describe the conformational changes in TTR upon the binding of the various metal ions. Furthermore, using bio-layer interferometry (BLI) with immobilized Aβ(1-28), we observe the binding of TTR only in the presence of copper. Such Cu²⁺-dependent binding suggests a recognition mechanism whereby Cu²⁺ modulates both the TTR conformation, induces a complementary Aβ structure and may participate in the interaction. Cu²⁺-soaked TTR crystals show a conformation different from that induced by Fe²⁺, and intriguingly, TTR crystals grown in presence of Aβ(1-28) show different positions for the copper sites from those grown its absence.

Retinol-Binding Protein Interferes with Transthyretin-Mediated β-Amyloid Aggregation Inhibition

β-Amyloid (Aβ) aggregation is causally linked to Alzheimer's disease. On the basis of in vitro and transgenic animal studies, transthyretin (TTR) is hypothesized to provide neuroprotection against Aβ toxicity by binding to Aβ and inhibiting its aggregation. TTR is a homotetrameric protein that circulates in blood and cerebrospinal fluid; its normal physiological role is as a carrier for thyroxine and retinol-binding protein (RBP). RBP forms a complex with retinol, and the holoprotein (hRBP) binds with high affinity to TTR. In this study, the role of TTR ligands in TTR-mediated inhibition of Aβ aggregation was investigated. hRBP strongly reduced the ability of TTR to inhibit Aβ aggregation. The effect was not due to competition between Aβ and hRBP for binding to TTR, as Aβ bound equally well to TTR-hRBP complexes and TTR. hRBP is known to stabilize the TTR tetrameric structure. We show that Aβ partially destabilizes TTR and that hRBP counteracts this destabilization. Taken together, our results support a mechanism wherein TTR-mediated inhibition of Aβ aggregation requires not only TTR-Aβ binding but also destabilization of TTR quaternary structure.

Inhibition of amyloid beta fibril formation by monomeric human transthyretin

Transthyretin (TTR) is a homotetrameric protein that is found in the plasma and cerebrospinal fluid. Dissociation of TTR tetramers sets off a downhill cascade of amyloid formation through polymerization of monomeric TTR. Interestingly, TTR has an additional, biologically relevant activity, which pertains to its ability to slow the progression of amyloid beta (Aβ) associated pathology in transgenic mice. In vitro, both TTR and a kinetically stable variant of monomeric TTR (M-TTR) inhibit the fibril formation of Aβ1-40/42 molecules. Published evidence suggests that tetrameric TTR binds preferentially to Aβ monomers, thus destabilizing fibril formation by depleting the pool of Aβ monomers from aggregating mixtures. Here, we investigate the effects of M-TTR on the in vitro aggregation of Aβ1-42 . Our data confirm previous observations that fibril formation of Aβ is suppressed in the presence of sub-stoichiometric amounts of M-TTR. Despite this, we find that sub-stoichiometric levels of M-TTR are not bona fide inhibitors of aggregation. Instead, they co-aggregate with Aβ to promote the formation of large, micron-scale insoluble, non-fibrillar amorphous deposits. Based on fluorescence correlation spectroscopy measurements, we find that M-TTR does not interact with monomeric Aβ. Two-color coincidence analysis of the fluorescence bursts of Aβ and M-TTR labeled with different fluorophores shows that M-TTR co-assembles with soluble Aβ aggregates and this appears to drive the co-aggregation into amorphous precipitates. Our results suggest that mimicking the co-aggregation activity with protein-based therapeutics might be a worthwhile strategy for rerouting amyloid beta peptides into inert, insoluble, and amorphous deposits.

Transthyretin Interferes with Aβ Amyloid Formation by Redirecting Oligomeric Nuclei into Non-Amyloid Aggregates

The pathological Aβ aggregates associated with Alzheimer's disease follow a nucleation-dependent path of formation. A nucleus represents an oligomeric assembly of Aβ peptides that acts as a template for subsequent incorporation of monomers to form a fibrillar structure. Nuclei can form de novo or via surface-catalyzed secondary nucleation, and the combined rates of elongation and nucleation control the overall rate of fibril formation. Transthyretin (TTR) obstructs Aβ fibril formation in favor of alternative non-fibrillar assemblies, but the mechanism behind this activity is not fully understood. This study shows that TTR does not significantly disturb fibril elongation; rather, it effectively interferes with the formation of oligomeric nuclei. We demonstrate that this interference can be modulated by altering the relative contribution of elongation and nucleation, and we show how TTR's effects can range from being essentially ineffective to almost complete inhibition of fibril formation without changing the concentration of TTR or monomeric Aβ.

Inhibition of curli assembly and Escherichia coli biofilm formation by the human systemic amyloid precursor transthyretin

During biofilm formation, Escherichia coli and other Enterobacteriaceae produce an extracellular matrix consisting of curli amyloid fibers and cellulose. The precursor of curli fibers is the amyloidogenic protein CsgA. The human systemic amyloid precursor protein transthyretin (TTR) is known to inhibit amyloid-β (Aβ) aggregation in vitro and suppress the Alzheimer's-like phenotypes in a transgenic mouse model of Aβ deposition. We hypothesized that TTR might have broad antiamyloid activity because the biophysical properties of amyloids are largely conserved across species and kingdoms. Here, we report that both human WT tetrameric TTR (WT-TTR) and its engineered nontetramer-forming monomer (M-TTR, F87M/L110M) inhibit CsgA amyloid formation in vitro, with M-TTR being the more efficient inhibitor. Preincubation of WT-TTR with small molecules that occupy the T4 binding site eliminated the inhibitory capacity of the tetramer; however, they did not significantly compromise the ability of M-TTR to inhibit CsgA amyloidogenesis. TTR also inhibited amyloid-dependent biofilm formation in two different bacterial species with no apparent bactericidal or bacteriostatic effects. These discoveries suggest that TTR is an effective antibiofilm agent that could potentiate antibiotic efficacy in infections associated with significant biofilm formation.

Transthyretin and BRICHOS: The Paradox of Amyloidogenic Proteins with Anti-Amyloidogenic Activity for Aβ in the Central Nervous System

Amyloid fibrils are physiologically insoluble biophysically specific β-sheet rich structures formed by the aggregation of misfolded proteins. In vivo tissue amyloid formation is responsible for more than 30 different disease states in humans and other mammals. One of these, Alzheimer's disease (AD), is the most common form of human dementia for which there is currently no definitive treatment. Amyloid fibril formation by the amyloid β-peptide (Aβ) is considered to be an underlying cause of AD, and strategies designed to reduce Aβ production and/or its toxic effects are being extensively investigated in both laboratory and clinical settings. Transthyretin (TTR) and proteins containing a BRICHOS domain are etiologically associated with specific amyloid diseases in the CNS and other organs. Nonetheless, it has been observed that TTR and BRICHOS structures are efficient inhibitors of Aβ fibril formation and toxicity in vitro and in vivo, raising the possibility that some amyloidogenic proteins, or their precursors, possess properties that may be harnessed for combating AD and other amyloidoses. Herein, we review properties of TTR and the BRICHOS domain and discuss how their abilities to interfere with amyloid formation may be employed in the development of novel treatments for AD.

Cerebrospinal Fluid Proteins as Regulators of Beta-amyloid Aggregation and Toxicity

Amyloid disorders, such as Alzheimer's, are almost invariably late-onset diseases. One defining diagnostic feature of Alzheimer's disease is the deposition of beta-amyloid as extracellular plaques, primarily in the hippocampus. This raises the question: are there natural protective agents that prevent beta-amyloid from depositing, and is it loss of this protection that leads to onset of disease? Proteins in cerebrospinal fluid (CSF) have been suggested to act as just such natural protective agents. Here, we describe some of the early evidence that led to this suggestion, and we discuss, in greater detail, two CSF proteins that have garnered the bulk of the attention.