Verification of the Intermolecular Parallel β-Sheet in E22K-Aβ42 Aggregates by Solid-State NMR Using Rotational Resonance: Implications for the Supramolecular Arrangement of the Toxic Conformer of Aβ42

Protein aggregation and its affecting mechanisms in neurodegenerative diseases

Protein aggregation serves as a critical pathological marker in a spectrum of neurodegenerative diseases (NDs), including the formation of amyloid β (Aβ) and Tau neurofibrillary tangles in Alzheimer's disease, as well as α-Synuclein (α-Syn) aggregates in Parkinson's disease, Parkinson's disease-related dementia (PDD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). A significant proportion of patients with amyotrophic lateral sclerosis (ALS) exhibit TDP-43 aggregates. Moreover, a confluence of brain protein pathologies, such as Aβ, Tau, α-Syn, and TDP-43, has been identified in individual NDs cases, highlighting the intricate interplay among these proteins that is garnering heightened scrutiny. Importantly, protein aggregation is modulated by an array of factors, with burgeoning evidence suggesting that it frequently results from perturbations in protein homeostasis, influenced by the cellular membrane milieu, metal ion concentrations, post-translational modifications, and genetic mutations. This review delves into the pathological underpinnings of protein aggregation across various NDs and elucidates the intercommunication among disparate proteins within the same disease context. Additionally, we examine the pathogenic mechanisms by which diverse factors impinge upon protein aggregation, offering fresh perspectives for the future therapeutic intervention of NDs.

PITRM1 interaction studies with amyloidogenic nonapeptide mutants of familial Alzheimer's disease

Amyloid β-protein (ABP) is found to be the major cause for the development of neurodegeneration which leads to Alzheimer's. The Aβ nonapeptide segment, QKLVFFAED (amino acids 15-23) is the highly amyloidogenic central region of Aβ. Familial mutation in Aβ increases the aggregation property of the peptide compared to the Native (Wild) amyloid-beta (Aβ) and these mutations fall on the Aβ nonapeptide segment. The catalytic activity of pitrilysin metallopeptidase 1(PITRM1) with familial mutant Aβ (Flemish, Arctic, Dutch, Italian and Iowa) during interaction is examined using molecular dynamic simulation. The molecular dynamics simulation of PITRM1 and the Aβ nonapeptide segment showed similar RMSD with respect to stability. The active site amino acid (AA) H108, hydrophobic pocket AA residues L111, F123, F124, and L127 and the basic pocket AA residues R888 and H896 showed similar interactions with both wild and familial Aβ. The molecular level interaction between amyloid beta and PITRM1 were similar in the wild and familial mutants except for the Arctic mutant. The hydrophobic interaction was commonly observed between the S1 hydrophobic pocket and the LVFF region, the Arctic mutant showed less hydrogen bond formation consistently when compared to other complexes. This molecular information on catalytic activity suggests that modulating inactive PITRM1 or an increase in expression of PITRM1 can help in eliminating different kinds of familial mutant Aβ in neurodegenerative cells.Communicated by Ramaswamy H. Sarma.

Effects of Familial Alzheimer's Disease Mutations on the Assembly of a β-Hairpin Peptide Derived from Aβ16-36

Familial Alzheimer's disease (FAD) is associated with mutations in the β-amyloid peptide (Aβ) or the amyloid precursor protein (APP). FAD mutations of Aβ were incorporated into a macrocyclic peptide that mimics a β-hairpin to study FAD point mutations K16N, A21G, E22Δ, E22G, E22Q, E22K, and L34V and their effect on assembly, membrane destabilization, and cytotoxicity. The X-ray crystallographic structures of the four E22 mutant peptides reveal that the peptides assemble to form the same compact hexamer. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) experiments reveal that the mutant FAD peptides assemble as trimers or hexamers, with peptides that have greater positive charge assembling as more stable hexamers. Mutations that increase the positive charge also increase the cytotoxicity of the peptides and their propensity to destabilize lipid membranes.

Bioactive 3D structures of naturally occurring peptides and their application in drug design

Naturally occurring peptides form unique 3D structures, which are critical for their bioactivities. To gain useful insights into drug design, the relationship between the 3D structure and bioactivity of the peptides has been studied. Solid-state nuclear magnetic resonance (NMR) analysis of the 42-residue amyloid β-protein (Aβ42) suggested the presence of toxic conformers with a turn structure at positions 22 and 23 in the aggregates. Antibodies specific to this turn structure could be utilized for immunotherapy and early diagnosis of Alzheimer's disease. Solution NMR analysis of apratoxin A, a cyclic depsipeptide with potent cytotoxicity, proposed an accurate structural model with an important bend structure, which led to the development of highly active mimetics. X-ray crystal analysis of PF1171F, a cyclic hexapeptide with insecticidal activity, indicated the formation of 4 intramolecular hydrogen bonds, which play an important role in cell membrane permeability of PF1171F.

New diagnostic method for Alzheimer’s disease based on the toxic conformation theory of amyloid β

Recent investigations suggest that soluble oligomeric amyloid β (Aβ) species may be involved in early onset of Alzheimer’s disease (AD). Using systematic proline replacement, solid-state NMR, and ESR, we identified a toxic turn at position 22 and 23 of Aβ42, the most potent neurotoxic Aβ species. Through radicalization, the toxic turn can induce formation of the C-terminal hydrophobic core to obtain putative Aβ42 dimers and trimers. Synthesized dimer and trimer models showed that the C-terminal hydrophobic core plays a critical role in the formation of high molecular weight oligomers with neurotoxicity. Accordingly, an anti-toxic turn antibody (24B3) that selectively recognizes a toxic dimer model of E22P-Aβ42 was developed. Sandwich enzyme-linked immunosorbent assay with 24B3 and 82E1 detected a significantly higher ratio of Aβ42 with a toxic turn to total Aβ42 in cerebrospinal fluid of AD patients compared with controls, suggesting that 24B3 could be useful for early onset of AD diagnosis.

The Phlorotannin-Rich Fraction of Ecklonia cava Extract Attenuated the Expressions of the Markers Related with Inflammation and Leptin Resistance in Adipose Tissue

Obesity is associated with systemic chronic inflammation, and it induces central leptin resistance which blocks the appetite-suppressing effect of leptin and leptin resistance in adipocytes. In the present study, we evaluated the effects of Ecklonia cava extract (ECE), which contained rich phlorotannins, on inflammation and leptin resistance in the adipose tissue of a diet-induced obese model. Effects of ECE on fat deposition, inflammation, M1/M2 macrophage, and T-cell infiltrations were investigated, and leptin resistance and SOCS3 were also measured in adipose tissue. Furthermore, ECE attenuated the expression of inflammation-related receptors such as TLR4 and RAGE and leptin resistance by reducing SOCS3 expression, increasing expression of leptin receptor in adipose tissue, and increasing lipolysis. ECE showed antiadiposity and anti-inflammatory effects, attenuated leptin resistance, and increased lipolysis in the diet-induced obese model. This study shows that ECE is a suitable dietary supplement candidate for the prevention or treatment of obesity or obesity-associated diseases, especially inflammation-related diseases.

Differential effect of amyloid beta peptides on mitochondrial axonal trafficking depends on their state of aggregation and binding to the plasma membrane

Inhibition of mitochondrial axonal trafficking by amyloid beta (Aβ) peptides has been implicated in early pathophysiology of Alzheimer's Disease (AD). Yet, it remains unclear whether the loss of motility inevitably induces the loss of mitochondrial function, and whether restoration of axonal trafficking represents a valid therapeutic target. Moreover, while some investigations identify Aβ oligomers as the culprit of trafficking inhibition, others propose that fibrils play the detrimental role. We have examined the effect of a panel of Aβ peptides with different mutations found in familial AD on mitochondrial motility in primary cortical mouse neurons. Peptides with higher propensity to aggregate inhibit mitochondrial trafficking to a greater extent with fibrils inducing the strongest inhibition. Binding of Aβ peptides to the plasma membrane was sufficient to induce trafficking inhibition where peptides with reduced plasma membrane binding and internalization had lesser effect on mitochondrial motility. We also found that Aβ peptide with Icelandic mutation A673T affects axonal trafficking of mitochondria but has very low rates of plasma membrane binding and internalization in neurons, which could explain its relatively low toxicity. Inhibition of mitochondrial dynamics caused by Aβ peptides or fibrils did not instantly affect mitochondrial bioenergetic and function. Our results support a mechanism where inhibition of axonal trafficking is initiated at the plasma membrane by soluble low molecular weight Aβ species and is exacerbated by fibrils. Since trafficking inhibition does not coincide with the loss of mitochondrial function, restoration of axonal transport could be beneficial at early stages of AD progression. However, strategies designed to block Aβ aggregation or fibril formation alone without ensuring the efficient clearance of soluble Aβ may not be sufficient to alleviate the trafficking phenotype.

Synthetic Models of Quasi-Stable Amyloid β40 Oligomers with Significant Neurotoxicity

The formation of soluble oligomers of amyloid β42 and 40 (Aβ42, Aβ40) is the initial event in the pathogenesis of Alzheimer's disease (AD). Based on previous systematic proline replacement and solid-state NMR, we proposed a toxic dimer structure of Aβ42, a highly aggregative alloform, with a turn at positions 22 and 23, and a hydrophobic core in the C-terminal region. However, in addition to Aβ42, Aβ40 dimers can also contribute to AD progression because of the more abundance of Aβ40 monomer in biological fluids. Here, we describe the synthesis and characterization of three dimer models of the toxic-conformation constrained E22P-Aβ40 using l,l-2,6-diaminopimeric acid (DAP) or l,l-2,8-diaminoazelaic acid (DAZ) linker at position 30, which is incorporated into the intermolecular parallel β-sheet region, and DAP at position 38 in the C-terminal hydrophobic core. E22P-A30DAP-Aβ40 dimer (1) and E22P-A30DAZ-Aβ40 dimer (2) existed mainly in oligomeric states even after 2 weeks incubation without forming fibrils, unlike the corresponding monomer. Their neurotoxicity toward SH-SY5Y neuroblastoma cells was very weak. In contrast, E22P-G38DAP-Aβ40 dimer (3) formed β-sheet-rich oligomeric aggregates, and exhibited more potent neurotoxicity than the corresponding monomer. Ion mobility-mass spectrometry suggested that high molecular-weight oligomers (12-24-mer) of 3 form, but not for 1 and 2 after 4 h incubation. These findings indicate that formation of the hydrophobic core at the C-terminus, rather than intermolecular parallel β-sheet, triggers the formation of toxic Aβ oligomers. Compound 3 may be a suitable model for studying the etiology of Alzheimer's disease.

Taxifolin inhibits amyloid-β oligomer formation and fully restores vascular integrity and memory in cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA) induces various forms of cerebral infarcts and hemorrhages from vascular amyloid-β accumulation, resulting in acceleration of cognitive impairment, which is currently untreatable. Soluble amyloid-β protein likely impairs cerebrovascular integrity as well as cognitive function in early stage Alzheimer’s disease. Taxifolin, a flavonol with strong anti-oxidative and anti-glycation activities, has been reported to disassemble amyloid-β in vitro but the in vivo relevance remains unknown. Here, we investigated whether taxifolin has therapeutic potential in attenuating CAA, hypothesizing that inhibiting amyloid-β assembly may facilitate its clearance through several elimination pathways. Vehicle- or taxifolin-treated Tg-SwDI mice (commonly used to model CAA) were used in this investigation. Cognitive and cerebrovascular function, as well as the solubility and oligomerization of brain amyloid-β proteins, were investigated. Spatial reference memory was assessed by water maze test. Cerebral blood flow was measured with laser speckle flowmetry and cerebrovascular reactivity evaluated by monitoring cerebral blood flow changes in response to hypercapnia. Significantly reduced cerebrovascular pan-amyloid-β and amyloid-β_1-40 accumulation was found in taxifolin-treated Tg-SwDI mice compared to vehicle-treated counterparts (n = 5). Spatial reference memory was severely impaired in vehicle-treated Tg-SwDI mice but normalized after taxifolin treatment, with scoring similar to wild type mice (n = 10–17). Furthermore, taxifolin completely restored decreased cerebral blood flow and cerebrovascular reactivity in Tg-SwDI mice (n = 4–6). An in vitro thioflavin-T assay showed taxifolin treatment resulted in efficient inhibition of amyloid-β_1-40 assembly. In addition, a filter trap assay and ELISA showed Tg-SwDI mouse brain homogenates exhibited significantly reduced levels of amyloid-β oligomers in vivo after taxifolin treatment (n = 4–5), suggesting the effects of taxifolin on CAA are attributable to the inhibition of amyloid-β oligomer formation. In conclusion, taxifolin prevents amyloid-β oligomer assembly and fully sustains cognitive and cerebrovascular function in a CAA model mice. Taxifolin thus appears a promising therapeutic approach for CAA.

Familial Alzheimer's Disease Mutations within the Amyloid Precursor Protein Alter the Aggregation and Conformation of the Amyloid-β Peptide

Most cases of Alzheimer's disease (AD) are sporadic, but a small percentage of AD cases, called familial AD (FAD), are associated with mutations in presenilin 1, presenilin 2, or the amyloid precursor protein. Amyloid precursor protein mutations falling within the amyloid-β (Aβ) sequence lead to a wide range of disease phenotypes. There is increasing evidence that distinct amyloid structures distinguished by amyloid conformation-dependent monoclonal antibodies have similarly distinct roles in pathology. It is possible that this phenotypic diversity of FAD associated with mutations within the Aβ sequence is due to differences in the conformations adopted by mutant Aβ peptides, but the effects of FAD mutations on aggregation kinetics and conformational and morphological changes of the Aβ peptide are poorly defined. To gain more insight into this possibility, we therefore investigated the effects of 11 FAD mutations on the aggregation kinetics of Aβ, as well as its ability to form distinct conformations recognized by a panel of amyloid conformation-specific monoclonal antibodies. We found that most FAD mutations increased the rate of aggregation of Aβ. The FAD mutations also led to the adoption of alternative amyloid conformations distinguished by monoclonal antibodies and resulted in the formation of distinct aggregate morphologies as determined by transmission electron microscopy. In addition, several of the mutant peptides displayed a large reduction in thioflavin T fluorescence, despite forming abundant fibrils indicating that thioflavin T is a probe of conformational polymorphisms rather than a reliable indicator of fibrillization. Taken together, these results indicate that FAD mutations falling within the Aβ sequence lead to dramatic changes in aggregation kinetics and influence the ability of Aβ to form immunologically and morphologically distinct amyloid structures.

Synthesis and characterization of the amyloid β40 dimer model with a linker at position 30 adjacent to the intermolecular β-sheet region

Amyloid fibrils in senile plaque mainly consist of the 40-mer and 42-mer amyloid β-proteins (Aβ40 and Aβ42). Although Aβ42 plays more important role in the pathogenesis of Alzheimer's disease (AD), Aβ40 could be involved in the progression of AD pathology because of its large amount. Recent studies revealed that variable sizes of Aβ oligomers contributed to the neuronal death and cognitive dysfunction. However, how large oligomeric species are responsible for AD pathogenesis remains unclear. We previously proposed a toxic dimer model of Aβ with turn structure at positions 22 and 23 using solid-state NMR and systematic proline replacement. Based on this model, we herein show the synthesis and biological activities of an E22P-Aβ40 dimer at position 30, which was connected to l,l-2,6-diaminopimeric acid. The E22P-Aβ40 dimer formed stable 6∼8-mer oligomers without amyloid fibrils, but was not neurotoxic on human neuroblastoma cells. On the other hand, E22P-Aβ40 generated high molecular-weight oligomers into fibrils, and showed the neurotoxicity. These results suggest that such kind of Aβ40 dimer with a parallel β-sheet might not be pathological.

Sensitive (13)C- (13)C correlation spectra of amyloid fibrils at very high spinning frequencies and magnetic fields

Sensitive 2D solid-state (13)C-(13)C correlation spectra of amyloid β fibrils have been recorded at very fast spinning frequencies and very high magnetic fields. It is demonstrated that PARIS-xy recoupling using moderate rf amplitudes can provide structural information by promoting efficient magnetization transfer even under such challenging experimental conditions. Furthermore, it has been shown both experimentally and by numerical simulations that the method is not very sensitive to dipolar truncation effects and can reveal direct transfer across distances of about 3.5-4 Å.