Computational insights into the cross-talk between medin and Aß: implications for age-related vascular risk factors in Alzheimer's disease.

The aggregation of medin forming aortic medial amyloid is linked to arterial wall degeneration and cerebrovascular dysfunction. Elevated levels of arteriolar medin are correlated with an increased presence of vascular amyloid-ß (Aß) aggregates, a hallmark of Alzheimer's disease (AD) and vascular dementia. The cross-interaction between medin and Aß results in the formation of heterologous fibrils through co-aggregation and cross-seeding processes both in vitro and in vivo. However, a comprehensive molecular understanding of the cross-interaction between medin and Aß-two intrinsically disordered proteins-is critically lacking. Here, we employed atomistic discrete molecular dynamics simulations to systematically investigate the self-association, co-aggregation and also the phenomenon of cross-seeding between these two proteins. Our results demonstrated that both Aß and medin were aggregation prone and their mixture tended to form ß-sheet-rich hetero-aggregates. The formation of Aß-medin hetero-aggregates did not hinder Aß and medin from recruiting additional Aß and medin peptides to grow into larger ß-sheet-rich aggregates. The ß-barrel oligomer intermediates observed in the self-aggregations of Aß and medin were also present during their co-aggregation. In cross-seeding simulations, preformed Aß fibrils could recruit isolated medin monomers to form elongated ß-sheets. Overall, our comprehensive simulations suggested that the cross-interaction between Aß and medin may contribute to their pathological aggregation, given the inherent amyloidogenic tendencies of both medin and Aß. Targeting medin, therefore, could offer a novel therapeutic approach to preserving brain function during aging and AD by improving vascular health.

Deciphering the influence of Y12L and N17H substitutions on the conformation and oligomerization of human calcitonin.

The abnormal aggregation of human calcitonin (hCT) hormone peptides impairs their physiological function, leading to harmful immune responses and cytotoxicity, which limits their clinical utility. Interestingly, a representative hCT analog incorporating Y12L and N17H substitutions (DM-hCT) has shown reduced aggregation tendencies while maintaining bioactivity. But the molecular mechanism of Y12L and N17H substitutions on the conformational dynamics of hCT remains unclear. Here, we systematically investigated the folding and self-assembly dynamics of hCT and DM-hCT using atomistic discrete molecular dynamics (DMD) simulations. Our findings revealed that hCT monomers predominantly adopted unstructured conformations with dynamic helices. Oligomerization of hCT resulted in the formation of ß-sheet-rich aggregates and ß-barrel intermediates. The Y12L and N17H substitutions enhanced helical conformations and suppressed ß-sheet formation in both monomers and oligomers. These substitutions stabilized the dynamic helices and disrupted aromatic interactions responsible for ß-sheet formation at residue 12. Notably, DM-hCT assemblies still exhibited ß-sheets in phenylalanine-rich and C-terminal hydrophobic regions, suggesting that future optimizations should focus on these areas. Our simulations provide insights into the molecular mechanisms underlying hCT aggregation and the amyloid-resistant effects of Y12L and N17H substitutions. These findings have valuable implications for the development of clinical hCT analogs.

Molecular Insights into the Differential Effects of Acetylation on the Aggregation of Tau Microtubule-Binding Repeats.

Aggregation of tau protein into intracellular fibrillary inclusions is characterized as the hallmark of tauopathies, including Alzheimer's disease and chronic traumatic encephalopathy. The microtubule-binding (MTB) domain of tau, containing either three or four repeats with sequence similarities, plays an important role in determining tau's aggregation. Previous studies have reported that abnormal acetylation of lysine residues displays a distinct effect on the formation of pathological tau aggregates. However, the underlying molecular mechanism remains mostly elusive. In this study, we performed extensive replica exchange molecular dynamics (REMD) simulations of 144 µs in total to systematically investigate the dimerization of four tau MTB repeats and explore the impacts of Lys280 (K280) or Lys321 (K321) acetylation on the conformational ensembles of the R2 or R3 dimer. Our results show that R3 is the most prone to aggregation among the four repeats, followed by R2 and R4, while R1 displays the weakest aggregation propensity with a disordered structure. Acetylation of K280 could promote the aggregation of R2 peptides by increasing the formation of ß-sheet structures and strengthening the interchain interaction. However, K321 acetylation decreases the ß-sheet content of the R3 dimer, reduces the ability of R3 peptides to form long ß-strands, and promotes the stable helix structure formation. The salt bridge and Y310-Y310 π-π stacking interactions of the R3 dimer are greatly weakened by K321 acetylation, resulting in the inhibition of dimerization. This study uncovers the structural ensembles of tau MTB repeats and provides mechanistic insights into the influences of acetylation on tau aggregation, which may deepen the understanding of the pathogenesis of tauopathies.

Molecular Insights into the Effects of F16L and F19L Substitutions on the Conformation and Aggregation Dynamics of Human Calcitonin.

Human calcitonin (hCT) regulates calcium-phosphorus metabolism, but its amyloid aggregation disrupts physiological activity, increases thyroid carcinoma risk, and hampers its clinical use for bone-related diseases like osteoporosis and Paget's disease. Improving hCT with targeted modifications to mitigate amyloid formation while maintaining its function holds promise as a strategy. Understanding how each residue in hCT's amyloidogenic core affects its structure and aggregation dynamics is crucial for designing effective analogues. Mutants F16L-hCT and F19L-hCT, where Phe residues in the core are replaced with Leu as in nonamyloidogenic salmon calcitonin, showed different aggregation kinetics. However, the molecular effects of these substitutions in hCT are still unclear. Here, we systematically investigated the folding and self-assembly conformational dynamics of hCT, F16L-hCT, and F19L-hCT through multiple long-time scale independent atomistic discrete molecular dynamics (DMD) simulations. Our results indicated that the hCT monomer primarily assumed unstructured conformations with dynamic helices around residues 4-12 and 14-21. During self-assembly, the amyloidogenic core of hCT14-21 converted from dynamic helices to ß-sheets. However, substituting F16L did not induce significant conformational changes, as F16L-hCT exhibited characteristics similar to those of wild-type hCT in both monomeric and oligomeric states. In contrast, F19L-hCT exhibited substantially more helices and fewer ß-sheets than did hCT, irrespective of their monomers or oligomers. The substitution of F19L significantly enhanced the stability of the helical conformation for hCT14-21, thereby suppressing the helix-to-ß-sheet conformational conversion. Overall, our findings elucidate the molecular mechanisms underlying hCT aggregation and the effects of F16L and F19L substitutions on the conformational dynamics of hCT, highlighting the critical role of F19 as an important target in the design of amyloid-resistant hCT analogs for future clinical applications.

Computational Investigation of Coaggregation and Cross-Seeding between Aß and hIAPP Underpinning the Cross-Talk in Alzheimer's Disease and Type 2 Diabetes.

The coexistence of amyloid-ß (Aß) and human islet amyloid polypeptide (hIAPP) in the brain and pancreas is associated with an increased risk of Alzheimer's disease (AD) and type 2 diabetes (T2D) due to their coaggregation and cross-seeding. Despite this, the molecular mechanisms underlying their interaction remain elusive. Here, we systematically investigated the cross-talk between Aß and hIAPP using atomistic discrete molecular dynamics (DMD) simulations. Our results revealed that the amyloidogenic core regions of both Aß (Aß10-21 and Aß30-41) and hIAPP (hIAPP8-20 and hIAPP22-29), driving their self-aggregation, also exhibited a strong tendency for cross-interaction. This propensity led to the formation of ß-sheet-rich heterocomplexes, including potentially toxic ß-barrel oligomers. The formation of Aß and hIAPP heteroaggregates did not impede the recruitment of additional peptides to grow into larger aggregates. Our cross-seeding simulations demonstrated that both Aß and hIAPP fibrils could mutually act as seeds, assisting each other's monomers in converting into ß-sheets at the exposed fibril elongation ends. The amyloidogenic core regions of Aß and hIAPP, in both oligomeric and fibrillar states, exhibited the ability to recruit isolated peptides, thereby extending the ß-sheet edges, with limited sensitivity to the amino acid sequence. These findings suggest that targeting these regions by capping them with amyloid-resistant peptide drugs may hold potential as a therapeutic approach for addressing AD, T2D, and their copathologies.

Direct Observation of Seeded Conformational Conversion of hIAPP In Silico Reveals the Mechanisms for Morphological Dependence and Asymmetry of Fibril Growth.

Rapid growth of amyloid fibrils via a seeded conformational conversion of monomers is a critical step of fibrillization and important for disease transmission and progression. Amyloid fibrils often display diverse morphologies with distinct populations, and yet the molecular mechanisms of fibril elongation and their corresponding morphological dependence remain poorly understood. Here, we computationally investigated the single-molecular growth of two experimentally resolved human islet amyloid polypeptide fibrils of different morphologies. In both cases, the incorporation of monomers into preformed fibrils was observed. The conformational conversion dynamics was characterized by a small number of fibril growth intermediates. Fibril morphology affected monomer binding at fibril elongation and lateral surfaces as well as the seeded conformational conversion dynamics at the fibril ends, resulting in different fibril elongation rates and populations. We also observed an asymmetric fibril growth as in our prior experiments, attributing to differences of two fibril ends in terms of their local surface curvatures and exposed hydrogen-bond donors and acceptors. Together, our mechanistic findings afforded a theoretical basis for delineating different amyloid strains-entailed divergent disease progression.

Structural Perturbation of Monomers Determines the Amyloid Aggregation Propensity of Calcitonin Variants.

Human calcitonin (hCT) is a polypeptide hormone that participates in calcium-phosphorus metabolism. Irreversible aggregation of 32-amino acid hCT into ß-sheet-rich amyloid fibrils impairs physiological activity and increases the risk of medullary carcinoma of the thyroid. Amyloid-resistant hCT derivatives substituting critical amyloidogenic residues are of particular interest for clinical applications as therapeutic drugs against bone-related diseases. Uncovering the aggregation mechanism of hCT at the molecular level, therefore, is important for the design of amyloid-resistant hCT analogues. Here, we investigated the aggregation dynamics of hCT, non-amyloidogenic salmon calcitonin (sCT), and two hCT analogues with reduced aggregation tendency─TL-hCT and phCT─using long timescale discrete molecular dynamics simulations. Our results showed that hCT monomers mainly adopted unstructured conformations with dynamically formed helices around the central region. hCT self-assembled into helix-rich oligomers first, followed by a conformational conversion into ß-sheet-rich oligomers with ß-sheets formed by residues 10-30 and stabilized by aromatic and hydrophobic interactions. Our simulations confirmed that TL-hCT and phCT oligomers featured more helices and fewer ß-sheets than hCT. Substitution of central aromatic residues with leucine in TL-hCT and replacing C-terminal hydrophobic residue with hydrophilic amino acid in phCT only locally suppressed ß-sheet propensities in the central region and C-terminus, respectively. Having mutations in both central and C-terminal regions, sCT monomers and dynamically formed oligomers predominantly adopted helices, confirming that both central aromatic and C-terminal hydrophobic residues played important roles in the fibrillization of hCT. We also observed the formation of ß-barrel intermediates, postulated as the toxic oligomers in amyloidosis, for hCT but not for sCT. Our computational study depicts a complete picture of the aggregation dynamics of hCT and the effects of mutations. The design of next-generation amyloid-resistant hCT analogues should consider the impact on both amyloidogenic regions and also take into account the amplification of transient ß-sheet population in monomers upon aggregation.

Dissecting the Self-assembly Dynamics of Imperfect Repeats in α-Synuclein.

The pathological aggregation of α-synuclein (αS) into amyloid fibrils is the hallmark of Parkinson's disease (PD). The self-assembly and membrane interactions of αS are mainly governed by the seven imperfect 11-residue repeats of the XKTKEGVXXXX motif around residues 1-95. However, the particular role of each repeat in αS fibrillization remains unclear. To answer this question, we studied the aggregation dynamics of each repeat with up to 10 peptides in silico by conducting multiple independent micro-second atomistic discrete molecular dynamics simulations. Our simulations revealed that only repeats R3 and R6 readily self-assembled into ß-sheet-rich oligomers, while the other repeats remained as unstructured monomers with weak self-assembly and ß-sheet propensities. The self-assembly process of R3 featured frequent conformational changes with ß-sheet formation mainly in the non-conserved hydrophobic tail, whereas R6 spontaneously self-assembled into extended and stable cross-ß structures. These results of seven repeats are consistent with their structures and organization in recently solved αS fibrils. As the primary amyloidogenic core, R6 was buried inside the central cross-ß core of all αS fibrils, attracting the hydrophobic tails of adjacent R4, R5, and R7 repeats forming ß-sheets around R6 in the core. Further away from R6 in the sequence but with a moderate amyloid aggregation propensity, the R3 tail could serve as a secondary amyloidogenic core and form independent ß-sheets in the fibril. Overall, our results demonstrate the critical role of R3 and R6 repeats in αS amyloid aggregation and suggest their potential as targets for the peptide-based and small-molecule amyloid inhibitors.

Unveiling Medin Folding and Dimerization Dynamics and Conformations via Atomistic Discrete Molecular Dynamics Simulations.

Medin is a principal component of localized amyloid found in the vasculature of individuals over 50 years old. Its amyloid aggregation has been linked to endothelial dysfunction and vascular inflammation, contributing to the pathogenesis of various vascular diseases. Despite its significance, the structures of the medin monomer, oligomer, and fibril remain elusive, and the dynamic processes of medin aggregation are not fully understood. In this study, we comprehensively investigated the medin folding and dimerization dynamics and conformations using atomistic discrete molecular dynamics simulations. Our simulation results suggested that the folding initiation of the medin involved the formation of ß-sheets around medin30-41 and medin42-50, with subsequent capping of other segments to their ß-sheet edges. Medin monomers typically consisted of three or four ß-strands, along with a dynamic N-terminal helix. Two isolated medin peptides readily aggregated into a ß-sheet-rich dimer, displaying a strong aggregation propensity. Dimerization of medin not only enhanced the ß-sheet conformations but also led to the formation of ß-barrel oligomers. The aggregation tendencies of medin1-18 and medin19-29 were relatively weak. However, the segments of medin30-41 and medin42-50 played a crucial role as they primarily formed a ß-sheet core and facilitated medin1-18 and medin19-29 to form intra- and interpeptide ß-sheets. The findings highlight the critical role of the medin30-41 and medin42-50 regions in stabilizing the monomer structure and driving the medin amyloid aggregation. These regions could potentially serve as promising targets for designing antiamyloid inhibitors against amyloid aggregation of medin. Additionally, our study provides a full picture of the monomer conformations and dimerization dynamics for medin, which will help better understand the pathology of medin aggregation.

SEVI Inhibits Aß Amyloid Aggregation by Capping the ß-Sheet Elongation Edges.

Inhibiting the aggregation of amyloid peptides with endogenous peptides has broad interest due to their intrinsically high biocompatibility and low immunogenicity. Here, we investigated the inhibition mechanism of the prostatic acidic phosphatase fragment SEVI (semen-derived enhancer of viral infection) against Aß42 fibrillization using atomistic discrete molecular dynamic simulations. Our result revealed that SEVI was intrinsically disordered with dynamic formation of residual helices. With a high positive net charge, the self-aggregation tendency of SEVI was weak. Aß42 had a strong aggregation propensity by readily self-assembling into ß-sheet-rich aggregates. SEVI preferred to interact with Aß42, rather than SEVI themselves. In the heteroaggregates, Aß42 mainly adopted ß-sheets buried inside and capped by SEVI in the outer layer. SEVI could bind to various Aß aggregation species─including monomers, dimers, and proto-fibrils─by capping the exposed ß-sheet elongation edges. The aggregation processes Aß42 from the formation of oligomers to conformational nucleation into fibrils and fibril growth should be inhibited as their ß-sheet elongation edges are being occupied by the highly charged SEVI. Overall, our computational study uncovered the molecular mechanism of experimentally observed inhibition of SEVI against Aß42 aggregation, providing novel insights into the development of therapeutic strategies against Alzheimer's disease.

Co-aggregation of α-synuclein with amyloid-ß stabilizes ß-sheet-rich oligomers and enhances the formation of ß-barrels.

Alzheimer's disease (AD) and Parkinson's disease (PD) are the two most common neurodegenerative diseases with markedly different pathological features of ß-amyloid (Aß) plaques and α-synuclein (αS) Lewy bodies (LBs), respectively. However, clinical overlaps in symptoms and pathologies between AD and PD are commonly observed caused by the cross-interaction between Aß and αS. To uncover the molecular mechanisms behind their overlapping symptoms and pathologies, we computationally investigated the impact of αS on an Aß monomer and dimerization using atomistic discrete molecular dynamics simulations (DMD). Our results revealed that αS could directly interact with Aß monomers and dimers, thus forming ß-sheet-rich oligomers, including potentially toxic ß-barrel intermediates. The binding hotspot involved the second half of the N-terminal domain and NAC region in αS, along with residues 10-21 and 31-42 in Aß. In their hetero-complex, the binding hotspot primarily assumed a ß-sheet core buried inside, which was dynamically shielded by the highly charged, amyloid-resistant C-terminus of αS. Because the amyloid prion region was the same as the binding hotspot being buried, their fibrillization may be delayed, causing the toxic oligomers to increase. This study sheds light on the intricate relationship between Aß and αS and provides insights into the overlapping pathology of AD and PD.

The role and mechanism of unfolded protein response signaling pathway in methylmercury-induced apoptosis of mouse spermatocytes germ cell-2 cells.

The study aimed to explore the role and mechanism of unfolded protein response (UPR) in methylmercury (MeHg)-induced Mouse Spermatocytes (GC-2spd[ts]) apoptosis. Methods such as MTT, flow cytometry, and Western Blot were used to evaluate the cell viability, membrane potential (MMP), reactive oxygen species (ROS), calcium ion (Ca2+ ), rate of cell apoptosis, and the expression of apoptosis-related and UPR-related protein. The results showed that with the increase of MeHg concentration, cell viability and MMP decreased, ROS, Ca2+ , rate of cell apoptosis, and the expression of apoptosis-related protein and UPR-related protein increased. To further explore the effect of ROS-induced oxidative damage on it, the ROS inhibitor N-acetyl-L-cysteine (NAC) was used. The effects of MeHg on germ cell (GC-2) cells were partially inhibited after NAC pretreatment. Our present study proved that MeHg might induce cell apoptosis by activating the UPR signaling pathway in GC-2 cells and affect normal reproductive function.

Evidence of Formation of Monolayer Hydrated Salts in Nanopores.

Despite much effort being devoted to the study of ionic aqueous solutions at the nanoscale, our fundamental understanding of the microscopic kinetic and thermodynamic behaviors in these systems remains largely incomplete. Herein, we reported the first 10 µs molecular dynamics simulation, providing evidence of the spontaneous formation of monolayer hexagonal honeycomb hydrated salts of XCl2·6H2O (X = Ba, Sr, Ca, and Mg) from electrolyte aqueous solutions confined in an angstrom-scale slit under ambient conditions. By using both the classical molecular dynamics simulations and the first-principles Born-Oppenheimer molecular dynamics simulations, we further demonstrated that the hydrated salts were stable not only at ambient temperature but also at elevated temperatures. This phenomenon of formation of hydrated salt in water is contrary to the conventional view. The free energy calculations and dehydration analyses indicated that the spontaneous formation of hydrated salts can be attributed to the interplay between ion hydration and Coulombic attractions in the highly confined water. In addition to providing molecular-level insights into the novel behavior of ionic aqueous solutions at the nanoscale, our findings may have implications for the future exploration of potential existence of water molecules in the saline deposits on hot planets.

Molecular Insights into the Self-Assembly of Block Copolymer Suckerin Polypeptides into Nanoconfined ß-Sheets.

Suckerin in squid sucker ring teeth is a block-copolymer peptide comprised of two repeating modules-the alanine and histidine-rich M1 and the glycine-rich M2. Suckerin self-assemblies display excellent thermo-plasticity and pH-responsive properties, along with the high biocompatibility, biodegradability, and sustainability. However, the self-assembly mechanism and the detailed role of each module are still elusive, limiting the capability of applying and manipulating such biomaterials. Here, the self-assembly dynamics of the two modules and two minimalist suckerin-mimetic block-copolymers, M1-M2-M1 and M2-M1-M2, in silico is investigated. The simulation results demonstrate that M2 has a stronger self-association but weaker ß-sheet propensities than M1. The high self-assembly propensity of M2 allows the minimalist block-copolymer peptides to coalesce with microphase separation, enabling the formation of nanoconfined ß-sheets in the matrix formed by M1-M2 contacts. Since these glycine-rich fragments with scatted hydrophobic and aromatic residues are building blocks of many other block-copolymer peptides, the study suggests that these modules function as the "molecular glue" in addition to the flexible linker or spacer to drive the self-assembly and microphase separation. The uncovered molecular insights may help understand the structure and function of suckerin and also aid in the design of functional block-copolymer peptides for nanotechnology and biomedicine applications.

Modulating Nanodroplet Formation En Route to Fibrillization of Amyloid Peptides with Designed Flanking Sequences.

Soluble oligomers populating early amyloid aggregation can be regarded as nanodroplets of liquid-liquid phase separation (LLPS). Amyloid peptides typically contain hydrophobic aggregation-prone regions connected by hydrophilic linkers and flanking sequences, and such a sequence hydropathy pattern drives the formation of supramolecular structures in the nanodroplets and modulates subsequent fibrillization. Here, we studied LLPS and fibrillization of coarse-grained amyloid peptides with increasing flanking sequences. Nanodroplets assumed lamellar, cylindrical micellar, and spherical micellar structures with increasing peptide hydrophilic/hydrophobic ratios, and such morphologies governed subsequent fibrillization processes. Adding glycine-serine repeats as flanking sequences to Aß16-22, the amyloidogenic core of amyloid-ß, our computational predictions of morphological transitions were corroborated experimentally. The uncovered inter-relationships between the peptide sequence pattern, oligomer/nanodroplet morphology, and fibrillization pathway, kinetics, and structure may contribute to our understanding of pathogenic amyloidosis in aging, facilitate future efforts ameliorating amyloidosis through peptide engineering, and aid in the design of novel amyloid-based functional nanobiomaterials and nanocomposites.

Hydrophobic/Hydrophilic Ratio of Amphiphilic Helix Mimetics Determines the Effects on Islet Amyloid Polypeptide Aggregation.

Amyloid depositions of human islet amyloid polypeptides (hIAPP) are associated with type II diabetes (T2D) impacting millions of people globally. Accordingly, strategies against hIAPP aggregation are essential for the prevention and eventual treatment of the disease. Helix mimetics, which modulate the protein-protein interaction by mimicking the side chain residues of a natural α-helix, were found to be a promising strategy for inhibiting hIAPP aggregation. Here, we applied molecular dynamics simulations to investigate two helix mimetics reported to have opposite effects on hIAPP aggregation in solution, the oligopyridylamide-based scaffold 1e promoted, whereas naphthalimide-appended oligopyridylamide scaffold DM 1 inhibited the aggregation of hIAPP in solution. We found that 1e promoted hIAPP aggregation because of the recruiting effects through binding with the N-termini of hIAPP peptides. In contrast, DM 1 with a higher hydrophobic/hydrophilic ratio effectively inhibited hIAPP aggregation by strongly binding with the C-termini of hIAPP peptides, which competed for the interpeptide contacts between amyloidogenic regions in the C-termini and impaired the fibrillization of hIAPP. Structural analyses revealed that DM 1 formed the core of hIAPP-DM 1 complexes and stabilized the off-pathway oligomers, whereas 1e formed the corona outside the hIAPP-1e complexes and remained active in recruiting free hIAPP peptides. The distinct interaction mechanisms of DM 1 and 1e, together with other reported potent antagonists in the literature, emphasized the effective small molecule-based amyloid inhibitors by disrupting peptide interactions that should reach a balanced hydrophobic/hydrophilic ratio, providing a viable and generic strategy for the rational design of novel anti-amyloid nanomedicine.

Mechanistic insight into the disruption of Tau R3-R4 protofibrils by curcumin and epinephrine: an all-atom molecular dynamics study.

The accumulation of Tau protein aggregates is a pathological hallmark of tauopathy, including chronic traumatic encephalopathy (CTE). Inhibiting Tau aggregation or disrupting preformed Tau fibrils is considered one of the rational therapeutic strategies to combat tauopathy. Previous studies reported that curcumin (Cur, a molecule of a labile natural product) and epinephrine (EP, an important neurotransmitter) could effectively inhibit the formation of Tau fibrillar aggregates and disassociate preformed fibrils. However, the underlying molecular mechanisms remain elusive. In this study, we performed multiple molecular dynamics simulations for 17.5 µs in total to investigate the influence of Cur and EP on the C-shaped Tau protofibril associated with CTE. Our simulations show that the protofibrillar pentamer is the smallest stable Tau R3-R4 protofibril. Taking the pentamer as a protofibril model, we found that both Cur and EP molecules could affect the shape of the Tau pentamer by changing the ß2-ß3 and ß7-ß8 angles, leading to a more extended structure. Cur and EP display a disruptive effect on the local ß-sheets and the formation of hydrogen bonds, and thus destabilize the global protofibril structure. The contact number analysis shows that Cur has a higher binding affinity with the Tau pentamer than EP, especially in the nucleating segment PHF6. Hydrophobic, π-π and cation-π interactions together facilitate the binding of Cur and EP with the Tau pentamer. Cur exhibits stronger hydrophobic and π-π interactions with Tau than EP, and EP displays a stronger cation-π interaction. Our findings provide molecular insights into the disruptive mechanisms of the Tau R3-R4 protofibrils by curcumin and epinephrine, which may be useful for the design of effective drug candidates for the treatment of CTE.

Molecular insights into the oligomerization dynamics and conformations of amyloidogenic and non-amyloidogenic amylin from discrete molecular dynamics simulations.

The amyloid aggregation of human islet amyloid polypeptide (hIAPP) is associated with pancreatic ß-cell death in type 2 diabetes. The S20G substitution of hIAPP (hIAPP(S20G)), found in Japanese and Chinese people, is more amyloidogenic and cytotoxic than wild-type hIAPP. Rat amylin (rIAPP) does not have aggregation propensity or cytotoxicity. Mounting evidence suggests that soluble low-molecular-weight amyloid oligomers formed during early aggregation are more cytotoxic than mature fibrils. The self-assembly dynamics and oligomeric conformations remain unknown because the oligomers are heterogeneous and transient. The molecular mechanism of sequence-variation rendering dramatically different aggregation propensity and cytotoxicity is also elusive. Here, we investigate the oligomerization dynamics and conformations of amyloidogenic hIAPP, hIAPP(S20G), and non-amyloidogenic rIAPP using atomistic discrete molecular dynamics (DMD) simulations. Our simulation results demonstrated that all three monomeric amylin peptides mainly adopted an unstructured formation with partial dynamical helices near the N-terminus. Relatively transient ß-hairpins were more abundant in hIAPP and hIAPP(S20G) than in rIAPP. The S20G-substituting mutant of hIAPP altered the turn region of the ß-hairpin motif, resulting in more hydrophobic residue-pairwise contacts within the ß-hairpin. Oligomerization dynamic investigation revealed that all three peptides spontaneously accumulated into helix-populated oligomers. The conformational conversion to form ß-sheet-rich oligomers was only observed in hIAPP and hIAPP(S20G). The population of high-ß-sheet-content oligomers was enhanced by S20G substitution. Interestingly, both hIAPP and hIAPP(S20G) could form ß-barrel formations, and the ß-barrel propensity of hIAPP(S20G) was three times larger than that of hIAPP. No ß-sheet-rich or ß-barrel formations were observed in rIAPP. Our direct observation of the correlation between ß-barrel oligomer formation and cytotoxicity suggests that ß-barrels might play a critically important role in the cytotoxicity of amyloidosis.

Direct Observation of ß-Barrel Intermediates in the Self-Assembly of Toxic SOD128-38 and Absence in Nontoxic Glycine Mutants.

Soluble low-molecular-weight oligomers formed during the early stage of amyloid aggregation are considered the major toxic species in amyloidosis. The structure-function relationship between oligomeric assemblies and the cytotoxicity in amyloid diseases are still elusive due to the heterogeneous and transient nature of these aggregation intermediates. To uncover the structural characteristics of toxic oligomeric intermediates, we compared the self-assembly dynamics and structures of SOD128-38, a cytotoxic fragment of the superoxide dismutase 1 (SOD1) associated with the amyotrophic lateral sclerosis, with its two nontoxic mutants G33V and G33W using molecular dynamics simulations. Single-point glycine substitutions in SOD128-38 have been reported to abolish the amyloid toxicity. Our simulation results showed that the toxic SOD128-38 and its nontoxic mutants followed different aggregation pathways featuring distinct aggregation intermediates. Specifically, wild-type SOD128-38 initially self-assembled into random-coil-rich oligomers, among which fibrillar aggregates composed of well-defined curved single-layer ß-sheets were nucleated via coil-to-sheet conversions and the formation of ß-barrels as intermediates. In contrast, the nontoxic G33V/G33W mutants readily assembled into small ß-sheet-rich oligomers and then coagulated with each other into cross-ß fibrils formed by two-layer ß-sheets without forming ß-barrels as the intermediates. The direct observation of ß-barrel oligomers during the assembly of toxic SOD128-38 fragments but not the nontoxic glycine-substitution mutants strongly supports ß-barrels as the toxic oligomers in amyloidosis, probably via interactions with the cell membrane and forming amyloid pores. With well-defined structures, the ß-barrel might serve as a novel therapeutic target against amyloid-related diseases.

Misfolding and Self-Assembly Dynamics of Microtubule-Binding Repeats of the Alzheimer-Related Protein Tau.

Pathological aggregation of intrinsically disordered tau protein, driven by the interactions between microtubule-binding (MTB) domains, is associated with Alzheimer's disease. The MTB domain contains either three or four repeats with sequence similarities. Compared to amyloid ß, many aspects of the misfolding and aggregation mechanisms of tau are largely unknown. In this study, we systematically investigated the dynamics of monomer misfolding and dimerization of each MTB repeat using atomistic discrete molecular dynamic simulations. Our results revealed that all the four repeat monomers (R1-R4) were very dynamic, featuring frequent conformational conversion and lacking stable conformations. While R1, R2, and R4 monomers occasionally adopted partially helical conformations, R3 monomers frequently formed ß-sheets. In dimerization simulations, R3 displayed the strongest aggregation propensity with high ß-sheet contents, while R1 was the least prone to aggregation. The R2 and R4 dimers contained both helix and ß-sheet structures. The ß-sheets in R4 assemblies were dominant with ß-hairpin conformation. In R2 and R3 dimers, intermolecular ß-sheets were mainly driven by residues around the paired helical filament (PHF) regions. Residues around the PHF6* in R2 and PHF6 in R3 had significantly higher intermolecular contacts than other regions, suggesting that these residues play a key role in the amyloid aggregation of tau. Our results on the structural ensembles and early aggregation dynamics of each tau MTB repeat will help understand the nucleation and fibrillization of tau.