Noel D. Lazo

Amyloid-β protein oligomerization and the importance of tetramers and dodecamers in the aetiology of Alzheimer’s disease

In recent years, small protein oligomers have been implicated in the aetiology of a number of important amyloid diseases, such as type 2 diabetes, Parkinsons disease and Alzheimers disease. As a consequence, research efforts are being directed away from traditional targets, such as amyloid plaques, and towards characterization of early oligomer states. Here we present a new analysis method, ion mobility coupled with mass spectrometry, for this challenging problem, which allows determination of in vitro oligomer distributions and the qualitative structure of each of the aggregates. We applied these methods to a number of the amyloid-β protein isoforms of Aβ40 and Aβ42 and showed that their oligomer-size distributions are very different. Our results are consistent with previous observations that Aβ40 and Aβ42 self-assemble via different pathways and provide a candidate in the Aβ42 dodecamer for the primary toxic species in Alzheimers disease.

On the nucleation of amyloid β‐protein monomer folding

Neurotoxic assemblies of the amyloid β‐protein (Aβ) have been linked strongly to the pathogenesis of Alzheimers disease (AD). Here, we sought to monitor the earliest step in Aβ assembly, the creation of a folding nucleus, from which oligomeric and fibrillar assemblies emanate. To do so, limited proteolysis/mass spectrometry was used to identify protease‐resistant segments within monomeric Aβ(1–40) and Aβ(1–42). The results revealed a 10‐residue, protease‐resistant segment, Ala21–Ala30, in both peptides. Remarkably, the homologous decapeptide, Aβ(21–30), displayed identical protease resistance, making it amenable to detailed structural study using solution‐state NMR. Structure calculations revealed a turn formed by residues Val24–Lys28. Three factors contribute to the stability of the turn, the intrinsic propensities of the Val‐Gly‐Ser‐Asn and Gly‐Ser‐Asn‐Lys sequences to form a β‐turn, long‐range Coulombic interactions between Lys28 and either Glu22 or Asp23, and hydrophobic interaction between the isopropyl and butyl side chains of Val24 and Lys28, respectively. We postulate that turn formation within the Val24–Lys28 region of Aβ nucleates the intramolecular folding of Aβ monomer, and from this step, subsequent assembly proceeds. This model provides a mechanistic basis for the pathologic effects of amino acid substitutions at Glu22 and Asp23 that are linked to familial forms of AD or cerebral amyloid angiopathy. Our studies also revealed that common C‐terminal peptide segments within Aβ(1–40) and Aβ(1–42) have distinct structures, an observation of relevance for understanding the strong disease association of increased Aβ(1–42) production. Our results suggest that therapeutic approaches targeting the Val24–Lys28 turn or the Aβ(1–42)‐specific C‐terminal fold may hold promise.

Short Amyloid-β (Aβ) Immunogens Reduce Cerebral Aβ Load and Learning Deficits in an Alzheimer's Disease Mouse Model in the Absence of an Aβ-Specific Cellular Immune Response

Amyloid-β (Aβ) immunotherapy lowers cerebral Aβ and improves cognition in mouse models of Alzheimers disease (AD). A clinical trial using active immunization with Aβ1–42 was suspended after ∼6% of patients developed meningoencephalitis, possibly because of a T-cell reaction against Aβ. Nevertheless, beneficial effects were reported in antibody responders. Consequently, alternatives are required for a safer vaccine. The Aβ1–15 sequence contains the antibody epitope(s) but lacks the T-cell reactive sites of full-length Aβ1–42. Therefore, we tested four alternative peptide immunogens encompassing either a tandem repeat of two lysine-linked Aβ1–15 sequences (2×Aβ1–15) or the Aβ1–15 sequence synthesized to a cross-species active T1 T-helper-cell epitope (T1-Aβ1–15) and each with the addition of a three-amino-acid RGD (Arg-Gly-Asp) motif (R-2×Aβ1–15; T1-R-Aβ1–15). High anti-Aβ antibody titers were observed in wild-type mice after intranasal immunization with R-2×Aβ1–15 or 2×Aβ1–15 plus mutant Escherichia coli heat-labile enterotoxin LT(R192G) adjuvant. Moderate antibody levels were induced after immunization with T1-R-Aβ1–15 or T1-Aβ1–15 plus LT(R192G). Restimulation of splenocytes with the corresponding immunogens resulted in moderate proliferative responses, whereas proliferation was absent after restimulation with full-length Aβ or Aβ1–15. Immunization of human amyloid precursor protein, familial AD (hAPPFAD) mice with R-2×Aβ1–15 or 2×Aβ1–15 resulted in high anti-Aβ titers of noninflammatory T-helper 2 isotypes (IgG1 and IgG2b), a lack of splenocyte proliferation against full-length Aβ, significantly reduced Aβ plaque load, and lower cerebral Aβ levels. In addition, 2×Aβ1–15-immunized hAPPFAD animals showed improved acquisition of memory compared with vehicle controls in a reference-memory Morris water-maze behavior test that approximately correlated with anti-Aβ titers. Thus, our novel immunogens show promise for future AD vaccines.

Structure of the 21-30 fragment of amyloid β-protein

Folding and self‐assembly of the 42‐residue amyloid β‐protein (Aβ) are linked to Alzheimers disease (AD). The 21–30 region of Aβ, Aβ(21–30), is resistant to proteolysis and is believed to nucleate the folding of full‐length Aβ. The conformational space accessible to the Aβ(21–30) peptide is investigated by using replica exchange molecular dynamics simulations in explicit solvent. Conformations belonging to the global free energy minimum (the “native” state) from simulation are in good agreement with reported NMR structures. These conformations possess a bend motif spanning the central residues V24–K28. This bend is stabilized by a network of hydrogen bonds involving the side chain of residue D23 and the amide hydrogens of adjacent residues G25, S26, N27, and K28, as well as by a salt bridge formed between side chains of K28 and E22. The non‐native states of this peptide are compact and retain a native‐like bend topology. The persistence of structure in the denatured state may account for the resistance of this peptide to protease degradation and aggregation, even at elevated temperatures.

Folding events in the 21-30 region of amyloid β-protein (Aβ) studied in silico

Oligomeric assemblies of the amyloid β-protein (Aβ) have been implicated in the pathogenesis of Alzheimers disease as a primary source of neurotoxicity. Recent in vitro studies have suggested that a 10-residue segment, Ala-21-Ala-30, forms a turn-like structure that nucleates the folding of the full-length Aβ. To gain a mechanistic insight, we simulated Aβ(21-30) folding by using a discrete molecular dynamics algorithm and a united-atom model incorporating implicit solvent and a variable electrostatic interaction strength (EIS). We found that Aβ(21-30) folds into a loop-like conformation driven by an effective hydrophobic attraction between Val-24 and the butyl portion of the Lys-28 side chain. At medium EIS [1.5 kcal/mol (1 cal = 4.18 J)], unfolded conformations almost disappear, in agreement with experimental observations. Under optimal conditions for folding, Glu-22 and Asp-23 form transient electrostatic interactions (EI) with Lys-28 that stabilize the loop conformations. Glu-22-Lys-28 is the most favored interaction. High EIS, as it occurs in the interior of proteins and aggregates, destabilizes the packing of Val-24 and Lys-28. Analysis of the unpacked structures reveals strong EI with predominance of the Asp-23-Lys-28 interaction, in agreement with studies of molecular modeling of full-length Aβ fibrils. The binary nature of the EI involving Lys-28 provides a mechanistic explanation for the linkage of amino acid substitutions at Glu-22 with Alzheimers disease and cerebral amyloid angiopathy. Substitutions may alter the frequency of Glu-22 or Asp-23 involvement in contact formation and affect the stability of the folding nucleus formed in the Aβ(21-30) region.

Familial Alzheimer's disease mutations alter the stability of the amyloid β-protein monomer folding nucleus

Amyloid β-protein (Aβ) oligomers may be the proximate neurotoxins in Alzheimers disease (AD). Recently, to elucidate the oligomerization pathway, we studied Aβ monomer folding and identified a decapeptide segment of Aβ, 21Ala–22Glu–23Asp–24Val–25Gly–26Ser–27Asn–28Lys–29Gly–30Ala, within which turn formation appears to nucleate monomer folding. The turn is stabilized by hydrophobic interactions between Val-24 and Lys-28 and by long-range electrostatic interactions between Lys-28 and either Glu-22 or Asp-23. We hypothesized that turn destabilization might explain the effects of amino acid substitutions at Glu-22 and Asp-23 that cause familial forms of AD and cerebral amyloid angiopathy. To test this hypothesis, limited proteolysis, mass spectrometry, and solution-state NMR spectroscopy were used here to determine and compare the structure and stability of the Aβ(21–30) turn within wild-type Aβ and seven clinically relevant homologues. In addition, we determined the relative differences in folding free energies (ΔΔGf) among the mutant peptides. We observed that all of the disease-associated amino acid substitutions at Glu-22 or Asp-23 destabilized the turn and that the magnitude of the destabilization correlated with oligomerization propensity. The Ala21Gly (Flemish) substitution, outside the turn proper (Glu-22–Lys-28), displayed a stability similar to that of the wild-type peptide. The implications of these findings for understanding Aβ monomer folding and disease causation are discussed.

Effects of Familial Alzheimer's Disease Mutations on the Folding Nucleation of the Amyloid β-Protein

The effect of single amino acid substitutions associated with the Italian (E22K), Arctic (E22G), Dutch (E22Q) and Iowa (D23N) familial forms of Alzheimers disease and cerebral amyloid angiopathy on the structure of the 21-30 fragment of the Alzheimer amyloid beta-protein (Abeta) is investigated by replica-exchange molecular dynamics simulations. The 21-30 segment has been shown in our earlier work to adopt a bend structure in solution that may serve as the folding nucleation site for Abeta. Our simulations reveal that the 24-28 bend motif is retained in all E22 mutants, suggesting that mutations involving residue E22 may not affect the structure of the folding nucleation site of Abeta. Enhanced aggregation in Abeta with familial Alzheimers disease substitutions may result from the depletion of the E22-K28 salt bridge, which destabilizes the bend structure. Alternately, the E22 mutations may affect longer-range interactions outside the 21-30 segment that can impact the aggregation of Abeta. Substituting at residue D23, on the other hand, leads to the formation of a turn rather than a bend motif, implying that in contrast to E22 mutants, the D23N mutant may affect monomer Abeta folding and subsequent aggregation. Our simulations suggest that the mechanisms by which E22 and D23 mutations affect the folding and aggregation of Abeta are fundamentally different.

Mechanistic studies of peptide self-assembly: transient α-helices to stable β-sheets.

The pathologic self-assembly of proteins is associated with typically late-onset disorders such as Alzheimers disease, Parkinsons disease, and type 2 diabetes. Important mechanistic details of the self-assembly are unknown, but there is increasing evidence supporting the role of transient α-helices in the early events. Islet amyloid polypeptide (IAPP) is a 37-residue polypeptide that self-assembles into aggregates that are toxic to the insulin-producing β cells. To elucidate early events in the self-assembly of IAPP, we used limited proteolysis to identify an exposed and flexible region in IAPP monomer. This region includes position 20 where a serine-to-glycine substitution (S20G) is associated with enhanced formation of amyloid fibrils and early onset type 2 diabetes. To perform detailed biophysical studies of the exposed and flexible region, we synthesized three peptides including IAPP(11-25)WT (wild type), IAPP(11-25)S20G, and IAPP(11-25)S20P. Solution-state NMR shows that all three peptides transiently populate the α-helical conformational space, but the S20P peptide, which does not self-assemble, transiently samples a broken helix. Under similar sample conditions, the WT and S20G peptides populate the α-helical intermediate state and β-sheet end state, respectively, of fibril formation. Our results suggest a mechanism for self-assembly that includes the stabilization of transient α-helices through the formation of NMR-invisible helical intermediates followed by an α-helix to β-sheet conformational rearrangement. Furthermore, our results suggest that reducing intermolecular helix-helix contacts as in the S20P peptide is an attractive strategy for the design of blockers of peptide self-assembly.

Amyloid β-protein: Experiment and theory on the 21-30 fragment

The structure of the 21-30 fragment of the amyloid beta-protein (Abeta) was investigated by ion mobility mass spectrometry and replica exchange dynamics simulations. Mutations associated with familial Alzheimers disease (E22G, E22Q, E22K, and D23N) of Abeta(21-30) were also studied, in order to understand any structural changes that might occur with these substitutions. The structure of the WT peptide shows a bend and a perpendicular turn in the backbone which is maintained by a network of D23 hydrogen bonding. Results for the mutants show that substitutions at E22 do little to alter the overall structure of the fragment. A substitution at D23 resulted in a change of structure for Abeta(21-30). A comparison of these gas-phase studies to previous solution-phase studies reveals that the peptide can fold in the absence of solvent to a structure also seen in solution, highlighting the important role of the D23 hydrogen bonding network in stabilizing the fragments folded structure.

Curcumin modulates the self-assembly of the islet amyloid polypeptide by disassembling α-helix.

Understanding how small molecules affect amyloid formation is of major biomedical and pharmaceutical importance due to the association of amyloid with incurable diseases including Alzheimers, Parkinsons, and type II diabetes. Using solution state (1)H NMR, we demonstrate that curcumin, a planar biphenolic compound found in the Indian spice turmeric, delays the self-assembly of islet amyloid polypeptide to NMR-invisible assemblies. Accompanying circular dichroism studies show that curcumin disassembles α-helix in maturing assemblies of IAPP. The amount of α-helix disassembled correlates with predicted and experimentally determined helical content of IAPP obtained by others. Taken together, these results indicate that curcumin modulates IAPP self-assembly by unfolding α-helix on pathway to amyloid. The implications of this work in the elucidation of the mechanism for amyloid formation by IAPP in the presence of curcumin are discussed.