A structural model for Alzheimer's β-amyloid fibrils based on experimental constraints from solid state NMR

Abstract

We present a structural model for amyloid fibrils formed by the 40-residue ␤-amyloid peptide associated with Alzheimer's disease (A␤ 1-40), based on a set of experimental constraints from solid state NMR spectroscopy. The model additionally incorporates the cross-␤ structural motif established by x-ray fiber diffraction and satisfies constraints on A␤ 1-40 fibril dimensions and mass-per-length determined from electron microscopy. Approximately the first 10 residues of A␤ 1-40 are structurally disordered in the fibrils. Residues 12-24 and 30 -40 adopt ␤-strand conformations and form parallel ␤-sheets through intermolecular hydrogen bonding. Residues 25-29 contain a bend of the peptide backbone that brings the two ␤-sheets in contact through sidechain-sidechain interactions. A single cross-␤ unit is then a double-layered ␤-sheet structure with a hydrophobic core and one hydrophobic face. The only charged sidechains in the core are those of D23 and K28, which form salt bridges. Fibrils with minimum mass-per-length and diameter consist of two cross-␤ units with their hydrophobic faces juxtaposed.A myloid fibrils are filamentous structures, with typical diameters of Ϸ10 nm and lengths up to several micrometers, formed by numerous peptides and proteins with disparate sequences and molecular weights. Biomedical interest in amyloid fibrils arises from their occurrence in amyloid diseases (1), including Alzheimer's disease, type 2 diabetes, Huntington's disease, and prion diseases. Current interest in the molecular structures of amyloid fibrils additionally arises from fundamental questions regarding the molecular mechanism of amyloid formation and the nature of the intermolecular interactions that stabilize these structures for an extremely diverse class of polypeptides.No high-resolution molecular structure of an amyloid fibril has yet been determined experimentally because amyloid fibrils are noncrystalline solid materials and are therefore incompatible with x-ray crystallography and liquid state NMR. X-ray fiber diffraction shows that amyloid fibrils contain cross-␤ structural motifs, i.e., extended ␤-sheets in which the ␤-strand segments run approximately perpendicular to, and the intermolecular hydrogen bonds run approximately parallel to, the long axis of the fibril (2, 3). Other molecular-level structural features of amyloid fibrils are not well established.In the case of fibrils formed by the full-length ␤-amyloid peptide associated with Alzheimer's disease (A␤), which ranges from 39 to 43 residues in length in vivo (4, 5), several molecular models have been proposed (6-10). These models exhibit many qualitative and quantitative differences, reflecting the paucity of experimental constraints. All of these models are inconsistent with recent measurements of 13 C-13 C nuclear magnetic dipole-dipole couplings (i.e., intermolecular distances) by solid state NMR (11-13), which imply an in-register parallel alignment of peptide chains within the cross-␤ motif in A␤ 1-40 and A␤ 1-42 fibrils (A␤ mϪn denotes residues m t...

Transformation of amyloid β(1–40) oligomers into fibrils is characterized by a major change in secondary structure

“…The b-turns content represents 15-20% for all aggregated forms. The secondary structure determination within Ab(1-40) fibrils is in excellent agreement with previous studies [25].…”

“…Tycko's group provided evidence for a parallel b-sheet organization in the fibrils using solid state NMR implicating residues 12-40 with a turn located around residues 21-30 [25,26,41]. This strand-turn-strand motif is packed by the strands side chains and the turn is called b-arc.…”

“…Biophysical studies describe fibrils as cross-b-structure filaments [25][26][27]. However, structural studies of oligomers and the conformational transitions taking place during the aggregation process are limited by the size range and transitory nature of these aggregates [28].…”

Sensitivity enhancement in 13C solid-state NMR of protein microcrystals by use of paramagnetic metal ions for optimizing 1H T1 relaxation

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Particularly, use of protein micro-/nano-crystals [17] has significantly improved resolution in high-resolution SSNMR of dilute spins such as 13 C and 15 N, permitting signal assignment and structural determination of various uniformly 13 C and/or 15 N-labeled proteins by SSNMR. [18][19][20][21][22][23][24][25] However, restricted sensitivity in 13 C and 15 N SSNMR has been still one of the major limiting factors in SSNMR analysis of proteins.…”

Proton-detected scalar coupling based assignment strategies in MAS solid-state NMR spectroscopy applied to perdeuterated proteins

“…Methodological and conceptual improvements, however, are still necessary for a straight forward and routine application of solid-state NMR to biological matter. Nevertheless, its applicability to a wide range of candidates for biomolecular investigation like uniformly isotopically enriched membrane proteins [3][4][5][6] or proteins forming aggregates [7][8][9][10][11][12] has been demonstrated.…”