**2.2 Structural properties of amyloid aggregates**

Basic structural architecture of amyloid fibrils consists of a characteristic 4.7 Å repeat through the cross-β structure [2]. Still, there exist some variations in morphology of different Aβ fibrils; the C-terminus in Aβ(1–40) is hidden within the core, while the corresponding residues are exposed on the surface of Aβ(1–42) [10]. Solid state NMR studies of amyloid fibril structures employ 2D dipolar-assisted rotation resonance (DARR) experiments and frequency selective rotational-echo double resonance (REDOR) experiments along with distance measurements between 13C-13C and 13C-15N nuclei [8]. Studies of Aβ(1–42) fibrils using ssNMR suggested unidirectional protofibril growth with two molecules coming together to form parallel intermolecular β-sheets. The ssNMR structure of homogeneous Aβ(1–42) fibrils (PDB ID: 2MXU) exhibits triple-β-strands encompassing residues 12–18, 24–33 and 36–40 respectively, connected by coil-and-turns at residues 19–23 and 34–42 [8]. The other structure of Aβ(1–42) fibrils with oxidized Met35 (PDB:2BEG), suggests structural inhomogeneity in N-terminal residues from 1 to 17 and presence of two β-strands in residues 18–26 and 31–42, respectively [17]. The AD related Aβ(1–42) polymorph shows double-horseshoe like cross-β structure where N terminus of monomeric Aβ(1–42) has L shape and the C terminus takes an S-shape. The structure comprises five in-register parallel strands with each fibril layer comprising of two molecules with hydrophobic side chains buried maximally [16]. In addition to hydrophobic core, parallel β-sheets also show polar zipper interactions through intermolecular hydrogen bonds. A recent 4.0 Å resolution ssNMRcryoEM hybrid structure (PDB ID: 5OQV) of intertwined Aβ(1–42) protofilaments showed an approximate 21 screw helical symmetry with 4.67 Å rise [18]. This implicated step-wise shift between the subunits is similar to tau dimers. Three hydrophobic clusters involving Ala, Val, Phe, Leu, Ile, and Met residues expand along the fibril axis and contribute to overall stability of fibrils [18]. Tertiary interactions in Aβ(1–42) fibrils are different compared to Aβ(1–40) fibrils owing to differences in side-chain packaging in hydrophobic core of protofilaments [17]. The N-terminus of Aβ(1–40) peptides is disordered and highly prone to proteolysis whereas the ordered region from tyrosine residue onwards acquires a double-layered structure with a "β-arch" motif where two β-strands are separated by a short loop [19]. This motif is stabilized by formation of a salt bridge between Asp23 and Lys28 across the bend. Interactions between Lys28 and Ala42 observed in these fibrils differ to those found in Aβ(1–40) [8]. Similarly, salt bridges between Asp1 and Lys28, Asp7 and Arg5, Glu11 and His6 and His13, and Asn-Gln ladders further contribute to stability of the Aβ(1–42) fibril structure [16, 18]. Point mutations introduced in basic amino acid sequences lead to varied fibril architecture compared to the wild-type Aβ40 fibrils [2].

NMR structure of tau monomers shows detectable propensity to β-sheet, poly-proline helices and transient α-helical conformations. Aggregation of the R3 fragment consisting of 26 amino acid residues is strongly associated with formation of fibrils in the presence of polyanions as well as during self-assembly of pristine tau [7]. Phosphorylation of serine and threonine residues stabilize the α-helical conformation. αS fibrils acquire an overall assembly that mimics a Greek key as seen in atomic resolution structure of purified protein [7].

**193**

extracellularly [33].

**2.4 Toxicity of amyloid aggregates**

*Neuroprotective Function of Non-Proteolytic Amyloid-β Chaperones in Alzheimer's Disease*

Amyloidosis in cerebrovascular system is mediated through aggregation of wild type proteins as the consequence of multivalent interactions in intrinsically disordered proteins or regions of proteins, mutations in amyloidogenic precursors, expansion of repeats in the amyloidogenic sequences, actions of proteases or chemical modifications on the precursor sequences, overexpression of the precursor, liquid–liquid phase separation, actions of small metabolites or age-related cell death [27–31]. The number of human proteins capable of causing amyloidosis has exceeded 50 and are associated with many neurological disorders based of variations in the disease precursors [2]. Different oligomeric and morphologically distinct species affect cells and tissues in different manner. Amyloid oligomers can be cytotoxic with the long-term potentiation disrupting membranes to cause ion permeability and homeostasis imbalance [2]. Fully matured fibrils contribute to the disease by disturbing lipid membranes and thus interfere with the general membrane bound cellular organelles, or by forming physical barriers which disrupt inter-cellular communications [2]. Structural inhomogeneity was observed in fibrils from the different clinical variants of amyloids in AD. Differences in morphology of Aβ aggregates can change the phenotype observed in different forms of AD. While a single Aβ structure is found in brain cortex in the cases of typical prolonged duration AD, different polymorphs are found in the cases of rapidly progressive form of AD [32]. There are six isoforms of tau protein in brain and CNS, longest one containing a stretch of 441 amino acids, with high tendency for phosphorylation and self-assembly. Tau filaments cause different tauopathies, including neurofibrillary tangles in AD [7]. Neurofibrillary tangles and neuropil threads are predominantly tau aggregates localized in neuronal cell bodies and processes, respectively, whereas the mature Aβ plaques are present

Aβ(1–42) is more potent fibrillogenic Aβ variant in human brain, yet its concentration accounts for only about 5–10% of Aβ(1–40) concentration produced [33]. Accumulation of Aβ(1–42) in brain and cerebrovascular system due to its aggregation propensity, and by extension, the relative ratio of Aβ(1–42): Aβ(1–40) is a biomarker for AD [33]. Aβ(1–40) fibrils are less toxic compared to Aβ(1–42) fibrils which show differential toxicity and Aβ(1–43) fibrils are most cytotoxic [10]. Soluble Aβ in the form of low-n oligomers, Aβ-derived diffusible ligands and protofibrils are among major toxic contributors towards AD pathology [34]. Interactions of oligomeric species with other molecular complexes, metal ions and cellular membranes also impact the extent of their toxic effects in brain. N-terminus truncated Aβ peptides (Aβ(n–42), where n can vary from 2 to 11) are highly toxic and are usually present in Aβ deposits in AD brain [7]. Peptides cleaved at positions 3 or 11 are prone to pyroglutamylation and prominent components of Aβ deposits, likely owing to their resistance to proteolytic degradation [35]. αS is a major component of Lewy bodies and neurites causing neurotoxicity through dopaminergic mechanism and these aggregates are key identifiers for PD and Lewy body dementia [7]. Detection of αS aggregates early in peripheral nervous system can serve as early biomarkers before motor disabilities develop in PD patients [36]. Horizontal transfer of amyloid aggregates and oligomers between cells also contributes to propagation of disease pathology [37]. IAPP oligomers can impair insulin secretion

in pancreatic cells and cause cell death and cellular uncoupling [7].

*DOI: http://dx.doi.org/10.5772/intechopen.84238*

**2.3 Role of amyloidosis in AD pathology**

*Neuroprotective Function of Non-Proteolytic Amyloid-β Chaperones in Alzheimer's Disease DOI: http://dx.doi.org/10.5772/intechopen.84238*

### **2.3 Role of amyloidosis in AD pathology**

*Amyloid Diseases*

based on symmetry and periodicity with these methods [16]. Aβ40 fibrils structure have either 2-fold (2A) or 3-fold (3Q ) rotational symmetry with a twisted morphology with periodicity of 120 ± 20 nm, and roughly 8 nm width [2, 11]. IAPP fibrils

Basic structural architecture of amyloid fibrils consists of a characteristic 4.7 Å repeat through the cross-β structure [2]. Still, there exist some variations in morphology of different Aβ fibrils; the C-terminus in Aβ(1–40) is hidden within the core, while the corresponding residues are exposed on the surface of Aβ(1–42) [10]. Solid state NMR studies of amyloid fibril structures employ 2D dipolar-assisted rotation resonance (DARR) experiments and frequency selective rotational-echo double resonance (REDOR) experiments along with distance measurements between 13C-13C and 13C-15N nuclei [8]. Studies of Aβ(1–42) fibrils using ssNMR suggested unidirectional protofibril growth with two molecules coming together to form parallel intermolecular β-sheets. The ssNMR structure of homogeneous Aβ(1–42) fibrils (PDB ID: 2MXU) exhibits triple-β-strands encompassing residues 12–18, 24–33 and 36–40 respectively, connected by coil-and-turns at residues 19–23 and 34–42 [8]. The other structure of Aβ(1–42) fibrils with oxidized Met35 (PDB:2BEG), suggests structural inhomogeneity in N-terminal residues from 1 to 17 and presence of two β-strands in residues 18–26 and 31–42, respectively [17]. The AD related Aβ(1–42) polymorph shows double-horseshoe like cross-β structure where N terminus of monomeric Aβ(1–42) has L shape and the C terminus takes an S-shape. The structure comprises five in-register parallel strands with each fibril layer comprising of two molecules with hydrophobic side chains buried maximally [16]. In addition to hydrophobic core, parallel β-sheets also show polar zipper interactions through intermolecular hydrogen bonds. A recent 4.0 Å resolution ssNMRcryoEM hybrid structure (PDB ID: 5OQV) of intertwined Aβ(1–42) protofilaments showed an approximate 21 screw helical symmetry with 4.67 Å rise [18]. This implicated step-wise shift between the subunits is similar to tau dimers. Three hydrophobic clusters involving Ala, Val, Phe, Leu, Ile, and Met residues expand along the fibril axis and contribute to overall stability of fibrils [18]. Tertiary interactions in Aβ(1–42) fibrils are different compared to Aβ(1–40) fibrils owing to differences in side-chain packaging in hydrophobic core of protofilaments [17]. The N-terminus of Aβ(1–40) peptides is disordered and highly prone to proteolysis whereas the ordered region from tyrosine residue onwards acquires a double-layered structure with a "β-arch" motif where two β-strands are separated by a short loop [19]. This motif is stabilized by formation of a salt bridge between Asp23 and Lys28 across the bend. Interactions between Lys28 and Ala42 observed in these fibrils differ to those found in Aβ(1–40) [8]. Similarly, salt bridges between Asp1 and Lys28, Asp7 and Arg5, Glu11 and His6 and His13, and Asn-Gln ladders further contribute to stability of the Aβ(1–42) fibril structure [16, 18]. Point mutations introduced in basic amino acid sequences lead to varied fibril architecture compared to the wild-type Aβ40

NMR structure of tau monomers shows detectable propensity to β-sheet, poly-proline helices and transient α-helical conformations. Aggregation of the R3 fragment consisting of 26 amino acid residues is strongly associated with formation of fibrils in the presence of polyanions as well as during self-assembly of pristine tau [7]. Phosphorylation of serine and threonine residues stabilize the α-helical conformation. αS fibrils acquire an overall assembly that mimics a

Greek key as seen in atomic resolution structure of purified protein [7].

also lead to formation of striated and twisted ribbons [11].

**2.2 Structural properties of amyloid aggregates**

**192**

fibrils [2].

Amyloidosis in cerebrovascular system is mediated through aggregation of wild type proteins as the consequence of multivalent interactions in intrinsically disordered proteins or regions of proteins, mutations in amyloidogenic precursors, expansion of repeats in the amyloidogenic sequences, actions of proteases or chemical modifications on the precursor sequences, overexpression of the precursor, liquid–liquid phase separation, actions of small metabolites or age-related cell death [27–31]. The number of human proteins capable of causing amyloidosis has exceeded 50 and are associated with many neurological disorders based of variations in the disease precursors [2]. Different oligomeric and morphologically distinct species affect cells and tissues in different manner. Amyloid oligomers can be cytotoxic with the long-term potentiation disrupting membranes to cause ion permeability and homeostasis imbalance [2]. Fully matured fibrils contribute to the disease by disturbing lipid membranes and thus interfere with the general membrane bound cellular organelles, or by forming physical barriers which disrupt inter-cellular communications [2]. Structural inhomogeneity was observed in fibrils from the different clinical variants of amyloids in AD. Differences in morphology of Aβ aggregates can change the phenotype observed in different forms of AD. While a single Aβ structure is found in brain cortex in the cases of typical prolonged duration AD, different polymorphs are found in the cases of rapidly progressive form of AD [32]. There are six isoforms of tau protein in brain and CNS, longest one containing a stretch of 441 amino acids, with high tendency for phosphorylation and self-assembly. Tau filaments cause different tauopathies, including neurofibrillary tangles in AD [7]. Neurofibrillary tangles and neuropil threads are predominantly tau aggregates localized in neuronal cell bodies and processes, respectively, whereas the mature Aβ plaques are present extracellularly [33].

### **2.4 Toxicity of amyloid aggregates**

Aβ(1–42) is more potent fibrillogenic Aβ variant in human brain, yet its concentration accounts for only about 5–10% of Aβ(1–40) concentration produced [33]. Accumulation of Aβ(1–42) in brain and cerebrovascular system due to its aggregation propensity, and by extension, the relative ratio of Aβ(1–42): Aβ(1–40) is a biomarker for AD [33]. Aβ(1–40) fibrils are less toxic compared to Aβ(1–42) fibrils which show differential toxicity and Aβ(1–43) fibrils are most cytotoxic [10]. Soluble Aβ in the form of low-n oligomers, Aβ-derived diffusible ligands and protofibrils are among major toxic contributors towards AD pathology [34]. Interactions of oligomeric species with other molecular complexes, metal ions and cellular membranes also impact the extent of their toxic effects in brain. N-terminus truncated Aβ peptides (Aβ(n–42), where n can vary from 2 to 11) are highly toxic and are usually present in Aβ deposits in AD brain [7]. Peptides cleaved at positions 3 or 11 are prone to pyroglutamylation and prominent components of Aβ deposits, likely owing to their resistance to proteolytic degradation [35]. αS is a major component of Lewy bodies and neurites causing neurotoxicity through dopaminergic mechanism and these aggregates are key identifiers for PD and Lewy body dementia [7]. Detection of αS aggregates early in peripheral nervous system can serve as early biomarkers before motor disabilities develop in PD patients [36]. Horizontal transfer of amyloid aggregates and oligomers between cells also contributes to propagation of disease pathology [37]. IAPP oligomers can impair insulin secretion in pancreatic cells and cause cell death and cellular uncoupling [7].
