αB-crystallin, a small heat-shock protein, prevents the amyloid fibril growth of an amyloid β-peptide and β2-microglobulin

Abstract

AlphaB-crystallin, a small heat-shock protein, exhibits molecular chaperone activity. We have studied the effect of alphaB-crystallin on the fibril growth of the Abeta (amyloid beta)-peptides Abeta-(1-40) and Abeta-(1-42). alphaB-crystallin, but not BSA or hen egg-white lysozyme, prevented the fibril growth of Abeta-(1-40), as revealed by thioflavin T binding, total internal reflection fluorescence microscopy and CD spectroscopy. Comparison of the activity of some mutants and chimaeric alpha-crystallins in preventing Abeta-(1-40) fibril growth with their previously reported chaperone ability in preventing dithiothreitol-induced aggregation of insulin suggests that there might be both common and distinct sites of interaction on alpha-crystallin involved in the prevention of amorphous aggregation of insulin and fibril growth of Abeta-(1-40). alphaB-crystallin also prevents the spontaneous fibril formation (without externally added seeds) of Abeta-(1-42), as well as the fibril growth of Abeta-(1-40) when seeded with the Abeta-(1-42) fibril seed. Sedimentation velocity measurements show that alphaB-crystallin does not form a stable complex with Abeta-(1-40). The mechanism by which it prevents the fibril growth differs from the known mechanism by which it prevents the amorphous aggregation of proteins. alphaB-crystallin binds to the amyloid fibrils of Abeta-(1-40), indicating that the preferential interaction of the chaperone with the fibril nucleus, which inhibits nucleation-dependent polymerization of amyloid fibrils, is the mechanism that is predominantly involved. We found that alphaB-crystallin prevents the fibril growth of beta2-microglobulin under acidic conditions. It also retards the depolymerization of beta2-microglobulin fibrils, indicating that it can interact with the fibrils. Our study sheds light on the role of small heat-shock proteins in protein conformational diseases, particularly in Alzheimer's disease.

AβPP-overexpression and proteasome inhibition increase αB-crystallin in cultured human muscle: Relevance to inclusion-body myositis

“…In Alzheimer brain, αBC binds to AβPP [26], and αBC mRNA is increased (referenced in [26]). In vitro, αBC prevents Aβ fibril growth and spontaneous fibril formation, binds Aβ, and prevents its aggregation [27][28][29]. However, when applied extracellularly to cultured rat neurons, concomitantly with Aβ, αBC increases Aβ cytotoxicity [28], possibly due to αBC's influence in maintaining Aβ in its soluble oligomeric, highly cytotoxic form [28].…”

Crystallin proteins and amyloid fibrils

“…Also, the chaperone activity of αB-crystallin can be easily compared with well-established data on its chaperone activity against amorphously aggregating target proteins. Thus, studies have shown that αB-crystallin inhibits the fibrillation of α-synuclein [48, 83,93], β2-microglobulin [94] (Esposito, Carver, et al, unpublished results), κ-casein [95,96], the ccβ-Trp peptide [95], apoC-II [97], and the prion protein (Ecroyd, unpublished results). In doing so, αB-crystallin acts as a chaperone under a range of solvent conditions, including at pH values as low as 2.5 against β2-microglobulin fibrillation [94].…”

“…They also found that the toxicity of aggregated DAβ(1-40) and Aβ(1-42) to human brain pericytes was completely abolished by co-incubation with αB-crystallin (at a 1.0: 0.2 molar ratio of DAβ(1-40): αB-crystallin and a 1.0: 0.02 molar ratio of Aβ(1-42): αB-crystallin). Raman et al [94] have reported that the increase in ThT fluorescence associated with fibril formation by the Aβ peptides is completely inhibited by a 1.0: 0.05 molar ratio of Aβ(1-40): αB-crystallin and a 1.0: 0.1 molar ratio of Aβ(1-42): αB-crystallin (αB-crystallin was also found to be more effective at preventing the fibril formation of Aβ(1-40) than αA-crystallin). These authors concluded that no stable complex forms between αB-crystallin and Aβ(1-40), but instead αB-crystallin acts primarily by forming a complex with the fibril nucleus to prevent its growth [94].…”

“…Raman et al [94] have reported that the increase in ThT fluorescence associated with fibril formation by the Aβ peptides is completely inhibited by a 1.0: 0.05 molar ratio of Aβ(1-40): αB-crystallin and a 1.0: 0.1 molar ratio of Aβ(1-42): αB-crystallin (αB-crystallin was also found to be more effective at preventing the fibril formation of Aβ(1-40) than αA-crystallin). These authors concluded that no stable complex forms between αB-crystallin and Aβ(1-40), but instead αB-crystallin acts primarily by forming a complex with the fibril nucleus to prevent its growth [94]. αA-Crystallin has also been shown to completely inhibit fibril formation by Aβ(1-40) at a 1.0: 0.2 molar ratio (Aβ(1-40): αA-crystallin) and when αA-crystallin was added together with Aβ(1-40) to rat neuronal (PC12) cells (at the same molar concentration), it suppressed the toxicity compared to when Aβ(1-40) was added on its own [102].…”

“…This conclusion is based on studies which have compared the chaperone activity of mutant and/or chimaeric forms of α-crystallin proteins, (i.e. proteins in which the N-terminal domain of αB-crystallin has been fused with the C-terminal domain of αA-crystallin and vice-versa) [94] against both amorphous and amyloid fibril forming target proteins. For example, the chaperone activity of the R120G αB-crystallin mutant (which is linked to desmin-related myopathy and early onset cataract) towards amorphously aggregating target proteins is significantly reduced compared to the wild-type protein [104,105], however, its ability to prevent fibril formation by Aβ(1-40) is only slightly lower [94].…”