Functional Amyloid Protection in the Eye Lens: Retention of α-Crystallin Molecular Chaperone Activity after Modification into Amyloid Fibrils

Abstract

Amyloid fibril formation occurs from a wide range of peptides and proteins and is typically associated with a loss of protein function and/or a gain of toxic function, as the native structure of the protein undergoes major alteration to form a cross β-sheet array. It is now well recognised that some amyloid fibrils have a biological function, which has led to increased interest in the potential that these so-called functional amyloids may either retain the function of the native protein, or gain function upon adopting a fibrillar structure. Herein, we investigate the molecular chaperone ability of α-crystallin, the predominant eye lens protein which is composed of two related subunits αA- and αB-crystallin, and its capacity to retain and even enhance its chaperone activity after forming aggregate structures under conditions of thermal and chemical stress. We demonstrate that both eye lens α-crystallin and αB-crystallin (which is also found extensively outside the lens) retain, to a significant degree, their molecular chaperone activity under conditions of structural change, including after formation into amyloid fibrils and amorphous aggregates. The results can be related directly to the effects of aging on the structure and chaperone function of α-crystallin in the eye lens, particularly its ability to prevent crystallin protein aggregation and hence lens opacification associated with cataract formation.

Proteostasis and the Regulation of Intra- and Extracellular Protein Aggregation by ATP-Independent Molecular Chaperones: Lens α-Crystallins and Milk Caseins

“…lack of metabolic activity) of lens fiber cells. Consistent with this, amyloid fibrillar forms of α-crystallin and αΒ-crystallin alone function well as molecular chaperones, 23 as do the amyloidogenic α-crystallin mini-chaperone peptides. 25 Thus, in the lens environment where significant temporal changes occur, i.e.…”

“…22 Despite the associated structural changes, aged α-crystallin retains functionality. For example, the highly aggregated form of α-crystallin has reasonable chaperone ability, 23 and the central ACD of αB-crystallin itself is an effective chaperone, 24 as are peptide fragments of the ACD. 25 What other factors may alter lens proteostatsis and lead to crystallin aggregation and hence lens opacification?…”

Structural and Functional Peculiarities of α-Crystallin

“…As shown, α-crystallin fibrils exhibit chaperone activity; so, both dimers and α-crystallin fibrils exhibit chaperone activity [48]. Current data suggest that, at a high α-crystallin concentration in the eye lens, α-crystallin has a gel-like state and, even in the form of amyloids, retains the chaperone function [48].…”

“…Separate oligomer complexes with parameters characteristic of α-crystallin from the nucleus can be observed in areas where the preparation was not adsorbed (less adsorption occurred) (Figure 2, Figure 3, and Figure 6D). Amyloidogenic fibrils of α-crystallin have been described previously [47,48]. However, for their formation, α-crystallin was treated with 1 M guanidine hydrochloride [47,48], i.e., destabilization of α-crystallin was used.…”

“…Amyloidogenic fibrils of α-crystallin have been described previously [47,48]. However, for their formation, α-crystallin was treated with 1 M guanidine hydrochloride [47,48], i.e., destabilization of α-crystallin was used.…”