Amyloid Fibril Formation by Lens Crystallin Proteins and Its Implications for Cataract Formation

Abstract

The ␣-, ␤-, and ␥-crystallins are the major structural proteins within the eye lens and are responsible for its exceptional stability and transparency. Under mildly denaturing conditions, all three types of bovine crystallin assemble into fibrillar structures in vitro. Characterization by transmission electron microscopy, dye binding assays, and x-ray fiber diffraction shows that these species have all of the characteristics of fibrils associated with the family of amyloid diseases. Moreover, the full-length proteins are incorporated into the fibrils, (i.e. no protein cleavage is required for these species to form), although for the ␥-crystallins some fragmentation occurs under the conditions employed in this study. Our findings indicate that the inherent stability of the ␤-sheet supramolecular structure adopted by the crystallins in the eye lens and the chaperone ability of ␣-crystallin must be crucial for preventing fibril formation in vivo. The crystallins are very stable proteins but undergo extensive post-translational modification with age that leads to their destabilization. The ability of the crystallins to convert into fibrils under destabilizing conditions suggests that this process could contribute to the development of cataract with aging.

Crystallin proteins and amyloid fibrils

“…Our work has shown that each of the crystallin protein classes (α, β and γ) readily forms amyloid fibrils under mildly denaturing conditions [153]. The fibrils have all the generic structural and physical characteristics of amyloid fibrils (Fig.…”

“…The advantage of crystallin proteins is their ready availability in significant quantity which is of particular importance if amyloid fibrils are to be used for commercial purposes. As described above, individual α-and γ-crystallin subunits readily form fibrils under mildly denaturing conditions [152,155] as do the individual α-, β-and γ-crystallin classes [153]. Our recent work has shown that semi-pure and crude mixtures of the crystallins from bovine, ovine and deer lenses formed amyloid fibrils under similar conditions employed for the preparation of crystallin fibrils, i.e.…”

“…If present, however, they may be mixed in with amorphously aggregated crystallin proteins. Interestingly, αA-and αB-crystallin are as susceptible to fibril formation as the β-and γ-crystallins [152,153]. Thus, even the molecular chaperones αA-and αB-crystallin, which are highly stable proteins (like the other crystallins), are prone to aggregate and form fibrils, despite their role in protein stabilization in the lens.…”

The functional roles of the unstructured N- and C-terminal regions in αB-crystallin and other mammalian small heat-shock proteins

“…(*)The C-terminal extensions for HspB1/Hsp27, HspB2/ MKBP and HspB6/Hsp20 were identified from 1 H NMR spectroscopic studies of sHsps from other mammals, i.e. mouse, rat and rat respectively Our work with αAc and αBc has shown that they form amyloid fibrils under slightly destabilising conditions, for example, in the presence of low concentrations of denaturant and elevated temperature (Meehan et al 2004;Meehan et al 2007). The E. coli sHsp IbpA forms amyloid fibrils under physiological conditions in vitro, which is prevented by the presence of its co-chaperone IbpB (Ratajczak et al 2010).…”

Small heat-shock proteins: important players in regulating cellular proteostasis

“…Under physiological conditions, human Bc forms large roughly spherical assemblies of 8-18 nm in diameter [7,23]. Under partially denaturing conditions and elevated temperature, Bc also assembles into amyloid fibrils as revealed by TEM and atomic force microscopy [24,25].…”