In Vitro Filament-like Formation upon Interaction between Lens α-Crystallin and βL-Crystallin Promoted by Stress

Abstract

purpose. To determine whether α-crystallin is capable of forming filament-like structures with other members of the crystallin family.

methods. Water-soluble crystallins were isolated from calf lenses and fractionated into α-, βH-, βL-, and γ-crystallins according to standard procedures. Chaperone-like activity of α-crystallin was determined in control and UV-A–irradiated lenses by the heat-induced aggregation assay of βL-crystallin. Protein samples from this assay were analyzed by electron microscopy. In vitro filament formation was examined by transmission immunoelectron microscopy using specific antibodies directed against the crystallins. Involvement of intermediate filament constituents was excluded by the results of Western blot analysis, which were all negative. Moreover, the in vitro amyloid fibril interaction test using thioflavin T (ThT) was also performed.

results. At the supramolecular level heating at 60°C has no effect on the morphologic appearance of α-crystallin as observed by transmission electron microscopy. Moreover α-crystallin obtained from UV-A–irradiated lenses shows a virtually identical shape. However, heating in the presence of βL-crystallin results in the formation of filament-like αβ-hybrids as demonstrated by immunoelectron microscopy using specific antibodies directed either against α- orβ L-crystallin. Parallel experiments with α-crystallin derived from UV-A–irradiated lenses showed even more pronounced filamentous structures, compared with the controls. Nonetheless, we were able to show that the UV-light treatment affected the chaperone-like capacity of α-crystallin, as revealed by a diminished ability to inhibit in vitro denaturation of βL-crystallin. To exclude the presence of cytoskeletal contamination in the crystallin preparations, vimentin antibodies were also tested. These latter experiments were negative. The filamentous nature of the hybrids was further confirmed by the results obtained with the ThT assay earlier applied for the detection of amyloid fibrils.

conclusions. Crystallin hybrids have previously been detected in the water-soluble lens crystallin fraction. Our findings indicate that such endogenous hybrids, formerly called “rods,” may result from stress-induced interaction between α-crystallin and other lens constituents such as βL-crystallin. Because the hybrid formation is enhanced when α-crystallin from UV-A–irradiated lenses is used as one of the two components of the hybrid, one can only speculate that this formation may be one of the factors leading to UV-A cataract.

Two members of the small heat shock protein family, ^{1 2} αA- and αB-crystallin, possess molecular chaperone properties. ³ A decade ago it became apparent that these two polypeptides, which form 800-kDa aggregates in the lens, also exist in a variety of other tissues. ^{4 5 6} Current studies indicate that small heat shock proteins like αB-crystallin are able to interact with intermediate filaments in response to stress and to function as molecular chaperones. ^{7 8 9 10} Earlier ultrastructural observations showed that crude fractions from chicken lens consisted of 5- to 6-nm-thick core filaments and irregularly sized globular particles 15 to 20 nm in diameter called “beaded filaments.” ¹¹ It was noted that the “beads” had dimensions that were similar to native α-crystallin. ¹² Moreover, two proteins with molecular masses of 115 and 49 kDa, respectively (named filensin and phakinin), have been localized in the beaded filament fraction of the lens with the aid of immunoelectron microscopy. ^{13 14} However, the question still remains whether or not other lens proteins may be involved in the formation of filamentous structures. In this report we demonstrate that water-soluble α-crystallin has the ability to form, in response to heat stress, in vitro filament-like structures with one other crystallin, namely βL-crystallin. This filament formation is enhanced by UV-A irradiation. 

Methods

UV-A Irradiation

Lenses, excised from 2- to 4-year-old bovine eyes, were irradiated in special organ culture glass vessels described previously by Dovrat and Weinreb. ¹⁵ Briefly, a 400 W UV lamp (Vilber, Lourmat Cedex, France) contained a filter that provided radiation of 33 J/cm² for 75 minutes at 365 nm. 

Fractionation of Crystallins

Lenses were dissected under a binocular stereomicroscope. The lens cortex was homogenized in 100 mM Tris buffer at pH 7.5 and spun at 4°C at 13,000g for 30 minutes. The supernatant comprises the water-soluble lens fraction. Separation of this fraction into α-,β H-, βL-, and γ-crystallin was carried out by gel filtration on a Sephacryl S-300 (Pharmacia-LKB, Uppsala, Sweden) HR column. ¹⁶  

Chaperone-like Activity

The chaperone-like activity of α-crystallin from control and UV-irradiated lenses was determined by the heat-induced aggregation assay of βL-crystallin at 60°C. ³ The proteins were dissolved in a solution of 20 mM sodium phosphate, 100 mM Na₂SO₄, 10 mM EDTA, at pH 6.9. The assay was performed at a concentration of 0.25 mg/ml substrate protein and 0.05 mg/ml α-crystallin. 

Electron Microscopy

Protein samples from the heating assay were also analyzed by electron transmission microscopy. Samples were negatively stained with uranyl acetate (1% v/v). The in vitro filament formation ofα -crystallin from control and UV-treated lenses with βL-crystallin was followed by immunoelectron microscopy using antibodies against vimentin, αA-, αB-, and βL-crystallin. The grids were examined with a transmission electron microscope (Jeol TEM1210, Tokyo, Japan) using 70 to 80 kV. 

Thioflavin T Interaction Assay

Fluorometric determinations were carried out using the thioflavin T (ThT) interaction assay at excitation and emission of 450 and 482 nm, respectively. ¹⁷  

Results

Electron Microscopy

Micrographs of mixtures of α- and βL-crystallin, obtained from control lenses and α-crystallin from UV-A–treated lenses, which were separately heated at 60°C, are shown in Figure 1 . Apparently heating has no effect on α-crystallin obtained from the control lenses (Fig. 1A) , because comparison with previously published electron micrographs of nonheated α-crystallin revealed no detectable morphologic differences. ¹⁸ Normal α-crystallin consists of molecules having an apparent spherical structure, with a diameter of approximately 17 nm. Likewise α-crystallin obtained from irradiated lenses shows a similar shape, albeit the size is somewhat smaller (Fig. 1B) . After incubation at 60°C, βL-crystallin (Fig. 1C) lost its irregular spherical shape when compared with nonheated β-crystallin as described earlier. ¹⁸  

The in vitro filament formation was also examined by immunoelectron microscopy (Fig. 2) . Anti–αB- and anti–βL-crystallin labeling yielded identical results (not shown). The formation of filament-like structures can be observed after the heating assay at 60°C using 0.05 mg α-crystallin obtained from control lenses with 0.25 mg βL-crystallin. The identical experiment with α-crystallin from irradiated lenses (Figs. 3A 3B) showed more pronounced filament-like structures compared with the control. The results show that UV-A irradiation promotes the filament-like formation. Experiments with anti-vimentin were negative showing that no intermediate filament component was involved in the crystallin hybrid formation. 

Chaperone-like Activity

The chaperone-like activity determined with the aid of the protein scattering at 360 nm is depicted in Figure 4 . It can be seen that this property of the water-soluble α-crystallin was affected by UV-A light. Compared with controls (curve II),α -crystallin derived from UV-A–irradiated lenses revealed a decreased ability to inhibit βL-crystallin denaturation in vitro (curve III). Curve IV represents βL-crystallin in the absence ofα -crystallin. Furthermore, α-crystallin obtained from control and UV- irradiated lenses did not denature during 30 minutes of incubation at 60°C (compare the coinciding curves Ia and Ib). These results are consistent with previous reports that described decreased chaperone-like activity of α-crystallin on UV-B irradiation. ^{19 20 21}  

ThT Interaction Assay

The results of the ThT test are depicted in Figure 5 This assay, in which fibrils convert to a β-sheet configuration in vitro, has previously been successfully applied for detection of amyloid fibrils. ¹⁷ It can be seen that heatedβ L-crystallin or heated βL-crystallin plus α-crystallin from control lenses produced a 10 times higher fluorescence value than heated α-crystallin obtained from control and UV-irradiated lenses alone. The amount of fluorescence increased when heated βL-crystallin is assembled with α-crystallin from UV-treated lenses. 

Conclusion

Previously Slingsby et al. ²² suggested a new model for crystallin assembly in lens fiber cells. In the highly hydrated solution-like region of the lens, it is envisaged that weak interaction between subunits such as those of β-crystallin will occur, forming elements of a network with dynamic branching. An open gel structure would maintain protein–protein interactions at a high concentration, covering the more prominent hydrophobic regions and preventing random aggregation of subunits. This may possibly explain the present observation that (heated) βL-crystallin assembles withα -crystallin, resulting in filament-like structures. It cannot be excluded that one or more of UV-A–provoked alterations ²³ are related to the ability of water-soluble α-crystallin to form filaments in vitro more efficiently than with α-crystallin derived from control lenses. The in vitro filament-like chains identified by electron microscopy after irradiation have a high degree of morphologic similarity to the αβ-hybrids that have been described previously after reconstitution of the dissociated total mixture of the water-soluble crystallins. ¹⁸ Dhir et al. ²⁴ have recently shown by in vitro UV-A irradiation of recombinantα A-crystallin that sensitized photooxidation can occur in amino acids other than Trp in the presence of kynurenine or 3-hydroxykynurenine with effects similar to, albeit smaller than, direct UV-B photooxidation. In the old lens, other types of sensitizers may be operative, such as advanced glycation end products (AGE). Finley et al., ²⁵ studying the photooxidation sites in bovineα A-crystallin, found that in addition to Trp, Met and His were photooxidized. Their conclusion is that the N-terminal region ofα A-crystallin is exposed to an aqueous environment and is in the vicinity of Trp from neighboring subunits. Albeit we did not try to identify the exact site of photooxidation being beyond the aim of our study, it might well be that particularly AGE could play a role as sensitizer because we used adult bovine lenses. Besides, the relatively large amount of NAD(P)H in bovine lens could also initiate photochemical processes as it does in human and rabbit lens cells. ²⁶ Furthermore, the ThT interaction assay, which is used as a method for the demonstration of β-sheet conformation and which appeared previously to be a useful tool for detection of amyloid fibrils in vitro, ¹⁷ provided additional evidence for possible αβ-crystallin filament formation (Fig. 5) . According to Levine, ²⁷ it is very likely that both the β-sheet conformation and the aggregation state provide the environment to stabilize the long wavelength ThT fluorescent complex, regardless of the identity of the participating peptides. Therefore, at least some of the endogenous filament-like structures that have been demonstrated in the lens may result from interaction of α-crystallin with other proteins such as βL-crystallin under stress conditions. This might provide a clue regarding the processes leading to the development of UV cataract. 

 Supported by a Marie Curie Research Training Grant, Biotechnology, European Commission Science Research Development (Bio.4-CT96 to 5121) (OW) and by Dr. Endre Balazs and The Biomatrix Institute (HB).

 Submitted for publication November 12, 1999; revised April 11, 2000 and June 12, 2000; accepted July 5, 2000.

 Commercial relationships policy: N.

 

Figure 1.

 

Electron micrographs of (A) α-crystallin obtained from control lenses (bar, 200 nm); (B) α-crystallin from UV-A–treated lenses (bar, 200 nm); (C) βL-crystallin obtained from control lenses separately heated at 60°C (bar, 500 nm). Complexes were visualized by negative staining with uranyl acetate.

Figure 2.

 

Immunogold labeling with anti–αA-crystallin of samples from the heating assay. 0.05 mg of α-crystallin from control lenses incubated with 0.25 mg βL-crystallin at 60°C. F, filament-like chains; αA, labeling (bar, 500 nm).

Figure 3.

 

(A) Filament samples from the heating assay. α-Crystallin, 0.05 mg, from UV-A–treated lenses incubated with 0.25 mgβ L-crystallin at 60°C (bar, 500 nm); (B) filaments at higher magnification (bar, 100 nm). F, filament-like structures; long arrows in (B), labeling with anti–αA-crystallin.

Figure 4.

 

Chaperone-like activity of water-soluble α-crystallin fraction obtained from the cortex of control and UV-A–irradiated bovine lenses determined by heating assay at 60°C with 0.25 mg ofβ L-crystallin and 0.05 mg of α-crystallin.

Figure 5.

 

Fluorescence determination using Thioflavin T (ThT) interaction with samples obtained from the heating assay, measured at excitation of 450 nm and emission of 482 nm. ThT concentration was 250 nM. The reaction buffer contained 50 mM glycine-NaOH at pH 6.0. Bars, SD (in four experiments).

We thank Wilfried W. de Jong for fruitful discussions and Lucio Benedetti for advice concerning electron microscopy.