Evidence for the participation of alpha B-crystallin in human age-related nuclear cataract

The Aggregation of αB-Crystallin under Crowding Conditions Is Prevented by αA-Crystallin: Implications for α-Crystallin Stability and Lens Transparency

One of the most crowded biological environments is the eye lens which contains a high concentration of crystallin proteins. The molecular chaperones αB-crystallin (αBc) with its lens partner αA-crystallin (αAc) prevent deleterious crystallin aggregation and cataract formation. However, some forms of cataract are associated with structural alteration and dysfunction of αBc. While many studies have investigated the structure and function of αBc under dilute in vitro conditions, the effect of crowding on these aspects is not well understood despite its in vivo relevance. The structure and chaperone ability of αBc under conditions that mimic the crowded lens environment were investigated using the polysaccharide Ficoll 400 and bovine γ-crystallin as crowding agents and a variety of biophysical methods, principally contrast variation small-angle neutron scattering. Under crowding conditions, αBc unfolds, increases its size/oligomeric state, decreases its thermal stability and chaperone ability, and forms kinetically distinct amorphous and fibrillar aggregates. However, the presence of αAc stabilizes αBc against aggregation. These observations provide a rationale, at the molecular level, for the aggregation of αBc in the crowded lens, a process that exhibits structural and functional similarities to the aggregation of cataract-associated αBc mutants R120G and D109A under dilute conditions. Strategies that maintain or restore αBc stability, as αAc does, may provide therapeutic avenues for the treatment of cataract.

Understanding curcumin-induced modulation of protein aggregation

Curcumin, a diarylheptanoid compound, found in spice turmeric is known to alter the aggregation of proteins and reduce the toxicity of the aggregates. This review looks at the molecular basis of modulating protein aggregation and toxicity of the aggregates. Foremost, we identify the interaction of curcumin and its derivatives with proteins/peptides and the effect of their interaction on the conformational stability and unfolding/folding pathway(s). The unfolding/folding processes generate partially folded/unfolded intermediate, which serve as aggregation precursor state. Secondly, we discuss the effect of curcumin binding on the kinetics parameters of the aggregation process, which give information about the mechanism of the aggregation inhibition. We describe, in addition, that curcumin can accelerate/promote fibril formation by binding to oligomeric intermediate(s) accumulated in the aggregation pathway. Finally, we discuss the correlation of curcumin-induced monomeric and/or oligomeric precursor states with aggregate structure and toxicity. On the basis of these discussions, we propose a model describing curcumin-induced inhibition/promotion of formation of amyloid-like fibrils.

Chaperone-independent mitochondrial translocation and protection by αB-crystallin in RPE cells

αB-crystallin is a small heat shock protein that exhibits chaperone activity and can protect multiple cell types against oxidative stress damage. Altered levels and specific mutations of αB-crystallin are associated with multiple degenerative diseases. We previously found that αB-crystallin translocates to lens and retinal cell mitochondria upon oxidative stress exposure where it provides protection against oxidative stress damage. To date, the role of the chaperone function of αB-crystallin in mitochondrial translocation and protection has not been established. Here, we sought to determine the relationship between the chaperone activity of αB-crystallin and its ability to translocate to and protect retinal cell mitochondria against oxidative stress damage. Our data provide evidence that three forms of αB-crystallin exhibiting different chaperone activity levels including wild-type, R120G (decreased chaperone activity) and M68A (increased chaperone activity) provide comparable levels of mitochondrial translocation and protection to retinal cells exposed to oxidative stress. The results provide evidence that mitochondrial translocation and protection by αB-crystallin is independent of its chaperone activity and that other functions of αB-crystallin may also be independent of its chaperone activity.

αB-crystallin/sHSP protects cytochrome c and mitochondrial function against oxidative stress in lens and retinal cells

BACKGROUND

αB-crystallin/sHSP protects cells against oxidative stress damage. Here, we mechanistically examined its ability to preserve mitochondrial function in lens and retinal cells and protect cytochrome c under oxidative stress conditions.

METHODS

αB-crystallin/sHSP was localized in human lens (HLE-B3) and retinal (ARPE-19) cells. αB-crystallin/sHSP was stably over-expressed and its ability to preserve mitochondrial membrane potential under oxidative stress conditions was monitored. Interactions between αB-crystallin/sHSP and cytochrome c were examined by fluorescent resonance energy transfer (FRET) and by co-immune precipitation. The ability of αB-crystallin/sHSP to protect cytochrome c against methionine-80 oxidation was monitored.

RESULTS

αB-crystallin/sHSP is present in the mitochondria of lens and retinal cells and is translocated to the mitochondria under oxidative conditions. αB-crystallin/sHSP specifically interacts with cytochrome c in vitro and in vivo and its overexpression preserves mitochondrial membrane potential under oxidative stress conditions. αB-crystallin/sHSP directly protects cytochrome c against oxidation.

GENERAL SIGNIFICANCE

These data demonstrate that αB-crystallin/sHSP maintains lens and retinal cells under oxidative stress conditions at least in part by preserving mitochondrial function and by protecting cytochrome c against oxidation. Since oxidative stress and loss of mitochondrial function are associated with eye lens cataract and age-related macular degeneration, loss of these αB-crystallin/sHSP functions likely plays a key role in the development of these diseases. αB-crystallin/sHSP is expressed throughout the body and its ability to maintain mitochondrial function is likely important for the prevention of multiple degenerative diseases.

Enhanced degradation and decreased stability of eye lens alpha-crystallin upon methylglyoxal modification

Methylglyoxal (MGO), a potent glycating agent, forms advanced glycation end products (AGEs) with proteins. Several diabetic complications including cataract are thought to be the result of accumulation of these protein-AGEs. alpha-Crystallin, molecular chaperone of the eye lens, plays an important role in maintaining the transparency of the lens by preventing the aggregation/inactivation of several proteins/enzymes in addition to its structural role. Binding of adenosine-5-triphosphate (ATP) to alpha-crystallin has been shown to enhance its chaperone-like function and protection against proteolytic degradation. In the earlier study, we have shown that modification of alpha-crystallin by MGO caused altered chaperone-like activity along with structural changes, cross-linking, coloration and subsequent insolubilization leading to scattering of light [Biochem. J. 379 (2004) 273]. In the present study, we have investigated ATP binding, stability and degradation of MGO-modified alpha-crystallin. Proteolytic digestion with trypsin and chymotrypsin showed that MGO-modified alpha-crystallin is more susceptible to degradation compared to native alpha-crystallin. Furthermore, ATP was able to protect native alpha-crystallin against proteolytic cleavage but not MGO-modified alpha-crystallin. Interestingly, binding studies indicate decreased ATP binding to MGO-modified alpha-crystallin and support the decreased protection by ATP against proteolysis. In addition, differential scanning calorimetric and denaturant-induced unfolding studies indicate that modification of alpha-crystallin by MGO leads to decreased stability. These results indicate that MGO-modification of alpha-crystallin causes partial unfolding and decreased stability leading to enhanced proteolysis. Cross-linking of these degraded products could result in aggregation and subsequent insolubilization as observed in senile and diabetic cataract lenses.

Photochemically modified alpha-crystallin: a model system for aging in the primate lens

The purpose of this study was to quantitatively study the changes that occur upon irradiation of 3-hydroxykynurenine (3-HK) in the presence of alpha-crystallin under conditions similar to those in the lens. The samples were prepared in 10 mM phosphate buffer at pH 7.4, bubbled with O2 or Ar and irradiated with 300-400 nm light. The amount of light absorbed by the samples (Iabs) was measured using azobenzene as an actinometer. Modifications to alpha-crystallin were monitored by ultraviolet-visible and fluorescence spectroscopy. Aerobic samples had increased absorption around 320 nm and above 400 nm while the 3-HK maximum at 368 nm decreased. The isolated modified protein showed that there was increased absorption throughout the spectrum. Changes in the anaerobic samples were similar to those of the aerobic but occurred more slowly. As irradiation time increased fluorescence emission of the isolated protein red shifted and quantum yields of fluorescence (phi f) were calculated at different irradiation time intervals by comparison to 3-HK. By comparing OD320/OD365 for the model system to values from primate lenses, Iabs can be correlated with age and transmission of the sample in the blue region of the spectrum and thus allows lenticular aging to be quantitated.

Mitotic misregulation and human aging

Messenger RNA levels were measured in actively dividing fibroblasts isolated from young, middle-age, and old-age humans and humans with progeria, a rare genetic disorder characterized by accelerated aging. Genes whose expression is associated with age-related phenotypes and diseases were identified. The data also suggest that an underlying mechanism of the aging process involves increasing errors in the mitotic machinery of dividing cells in the postreproductive stage of life. We propose that this dysfunction leads to chromosomal pathologies that result in misregulation of genes involved in the aging process.

The Nitrite/alpha crystallin reaction: a possible mechanism in lens matrix damage

Recent work has explored the potential deleterious role that nitric oxide (NO) and its derivatives may have in human disease. The many by-products of NO include nitrite ion, which accumulates in the anterior chamber during ocular inflammation and can be derived from cigarette smoking. Cigarette smoking has been strongly linked to nuclear cataract formation, although the mechanism remains unknown. We have previously observed that nitrite reactions with the matrix proteins elastin and collagen produce damaging effects that mimic those observed in age- and smoking-related illnesses. In the present study we report on the reaction of nitrite with alpha crystallin, the major lens matrix protein. Incubations at neutral pH and body temperature of nitrite with alpha crystallin resulted in protein modifications indicative of oxidative damage and similar to changes seen in the aging lens as well as cataracts. These include increased fluorescence, yellowing and protein cross-linking. L-kynurenine, a tryptophan derivative, was identified as a reaction product. L-kynurenine was also formed from the reaction of nitrite with free tryptophan. Thus, this non-enzymatic nitration of alpha crystallin provides a novel mechanism by which lens proteins may be damaged in vivo. Since human exposure to nitrite is increased by cigarette smoking, this reaction could provide an explanation for the association between nuclear cataracts and smoking.