Photoreactions of human lens monomeric crystallins

Negative charge at aspartate 151 is important for human lens αA-crystallin stability and chaperone function

Aggregation of lens protein is a major cause of senile cataract. Lens crystallins contain many kinds of modification that accumulate over lifespan. In particular, isomerization of Asp 151 in αA-crystallin has been found in aged lenses; however, its significance is unknown. The purpose of this study was to determine the effects of isomerization of Asp 151 in αA-crystallin. Trypsin digestion followed by liquid chromatography-mass spectrometry analysis of the water-soluble high molecular weight (HMW) fraction from human lens samples showed that isomerization of Asp 151 in αA-crystallin is age-independent, and that 50% of isomerization occurs shortly after birth. However, the extent of Asp 151 isomerization varied with the size of αA-crystallin oligomer species separated from the HMW fraction from aged lens. To evaluate the effects of modification, Asp 151 of αA-crystallin was replaced by glycine, alanine, isoleucine, asparagine, glutamate, or lysine by site-directed mutagenesis. All substitutions except for glutamate decreased heat stability and chaperone function as compared with wild-type αA-crystallin. In particular, abnormal hydrophobicity and alteration of the charge state at Asp 151 caused loss of stability and chaperone activity of αA-crystallin; these properties were recovered to some extent when the mutant protein was mixed 1:1 with wild-type αA-crystallin. The results suggest that, by itself, age-independent isomerization of Asp 151 in αA-crystallin may not contribute to cataract formation. However, the long-term deleterious effect of Asp 151 isomerization on the structure and function of αA-crystallin might cooperatively contribute to the loss of transparency of aged human lens.

Asp 58 modulates lens αA-crystallin oligomer formation and chaperone function

Senile cataract onset is caused by insolubilization of lens proteins. The lens crystallin protein family correctly orders the formation of homo- or hetero-oligomers in lens fiber cells. Because lens fiber cells do not divide, covalent post-translational modifications, such as isomerization of aspartate residues, accumulate with aging. Although many isomerization sites of αA-crystallin have been reported, their structural and functional contributions have never been identified. In this study, αA-crystallin was extracted from aged human lens and separated into each oligomeric state by size exclusion chromatography and electrophoresis. The novel combination methodology of in-solution/gel tryptic digestion with liquid chromatography equipped with mass spectrometry (LC-MS/MS) was used to evaluate the isomerization of Asp 58. The contributions of isomerization to assembly, solubility, and chaperone functions of αA-crystallin were estimated using a series of mutations of Asp 58 in αA-crystallin. The results indicated that the isomerization of Asp 58 depended on the oligomer size and age of the lens. The substitution of Asp 58 for hydrophobic residues increased αA-crystallin oligomer size and decreased solubility. All substitutions decreased the chaperone function of αA-crystallin for aggregates of bovine βL-crystallin and alcohol dehydrogenase. The data indicated that Asp 58 in αA-crystallin was critical for intermolecular interactions in the lens. Our results also suggested that LC-MS/MS-based isomerization analyses of in-gel-digested products could be useful for investigating the isomerization of Asp residues in oligomeric states. This method could also be used to analyze d/l ratios of amino acid residues in soluble protein aggregates.

The role of Prdx6 in the protection of cells of the crystalline lens from oxidative stress induced by UV exposure

PURPOSE

The immediate aim of this study was to investigate alterations in peroxiredoxin (Prdx) 6 at posttranslational levels, and the levels of protein oxidation, lipid peroxidation, and reactive oxygen species (ROS) in lens epithelial cells (LECs) after exposure to severe oxidative stress, such as ultraviolet-B (UV-B). Our ultimate aim was to provide new information on antioxidant defenses in the lens and their regulation, thereby broadening existing knowledge of the role of Prdx6 in lens physiology and pathophysiology.

METHODS

The expression of the hyperoxidized form of Prdx6 and oxidation of protein were analyzed by western blotting and the OxyBlot assay in human LECs (hLECs). ROS levels were quantified using DCFH-DA dye, and cell viability was quantified by the MTS and TUNEL assays. To evaluate the protective effect of Prdx6, we cultured lenses with or without the TAT transduction domain (TAT-HA-Prdx6) and observed (and photographed) the cultures at specified time-points after the exposure to UV-B for the development of opacity.

RESULTS

Prdx6 in hLECs was hyperoxidized after exposure to high amounts of UV-B. UV-B treatment of hLECs increased the levels of cell death, protein oxidation, and ROS. hLECs exposed to UV-B showed higher levels of ROS, which could be reduced by the application of extrinsic TAT-HA-Prdx6, attenuating UV-B-induced lens opacity and apoptotic cell death.

CONCLUSION

Excessive oxidative stress induces the hyperoxidation of Prdx6 and may reduce the ability of Prdx6 to protect LECs against ROS or stresses. Because extrinsic Prdx6 could attenuate UV-B-induced abuse, this molecule may have a potential in preventing cataractogenesis.

Tryptophan cluster protects human γD-crystallin from ultraviolet radiation-induced photoaggregation in vitro

Exposure to ultraviolet radiation (UVR) is a significant risk factor for age-related cataract, a disease of the human lens and the most prevalent cause of blindness in the world. Cataract pathology involves protein misfolding and aggregation of the primary proteins of the lens, the crystallins. Human γD-crystallin (HγD-Crys) is a major γ-crystallin in the nucleus of the human lens. We report here analysis of UVR-induced damage to HγD-Crys in vitro. Irradiation of solutions of recombinant HγD-Crys with UVA/UVB light produced a rise in solution turbidity due to polymerization of the monomeric crystallins into higher molecular weight aggregates. A significant fraction of this polymerized protein was covalently linked. Photoaggregation of HγD-Crys required oxygen and its rate was protein concentration and UVR dose dependent. To investigate the potential roles of individual tryptophan residues in photoaggregation, triple W:F mutants of HγD-Crys were irradiated. Surprisingly, despite reducing UVR absorbing capacity, multiple W:F HγD-Crys mutant proteins photoaggregated more quickly and extensively than wild type. The results reported here are consistent with previous studies that postulated that an energy transfer mechanism between the highly conserved pairs of tryptophan residues in HγD-Crys could be protective against UVR-induced photodamage.

Identification of tryptophan oxidation products in bovine alpha-crystallin

Oxidation is known to affect the structure, activity, and rate of degradation of proteins, and is believed to contribute to a variety of pathological conditions. Metal-catalyzed oxidation (MCO) is a primary oxidizing system in many cell types. In this study, the oxidative effects of a MCO system (the Fenton reaction) on the structure of the tryptophan residues of alpha-crystallin were determined. Tandem mass spectrometry (MS/MS) was utilized to identify specific tryptophan and methionine oxidation products in the bovine alpha-crystallin sequence. After oxidative exposure, alpha-crystallin was digested with trypsin, and the resulting peptides were fractionated by reverse-phase HPLC. Structural analysis by mass spectrometry revealed that tryptophan 9 of alphaA- and tryptophan 60 of alphaB-crystallin were each converted into hydroxytryptophans (HTRP), N-formylkynurenine (NFK), and kynurenine (KYN). However, only HTRP and KYN formation were detected at residue 9 of alphaB-crystallin. Oxidation of methionine 1 of alphaA- and methionine 1 and 68 of alphaB-crystallin was also detected. The products NFK and KYN are of particular importance in the lens, as they themselves are photosensitizers that can generate reactive oxygen species (ROS) upon UV light absorption. The unambiguous identification of HTRP, NFK, and KYN in intact alpha-crystallin represents the first structural proof of the formation of these products in an intact protein, and provides a basis for detailed structural analysis of oxidized proteins generated in numerous pathological conditions.

Sulfomucins in the human body

Mucins are widely distributed in mucous secretion fluids or are associated with plasma membranes. Up to now 9 genes of epithelial mucins have been identified, distributed over five chromosomes. Superposed on the genetic diversity, each type of mucin displays heterogeneity in oligosaccharide composition, including the terminal sugar residues. On top of that there is variation between individuals brought about by blood group antigens. Heterogeneity is further incited by the degree of sulfation. This tremendous structural heterogeneity endows mucin molecules with properties suggestive for a multifunctional role. The major biological function assigned to mucins is still the protection of tissues covered by the mucous gel. Current knowledge on the specific biological functions of the sulfate residues is fragmentary and periphrastic. Glycosylation including sulfation appears to be subject to modification under pathological conditions. There is evidence that sulfation rate-limits bacterial degradation of mucins. Moreover, accumulating data focus towards their involvement in recognition phenomena. Sulfate residues on blood group related structures provoke specific epitopes for selective interaction with microorganisms e.g. Helicobacter pylori. A distinct class of mucins acts as ligands for selectins, crucial in cellular recognition processes like cellular homing of lymphocytes. Whereas in earlier days mucins were only seen as water-binding molecules, protecting the underlying mucosa against harmful agents, the current picture of these molecules is characterized by the selective interaction with their environment, including epithelial-, and endothelial cells and microorganisms, thereby regulating a great number of biological processes. However, the specific role of sulfate remains to be further elucidated.