Differential rate constants of racemization of aspartyl and asparaginyl residues in human alpha A-crystallin mutants

Oxidative Stress in the Anterior Ocular Diseases: Diagnostic and Treatment

The eye is a metabolically active structure, constantly exposed to solar radiations making its structure vulnerable to the high burden of reactive oxygen species (ROS), presenting many molecular interactions. The biomolecular cascade modification is caused especially in diseases of the ocular surface, cornea, conjunctiva, uvea, and lens. In fact, the injury in the anterior segment of the eye takes its origin from the perturbation of the pro-oxidant/antioxidant balance and leads to increased oxidative damage, especially when the first line of antioxidant defence weakens with age. Furthermore, oxidative stress is related to mitochondrial dysfunction, DNA damage, lipid peroxidation, protein modification, apoptosis, and inflammation, which are involved in anterior ocular disease progression such as dry eye, keratoconus, uveitis, and cataract. The different pathologies are interconnected through various mechanisms such as inflammation, oxidative stress making the diagnostics more relevant in early stages. The end point of the molecular pathway is the release of different antioxidant biomarkers offering the potential of predictive diagnostics of the pathology. In this review, we have analysed the oxidative stress and inflammatory processes in the front of the eye to provide a better understanding of the pathomechanism, the importance of biomarkers for the diagnosis of eye diseases, and the recent treatment of anterior ocular diseases.

Racemization in cataractous lens from diabetic and aging individuals: analysis of Asp 58 residue in αA-crystallin

Cataract is the leading cause of visual impairment globally. Racemization of lens proteins may contribute to cataract formation in aging individuals. As a special type of age-related cataract (ARC), diabetic cataract (DC) is characterized by the early onset of cortical opacification and finally developed into a mixed type of cortical and nuclear opacification. We compared racemization of Asp 58 residue, a hotspot position in αA-crystallin, from the cortex and nucleus of diabetic and age-matched senile cataractous lenses, by identifying L-Asp/L-isoAsp/D-Asp/D-isoAsp by mass spectrometry. Compared to nondiabetic cataractous lenses, DC lenses showed a significantly increased cortex/nucleus ratio of D-Asp 58, which originated primarily from an increased percentage of D-Asp 58 in the lens cortex of DC. Moreover, patients diagnosed with diabetes for over 10 years showed a lower cortex/nucleus ratio of D-isoAsp 58 in the lens compared with those who had a shorter duration of diabetes, which originated mainly from an increased percentage of D-isoAsp 58 in the lens nucleus of DC with increasing time of hyperglycemia. Further analysis confirmed decreased protein solubility in diabetic cataractous lenses. The different racemization pattern in DC may be distinguished from ARC and influence its phenotype over the protracted duration of diabetes.

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.

The Proteome of Cataract Markers: Focus on Crystallins

Cataract is a major cause of blindness worldwide. It is characterized by lens opacification and is accompanied by extensive posttranslational modifications (PTMs) in various proteins. PTMs play an essential role in lens opacification. Several PTMs have been described in proteins isolated from relatively old human lenses, including phosphorylation, deamidation, racemization, truncation, acetylation, and methylation. An overwhelming majority of previous cataract proteomic studies have exclusively focused on crystallin proteins, which are the most abundant proteome components of the lens. To investigate the proteome of cataract markers, this chapter focuses on the proteomic research on the functional relevance of the major PTMs in crystallins of human cataractous lenses. Elucidating the role of these modifications in cataract formation has been a challenging task because they are among the most difficult PTMs to study analytically. The proteomic status of some amides presents similar properties in normal aged and cataractous lenses, whereas some may undergo greater PTMs in cataract. Therefore, it is of great importance to review the current proteomic research on crystallins, the major protein markers in different types of cataract, to elucidate the pathogenesis of this major human-blinding condition.

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.

Racemization at the Asp 58 residue in αA-crystallin from the lens of high myopic cataract patients

Post-translational modifications in lens proteins are key causal factors in cataract. As the most abundant post-translational modification in the lens, racemization may be closely related to the pathogenesis of cataract. Racemization of αA-crystallin, a crucial structural and heat shock protein in the human lens, could significantly influence its structure and function. In previous studies, elevated racemization from l-Asp 58 to d-isoAsp58 in αA-crystallin has been found in age-related cataract (ARC) lenses compared to normal aged human lenses. However, the role of racemization in high myopic cataract (HMC), which is characterized by an early onset of nuclear cataract, remains unknown. In the current study, apparently different from ARC, significantly increased racemization from l-Asp 58 to d-Asp 58 in αA-crystallin was identified in HMC lenses. The average racemization rates for each Asp isoform were calculated in ARC and HMC group. In ARC patients, the conversion of l-Asp 58 to d-isoAsp 58, up to 31.89%, accounted for the main proportion in racemization, which was in accordance with the previous studies. However, in HMC lenses, the conversion of l-Asp 58 to d-Asp 58, as high as 35.44%, accounted for the largest proportion of racemization in αA-crystallin. The different trend in the conversion of αA-crystallin by racemization, especially the elevated level of d-Asp 58 in HMC lenses, might prompt early cataractogenesis and a possible explanation of distinct phenotypes of cataract in HMC.

Multidimensional significance of crystallin protein-protein interactions and their implications in various human diseases

BACKGROUND

Crystallins are the important structural and functional proteins in the eye lens responsible for refractive index. Post-translational modifications (PTMs) and mutations are major causative factors that affect crystallin structural conformation and functional characteristics thus playing a vital role in the etiology of cataractogenesis.

SCOPE OF REVIEW

The significance of crystallin protein-protein interactions (PPIs) in the lens and non-lenticular tissues is summarized.

MAJOR CONCLUSIONS

Aberrancy of PPIs between crystallin, its associated protein and metal ions has been accomplished in various human diseases including cataract. A detailed account on multidimensional structural and functional significance of crystallin PPI in humans must be brought into limelight, in order to understand the biochemical and molecular basis augmenting the aberrancies of such interaction. In this scenario, the present review is focused to shed light on studies which will aid to expand our present understanding on disease pathogenesis related to loss of PPI thereby paving the way for putative future therapeutic targets to curb such diseases.

GENERAL SIGNIFICANCE

The interactions with α-crystallins always aid to protect their structural and functional characteristics. The up-regulation of αB-crystallin in the non-lenticular tissues always decodes as biomarker for various stress related disorders. For better understanding and treatment of various diseases, PPI studies provide overall outline about the structural and functional characteristics of the proteins. This information not only helps to find out the route of cataractogenesis but also aid to identify potential molecules to inhibit/prevent the further development of such complicated phenomenon. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Overview of the Lens

In order to accomplish its function of transmitting and focusing light, the crystalline lens of the vertebrate eye has evolved a unique cellular structure and protein complement. These distinct adaptations have provided a rich source of scientific discovery ranging from biochemistry and genetics to optics and physics. In addition, because of these adaptations, lens cells persist for the lifetime of an organism, providing an excellent model of the aging process. The chapters dealing with the lens will demonstrate how the different aspects of lens biology and biochemistry combine in this singular refractive organ to accomplish its critical role in the visual system.