Methionine sulfoxide reductase A (MsrA) restores α-crystallin chaperone activity lost upon methionine oxidation

Abstract

Background Lens cataract is associated with protein oxidation and aggregation. Two proteins that cause cataract when deleted from the lens are methionine sulfoxide reductase A (MsrA) that repairs protein methionine sulfoxide (PMSO) oxidized proteins and α-crystallin which is a two subunit (αA and αB) chaperone. Here, we tested whether PMSO formation damages α-crystallin chaperone function and whether MsrA could repair PMSO-α-crystallin. Methods Total α-crystallin was oxidized to PMSO and evaluated by CNBr-cleavage and mass spectrometry. Chaperone activity was measured by light scattering using lysozyme as target. PMSO-α-crystallin was treated with MsrA, and repair was assessed by CNBr cleavage, mass spectrometry and recovery of chaperone function. The levels of α-crystallin-PMSO in the lenses of MsrA-knockout relative to wild-type mice were determined. Results PMSO oxidation of total α-crystallin (met 138 of αA and met 68 of αB) resulted in loss of α-crystallin chaperone activity. MsrA treatment of PMSO-α-crystallin repaired its chaperone activity through reduction of PMSO. Deletion of MsrA in mice resulted in increased levels of PMSO-α-crystallin. Conclusions Methionine oxidation damages α-crystallin chaperone function and MsrA can repair PMSO-α-crystallin restoring its chaperone function. MsrA is required for maintaining the reduced state of α-crystallin methionines in the lens. Significance Methionine oxidation of α-crystallin in combination with loss of MsrA repair causes loss of α-crystallin chaperone function. Since increased PMSO levels and loss of α-crystallin function are hallmarks of cataract, these results provide insight into the mechanisms of cataract development and likely those of other age-related diseases.

Methionine sulfoxide reductase A: Structure, function and role in ocular pathology

“…The chaperone activity is essential for a number of significant functions ranging from protein folding to cytoskeletal remodeling, apoptotic control, neuroprotection, and angiogenesis regulation [18,21,70] . Loss of α-crystallin chaperone function probably results in protein aggregation, which may account for the cataract formation found in the absence of MsrA in mice [65] . Given the role of MsrA and α-crystallin in the health and disease of retinal and other tissues, these results are probably applicable to our understanding of the oxidative-stress-associated and aging disease mechanisms, and these results may provide a basis for the development of well-designed interventions for these conditions.…”

“…This finding is of significance because of emerging evidence about the role of α-crystallins in ocular health. A direct link has been identified between α-crystallin methionine oxidation and age-related cataract formation [65] . α-crystallins are major proteins of the small heat shock protein family and are expressed in several tissues [66][67][68] .…”

“…Development of cataract could be the result of protein aggregation caused by loss of α-crystallin chaperone function. Methionine oxidation damages α-crystallin chaperone activity, but MsrA can also repair oxidized methionines in α-crystallin and restore the chaperone function (Figure 1) [65] . Therefore, loss of MsrA activity upon aging or oxidative stress could result in loss of α-crystallin chaperone function and contribute to the development of cataract and other age-related oxidative stress-associated disorders, such as AMD [78] , Parkinson's disease [79] , Alzheimer's disease [80] and desmin-related myopathy [81] .…”

“…α-crystallin and cytochrome C have been identified as major targets of MsrA in the lens [59,65] . This finding is of significance because of emerging evidence about the role of α-crystallins in ocular health.…”

Integrin αVβ5-mediated Removal of Apoptotic Cell Debris by the Eye Lens and Its Inhibition by UV Light Exposure

“…Consistently, the eye lens has evolved multiple protective and repair systems to preserve its transparent function in the face of environmental insults (63). These include abundant anti-oxidant systems (64 -66), protein protective systems (67)(68)(69), protein salvaging systems (70,71), protein repair systems (72)(73)(74), as well autophagy systems for clearance of damaged organelles (41)(42)(43). Loss of the functions of these systems results in a range of lens effects including, but not limited to, accumulation of damaged and modified proteins (75)(76)(77), apoptosis of lens cells (78,79), and cataract formation (73,80).…”