Cataracts and copper: A spectroscopic perspective of copper-induced aggregation of betaB2-crystallin

Cataracts disease is the principal cause of visual impairment worldwide. It is caused by damage, unfolding and subsequent non-amyloid aggregation of the highly soluble lens proteins: the crystallins. It has been demonstrated that essential metal ions, such as copper and zinc, can induce the aggregation of human gamma-crystallins. Here, we report the effect of metal ions one the aggregation of betaB2-crystallin: Pb(II), Hg(II), Cu(II) and Zn(II) ions can induce the non amyloid aggregation of this protein and the chelating agent EDTA can partially revert this effect. Furthermore, SDS-PAGE analysis of Cu(II)-induced protein aggregates reveal disulfide-bridged oligomerization. Spectroscopic studies of the Cu(II)-betaB2-crystallin interaction by electron paramagnetic resonance (EPR), circular dichroism (CD) and X-Ray absorption spectroscopy (XAS) show the presence of at least three metal binding sites. Surprisingly, one of them resembles that of Cu(II) bound to an amino-terminal copper and nickel binding motif (ATCUN), which is found in proteins and peptides that bind copper with high affinity and are involved in copper cellular trafficking. On the other hand, copper-induced formation of sulfur oxidized oligomers of betaB2-crystallin and a decreased EPR Cu(II) spin quantitation suggests the reduction of Cu(II) ions to Cu(I), as confirmed by XAS. Overall, these results suggests that copper-induced aggregation of betaB2-crystallin involves several mechanisms, including the formation of metal-bridged and disulfide-bridged species, and a very interesting copper-dependent redox process. This study expands the bioinorganic facet of cataract disease.