Profiling of lens protease involved in generation of αA-66-80 crystallin peptide using an internally quenched protease substrate

Aggrelyte-2 promotes protein solubility and decreases lens stiffness through lysine acetylation and disulfide reduction: Implications for treating presbyopia

Aging proteins in the lens become increasingly aggregated and insoluble, contributing to presbyopia. In this study, we investigated the ability of aggrelyte-2 (N,S-diacetyl-L-cysteine methyl ester) to reverse the water insolubility of aged human lens proteins and to decrease stiffness in cultured human and mouse lenses. Water-insoluble proteins (WI) of aged human lenses (65-75 years) were incubated with aggrelyte-2 (500 μM) for 24 or 48 h. A control compound that lacked the S-acetyl group (aggrelyte-2C) was also tested. We observed 19%-30% solubility of WI upon treatment with aggrelyte-2. Aggrelyte-2C also increased protein solubility, but its effect was approximately 1.4-fold lower than that of aggrelyte-2. The protein thiol contents were 1.9- to 4.9-fold higher in the aggrelyte-2- and aggrelyte-2C-treated samples than in the untreated samples. The LC-MS/MS results showed Nε -acetyllysine (AcK) levels of 1.5 to 2.1 nmol/mg protein and 0.6 to 0.9 nmol/mg protein in the aggrelyte-2- and aggrelyte-2C-treated samples. Mouse (C57BL/6J) lenses (incubated for 24 h) and human lenses (incubated for 72 h) with 1.0 mM aggrelyte-2 showed significant decreases in stiffness with simultaneous increases in soluble proteins (human lenses) and protein-AcK levels, and such changes were not observed in aggrelyte-2C-treated lenses. Mass spectrometry of the solubilized protein revealed AcK in all crystallins, but more was observed in α-crystallins. These results suggest that aggrelyte-2 increases protein solubility and decreases lens stiffness through acetylation and disulfide reduction. Aggrelyte-2 might be useful in treating presbyopia in humans.

Age-related cleavages of crystallins in human lens cortical fiber cells generate a plethora of endogenous peptides and high molecular weight complexes

Low molecular weight peptides derived from the breakdown of crystallins have been reported in adult human lenses. The proliferation of these LMW peptides coincides with the earliest stages of cataract formation, suggesting that the protein cleavages involved may contribute to the aggregation and insolubilization of crystallins. This study reports the identification of 238 endogenous LMW crystallin peptides from the cortical extracts of four human lenses representing young, middle and old-age human lenses. Analysis of the peptide terminal amino acids showed that Lys and Arg were situated at the C-terminus with significantly higher frequency compared to other residues, suggesting that trypsin-like proteolysis may be active in the lens cortical fiber cells. Selected reaction monitoring analysis of an endogenous αA-crystallin peptide (αA(57-65)) showed that the concentration of this peptide in the human lens increased gradually to middle age, after which the rate of αA(57-65) formation escalated significantly. Using 2D gel electrophoresis/nanoLC-ESI-MS/MS, 12 protein complexes of 40-150 kDa consisting of multiple crystallin components were characterized from the water soluble cortical extracts of an adult human lens. The detection of these protein complexes suggested the possibility of crystallin cross-linking, with these complexes potentially acting to stabilize degraded crystallins by sequestration into water soluble complexes.

Role of αA-crystallin-derived αA66-80 peptide in guinea pig lens crystallin aggregation and insolubilization

Earlier we reported that low molecular weight (LMW) peptides accumulate in aging human lens tissue and that among the LMW peptides, the chaperone inhibitor peptide αA66-80, derived from α-crystallin protein, is one of the predominant peptides. We showed that in vitro αA66-80 induces protein aggregation. The current study was undertaken to determine whether LMW peptides are also present in guinea pig lens tissue subjected to hyperbaric oxygen (HBO) in vivo. The nuclear opacity induced by HBO in guinea pig lens is the closest animal model for studying age-related cataract formation in humans. A LMW peptide profile by mass spectrometry showed the presence of an increased amount of LMW peptides in HBO-treated guinea pig lenses compared to age-matched controls. Interestingly, the mass spectrometric data also showed that the chaperone inhibitor peptide αA66-80 accumulates in HBO-treated guinea pig lens. Following incubation of synthetic chaperone inhibitor peptide αA66-80 with α-crystallin from guinea pig lens extracts, we observed a decreased ability of α-crystallin to inhibit the amorphous aggregation of the target protein alcohol dehydrogenase and the formation of large light scattering aggregates, similar to those we have observed with human α-crystallin and αA66-80 peptide. Further, time-lapse recordings showed that a preformed complex of α-crystallin and αA66-80 attracted additional crystallin molecules to form even larger aggregates. These results demonstrate that LMW peptide-mediated cataract development in aged human lens and in HBO-induced lens opacity in the guinea pig may have common molecular pathways.

Lens crystallin modifications and cataract in transgenic mice overexpressing acylpeptide hydrolase

The accumulation of crystallin fragments in vivo and their subsequent interaction with crystallins are responsible, in part, for protein aggregation in cataracts. Transgenic mice overexpressing acylpeptide hydrolase (APH) specifically in the lens were prepared to test the role of protease in the generation and accumulation of peptides. Cataract development was seen at various postnatal days in the majority of mice expressing active APH (wt-APH). Cataract onset and severity of the cataracts correlated with the APH protein levels. Lens opacity occurred when APH protein levels were >2.6% of the total lens protein and the specific activity, assayed using Ac-Ala-p-nitroanilide substrate, was >1 unit. Transgenic mice carrying inactive APH (mt-APH) did not develop cataract. Cataract development also correlated with N-terminal cleavage of the APH to generate a 57-kDa protein, along with an increased accumulation of low molecular weight (LMW) peptides, similar to those found in aging human and cataract lenses. Nontransgenic mouse lens proteins incubated with purified wt-APH in vitro resulted in a >20% increase in LMW peptides. Crystallin modifications and cleavage were quite dramatic in transgenic mouse lenses with mature cataract. Affected lenses showed capsule rupture at the posterior pole, with expulsion of the lens nucleus and degenerating fiber cells. Our study suggests that the cleaved APH fragment might exert catalytic activity against crystallins, resulting in the accumulation of distinct LMW peptides that promote protein aggregation in lenses expressing wt-APH. The APH transgenic model we developed will enable in vivo testing of the roles of crystallin fragments in protein aggregation.

The αA66-80 peptide interacts with soluble α-crystallin and induces its aggregation and precipitation: a contribution to age-related cataract formation

Formation of protein aggregates in the aging eye lens has been shown to correlate with progressive accumulation of specific low-molecular weight (LMW) peptides derived from crystallins. Prominent among the LMW fragments is αA66-80, a peptide derived from αA-crystallin and present at higher concentrations in the water-insoluble nuclear fractions of the aging lens. The αA66-80 peptide has amyloid-like properties and preferentially insolubilizes α-crystallin from soluble lens fractions. However, the specific interactions and mechanisms by which the peptide induces α-crystallin aggregation have not been delineated. To gain insight into the mechanisms of peptide-induced aggregation, we investigated the interactions of the peptide with α-crystallin by various biochemical approaches. The peptide weakens α-crystallin chaperone ability and drastically promotes α-crystallin aggregation via the formation of insoluble peptide-protein complexes through transient intermediates. 4,4'-Dianilino-1,1'-binaphthyl-5,5'-disulfonic acid studies suggest that the peptide induces changes in the hydrophobicity of α-crystallin that could trigger the formation and growth of aggregates. The peptide-α-crystallin aggregates were found to be resistant to dissociation by high ionic strengths, whereas guanidinium hydrochloride and urea were effective dissociating agents. We conclude that the αA66-80 peptide forms a hydrophobically driven, stable complex with α-crystallin and reduces its solubility. Using isotope-labeled chemical cross-linking and mass spectrometry, we show that the peptide binds to multiple sites, including the chaperone site, the C-terminal extension, and subunit interaction sites in αB-crystallin, which may explain the antichaperone property of the peptide and the consequential age-related accumulation of aggregated proteins. Thus, the α-crystallin-derived peptide could play a role in the pathogenesis of cataract formation in the aging lens.