Lens β-crystallins: The role of deamidation and related modifications in aging and cataract

Abstract

Crystallins are the major proteins in the lens of the eye and function to maintain transparency of the lens. Of the human crystallins, α, β, and γ, the β-crystallins remain the most elusive in their structural significance due to their greater number of subunits and possible oligomer formations. The β-crystallins are also heavily modified during aging. This review focuses on the functional significance of deamidation and the related modifications of racemization and isomerization, the major modifications in β-crystallins of the aged human lens. Elucidating the role of these modifications in cataract formation has been slow, because they are analytically among the most difficult post-translational modifications to study. Recent results suggest that many amides deamidate to similar extent in normal aged and cataractous lenses, while others may undergo greater deamidation in cataract. Mimicking deamidation at critical structural regions induces structural changes that disrupt the stability of the β-crystallins and lead to their aggregation in vitro. Deamidations at the surface disrupt interactions with other crystallins. Additionally, the α-crystallin chaperone is unable to completely prevent deamidated β-crystallins from insolubilization. Therefore, deamidation of β-crystallins may enhance their precipitation and light scattering in vivo contributing to cataract formation. Future experiments are needed to quantify differences in deamidation rates at all Asn and Gln residues within crystallins from aged and cataractous lenses, as well as racemization and isomerization which potentially perturb protein structure greater than deamidation alone. Quantitative data is greatly needed to investigate the importance of these major age-related modifications in cataract formation.

An Internal Disulfide Locks a Misfolded Aggregation-Prone Intermediate in Cataract-Linked Mutants of Human Gamma-D Crystallin

“…The result is generation of partially unfolded intermediate conformational states and progressive increase in light scattering (lens turbidity) due to aggregation (18)(19)(20)(21). No long-range structure, amyloid or otherwise, has been found in the cataractous aggregates (22,23), except in certain rare congenital cases (24,25), but disulfide bonds and other covalent modifications are common (13,(26)(27)(28)(29).…”

Biophysical chemistry of the ageing eye lens

“…This modification is so ubiquitous in longerlived tissues that it is considered one of the factors limiting protein lifetimes (Robinson and Robinson 2004). Deamidation is one of the most common post-translational modification in the eye lens, occurring early in life and building up with age (Hains and Truscott 2010;Lampi et al 2014;Takemoto et al 2001). There are some sites of deamidation that occur more frequently in cataract-affected lenses compared to healthy lenses of the same age (Hooi et al 2012).…”

“…Deamidation modifies asparagine and glutamine residues by hydrolysing the amide side chain, producing the corresponding carboxylic acid residue (Clarke 1987). The major effect of this modification is the introduction of a negative charge at a formerly neutral location in a protein, and thus it has the potential to destabilise protein structures in the vicinity, including in the lens crystallins (Hains and Truscott 2010;Lampi et al 2014).…”

“…Figure 9 shows the deamidation pathway for asparagine. Both asparagine and aspartic acid are capable of ring closing to form a succinimide intermediate, with the intermediate ring opening in both directions to subsequently produce the aspartate and isoaspartate residues (Lampi et al 2014;Takemoto et al 2001). The nature of the residue adjacent to the Asn/Gln is an important factor in both the rate of modification and propensity for deamidation.…”

“…These residues show the largest tendency to racemise of any amino acid in the eye due to the existence of a chemical pathway that allows for steric inversion (Lampi et al 2014;Wakankar and Borchardt 2006). This is the same rearrangement that causes deamidation via a succinimide intermediate ring Fig.…”