The etiology of human age-related cataract. Proteins don't last forever

Abstract

Background: It is probable that the great majority of human cataract results from the spontaneous decomposition of long-lived macromolecules in the human lens. Breakdown/reaction of long-lived proteins is of primary importance and recent proteomic analysis has enabled the identification of the particular crystallins, and their exact sites of amino acid modification.

Scope of review: Analysis of proteins from cataractous lenses revealed that there are sites on some structural proteins that show a consistently greater degree of deterioration than age-matched normal lenses.

Major conclusions: The most abundant posttranslational modification of aged lens proteins is racemization. Deamidation, truncation and crosslinking, each arising from the spontaneous breakdown of susceptible amino acids within proteins, are also present. Fundamental to an understanding of nuclear cataract etiology, it is proposed that once a certain degree of modification at key sites occurs, that protein-protein interactions are disrupted and lens opacification ensues.

General significance: Since long-lived proteins are now recognized to be present in many other sites of the body, such as the brain, the information gleaned from detailed analyses of degraded proteins from aged lenses will apply more widely to other age-related human diseases. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Keywords: Crystallins; Human aging; Lens; Lifespan; Protein degradation.

Figure 1

Long-lived proteins decompose in the body. In lens proteins, and other long-lived proteins, major degradative processes involve racemisation. These are spontaneous events that particularly affect aspartate, asparagine and serine residues in unstructured regions of the protein. In lens proteins the major end product of L-Asn and L-Asp breakdown is D-isoAsp, which arises via succinimide intermediates. isoAsp peptides appear to be stable and undergo little interconversion [20]. In most cells (but not the human lens nucleus) protein isoaspartate methyl transferase (PIMT) can partially ameliorate Asp racemization in long-lived proteins.

Figure 2

Overall racemisation of proteins is greater in cataractous lenses than in normal lenses. Racemisation of Asx (i.e. Asp + Asn) as a function of age, in normal and cataractous lens proteins. Racemisation expressed as a % of D/(D+L). From ref [13] and used with permission.

Figure 3

Racemisation at specific sites on crystallins may be cataractogenic. Deamidation of Asn 76 in γS crystallin as a function of age, in normal and cataractous lens proteins. The content of (L+D)-isoaspartic acid (isoAsp) at residue 76 following tryptic digestion of whole nuclear lens protein and LC/MS/MS. If deamidation of Asn occurs via a cyclic intermediate, four Asp isoforms are produced, two of which are isoAsp peptides – see Fig 1. (Symbols: □ Normal ◆ Cataract). From ref [28] and used with permission.

Figure 4

Old proteins undergo peptide bond cleavage. Mechanisms to account for spontaneous cleavage; a) on the C-terminal side of Asn, and b) on the N-terminal side of Ser residues.