Massive increase in the stiffness of the human lens nucleus with age: the basis for presbyopia?

Purpose: To determine the stiffness of different regions of human lenses as a function of age, and to correlate the biophysical measurements in the lens center with nuclear water content. Methods: A custom made probe fitted to a dynamic mechanical analyzer was employed to measure stiffness values at 1 mm increments across equatorial sections of individual human lenses. Thermogravimetric analysis was used to determine the percentage water content in the nuclei of human lenses. Results: There was a pronounced increase in lens stiffness over the age range from 14 to 78. In the nucleus, stiffness values varied almost 1,000 fold over this age range, with the largest change observed in lenses between the ages of 20 to 60. Nuclear stiffness values increased on average by a factor of 450. By contrast, in the cortex the average increase in stiffness was approximately 20 fold over this same time period. In lenses younger than age 30, the nucleus was found to be softer than the cortex. This was true for all six lenses examined. In contrast all lenses older than 30 were characterized by having nuclear values higher than those of the cortex. In lenses over the age of 50, the lens nucleus was typically an order of magnitude more rigid than that of the cortex. The crossover age, when the cortical and nuclear stiffness values were similar, was in the 30s. There was no significant change in the water content of the human lens nucleus from age 13 to age 82. Conclusions: There is a marked increase in the stiffness of the human lens with age. This is most pronounced in the nucleus. Since in vivo data indicate that the nucleus must change shape significantly during accommodation, it is highly likely that these measured changes in physical properties will markedly diminish the ability of the lens to accommodate, and thus may be a major contributing factor to presbyopia. Since there was no measurable difference in the water contents of the nuclear regions of the lenses, this marked increase in stiffness is not due to compaction of the lens nucleus

Proteostasis and the Regulation of Intra- and Extracellular Protein Aggregation by ATP-Independent Molecular Chaperones: Lens α-Crystallins and Milk Caseins

Conspectus: Molecular chaperone proteins perform a diversity of roles inside and outside the cell. One of the most important is the stabilization of misfolding proteins to prevent their aggregation, a process that is potentially detrimental to cell viability. Diseases such as Alzheimer\u27s, Parkinson\u27s, and cataract are characterized by the accumulation of protein aggregates. In vivo, many proteins are metastable and therefore under mild destabilizing conditions have an inherent tendency to misfold, aggregate, and hence lose functionality. As a result, protein levels are tightly regulated inside and outside the cell. Protein homeostasis, or proteostasis, describes the network of biological pathways that ensures the proteome remains folded and functional. Proteostasis is a major factor in maintaining cell, tissue, and organismal viability. We have extensively investigated the structure and function of intra- and extracellular molecular chaperones that operate in an ATP-independent manner to stabilize proteins and prevent their misfolding and subsequent aggregation into amorphous particles or highly ordered amyloid fibrils. These types of chaperones are therefore crucial in maintaining proteostasis under normal and stress (e.g., elevated temperature) conditions. Despite their lack of sequence similarity, they exhibit many common features, i.e., extensive structural disorder, dynamism, malleability, heterogeneity, oligomerization, and similar mechanisms of chaperone action. In this Account, we concentrate on the chaperone roles of α-crystallins and caseins, the predominant proteins in the eye lens and milk, respectively. Intracellularly, the principal ATP-independent chaperones are the small heat-shock proteins (sHsps). In vivo, sHsps are the first line of defense in preventing intracellular protein aggregation. The lens proteins αA- and αB-crystallin are sHsps. They play a crucial role in maintaining solubility of the crystallins (including themselves) with age and hence in lens proteostasis and, ultimately, lens transparency. As there is little metabolic activity and no protein turnover in the lens, crystallins are very long lived proteins. Lens proteostasis is therefore very different to that in normal, metabolically active cells. Crystallins undergo extensive post-translational modification (PTM), including deamidation, racemization, phosphorylation, and truncation, which can alter their stability. Despite this, the lens remains transparent for tens of years, implying that lens proteostasis is intimately integrated with crystallin PTMs. Many PTMs do not significantly alter crystallin stability, solubility, and functionality, which thereby facilitates lens transparency. In the long term, however, extensive accumulation of crystallin PTMs leads to large-scale crystallin aggregation, lens opacification, and cataract formation. Extracellularly, various ATP-independent molecular chaperones exist that exhibit sHsp-like structural and functional features. For example, caseins, the major milk proteins, exhibit chaperone ability by inhibiting the amorphous and amyloid fibrillar aggregation of a diversity of destabilized proteins. Caseins maintain proteostasis within milk by preventing deleterious casein amyloid fibril formation via incorporation of thousands of individual caseins into an amorphous structure known as the casein micelle. Hundreds of nanoclusters of calcium phosphate are sequestered within each casein micelle through interactions with short, highly phosphorylated casein sequences. This results in a stable biofluid that contains a high concentration of potentially amyloidogenic caseins and concentrations of calcium and phosphate that can be far in excess of the solubility of calcium phosphate. Casein micelle formation therefore performs vital roles in neonatal nutrition and calcium homeostasis in the mammary gland

Molecular Processes Implicated in Human Age-Related Nuclear Cataract

Human age-related nuclear cataract is commonly characterized by four biochemical features that involve modifications to the structural proteins that constitute the bulk of the lens: coloration, oxidation, insolubility, and covalent cross-linking. Each of these is progressive and increases as the cataract worsens. Significant progress has been made in understanding the origin of the factors that underpin the loss of lens transparency. Of these four hallmarks of cataract, it is protein-protein cross-linking that has been the most intransigent, and it is only recently, with the advent of proteomic methodology, that mechanisms are being elucidated. A diverse range of cross-linking processes involving several amino acids have been uncovered. Although other hypotheses for the etiology of cataract have been advanced, it is likely that spontaneous decomposition of the structural proteins of the lens, which do not turn over, is responsible for the age-related changes to the properties of the lens and, ultimately, for cataract. Cataract may represent the first and best characterized of a number of human age-related diseases where spontaneous protein modification leads to ongoing deterioration and, ultimately, a loss of tissue function

Protein-Bound UV Filters in Normal Human Lenses: The Concentration of Bound UV Filters Equals That of Free UV Filters in the Center of Older Lenses

PURPOSE. To survey the levels of protein-bound UV filters in the cortices and nuclei of normal human lenses as a function of age and to relate this to the concentration of free UV filters. METHODS. Levels of each of the three kynurenine (Kyn) UV filters, 3-hydroxykynurenine glucoside (3OHKG), Kyn, and 3-hydroxykynurenine (3OHKyn), covalently attached to proteins, were determined by using a newly developed method of reductive capture, after base treatment of the intact lens proteins. RESULTS. The data show that, in the normal lens, each of the three UV filters became bound to proteins to a significant extent only after age 50 and, further, that the levels in the nucleus were much higher than in the cortex. These findings are consistent with the lens barrier that forms in middle age. 3OHKG was present at the highest levels followed by Kyn, with 3OHKyn being attached in the lowest amount. The ratio was 145:4:1 (3OHKG-Kyn-3OHKyn), with a total proteinbound UV filter concentration in the lens nucleus after age 50 of approximately 1300 picomoles/mg protein. This ratio is in agreement with 3OHKG being the most abundant free UV filter in the human lens and 3OHKyn being present in the lowest concentration with free Kyn present in intermediate amounts. CONCLUSIONS. The three Kyn UV filters are bound to the nuclear proteins of all normal lenses over the age of 50. Indeed in the center of older normal lenses, the concentration of UV filters bound to proteins is approximately equal to that of the free filters. Since bound UV filters promote oxidation of proteins after exposure to wavelengths of light that penetrate the cornea, lenses in middle-aged and older individuals may be more prone to photooxidation than those of young people. (Invest Ophthalmol Vis Sci. 2007;48:1718 -1723 DOI: 10.1167DOI: 10. /iovs.06-1134 T he primate lens is unique among both other primate tissues and the lenses of other species, in that it synthesizes 3-hydroxykynurenine glucoside (3OHKG) as a UV filter from the amino acid tryptophan (Trp). 1 The immediate precursors kynurenine (Kyn) and 3-hydroxykynurenine (3OHKyn) are metabolites found in all organs of the body. All three of these Trp metabolites are unstable at physiological pH and undergo side chain deamination to yield ␣,␤-unsaturated ketones that are prone to nucleophilic attack. 2 In tissues that contain sufficient glutathione (GSH), it is likely that GSH will react with the deamination products before they bind to proteins 3 and that the adducts thus formed will diffuse out of the lens. In the interior of the lens, once the barrier to diffusion forms at middle age, 4 -6 the nucleus becomes a partially uncoupled region in which metabolites spend a longer time than in the young lens. This factor leads to an environment in the older lens nucleus that favors greater decomposition of intrinsically unstable molecules. Coupled with a diminished flux of GSH from the cortex, this results in increased covalent binding of UV filters to nuclear proteins after middle age. In this study, we set out to examine the levels of all three bound UV filters in normal lenses using a novel assay system in which lens proteins were incubated with excess GSH at pH 9.5. 9 Under these conditions, the UV filters that are attached to proteins are released, and the GSH adducts thus formed can be quantified by HPLC. UV filters were found to be covalently bound to proteins from all lenses older than 50 years. Such posttranslational modifications may have important consequences in terms of the susceptibility of the proteins to photooxidation and, in the case of 3OHKyn, the sensitivity to an oxidative environment such as that in the nuclei of lenses with age-related nuclear cataract. METHODS Purified water (purified to 18.2 M⍀/cm 2 ; Milli-Q; Millipore, Bedford, MA) was used in the preparation of all solutions. All organic solvents were HPLC grade (Ajax, Auburn, NSW, Australia). 3OHKyn, reduced GSH, trifluoroacetic acid (TFA), and guanidine HCl were obtained from Sigma-Aldrich (St. Louis, MO). Reversed-Phase HPLC Reversed-phase (RP)-HPLC was performed on a Shimadzu system (Kyoto, Japan). For analytical scale separations, a column (Jupiter, 5 m, C18, 300 Å, 250 ϫ 4.6 mm; Phenomenex, Torrance, CA) was used with the following mobile phase conditions: solvent A (aqueous 0.1% vol/vol TFA) for 5 minutes followed by a linear gradient of 0% to 50% solvent B (80% vol/vol acetonitrile/H 2 O, 0.1% vol/vol TFA) over 20 minutes, followed by a linear gradient of 50% to 100% B over 15 minutes and re-equilibration in the aqueous phase for 15 minutes. The flow rate was 0.5 mL/min and with detection at 360 nm. From th

Spontaneous cleavage of proteins at serine and threonine is facilitated by zinc

Old proteins are widely distributed in the body. Over time, they deteriorate and many spontaneous reactions, for example isomerisation of Asp and Asn, can be replicated by incubation of peptides under physiological conditions. One of the signatures of long-lived proteins that has proven to be difficult to replicate in vitro is cleavage on the N-terminal side of Ser residues, and this is important since cleavage at Ser, and also Thr, has been observed in a number of human proteins. In this study, the autolysis of Ser- and Thr-containing peptides was investigated with particular reference to discovering factors that promote cleavage adjacent to Ser/Thr at neutral pH. It was found that zinc catalyses cleavage of the peptide bond on the N-terminal side of Ser residues and further that this process is markedly accelerated if a His residue is adjacent to the Ser. NMR analysis indicated that the imidazole group co-ordinates zinc and that once zinc is co-ordinated, it can polarize the carbonyl group of the peptide bond in a manner analogous to that observed in the active site of the metalloexopeptidase, carboxypeptidase A. The hydroxyl side chain of Ser/Thr is then able to cleave the adjacent peptide bond. These observations enable an understanding of the origin of common truncations observed in long-lived proteins, for example truncation on the N-terminal side of Ser 8 in Abeta, Ser 19 in alpha B crystallin and Ser 66 in alpha A crystallin. The presence of zinc may therefore significantly affect the long-term stability of cellular proteins

Isoaspartic acid is present at specific sites in myelin basic protein from multiple sclerosis patients: could this represent a trigger for disease onset?

Multiple sclerosis (MS) is associated with breakdown of the myelin sheath that coats neurons in the central nervous system. The cause of MS is not known, although the pathogenesis involves destruction of myelin by the immune system. It was the aim of this study to examine the abundant myelin protein, myelin basic protein (MBP), to determine if there are sites of modification that may be characteristic for MS. MBP from the cerebellum was examined from controls and MS patients across the age range using mass spectrometry and amino acid analysis. Amino acid racemization data indicated that myelin basic protein is long-lived and proteomic analysis of MBP showed it to be highly modified. A common modification of MBP was racemization of Asp and this was significantly greater in MS patients. In long-lived proteins, L-Asp and L-Asn can racemize to three other isomers, D-isoAsp, L-isoAsp and D-Asp and this is significant because isoAsp formation in peptides renders them immunogenic.Proteomic analysis revealed widespread modifications of MBP with two surface regions that are altered in MS. In particular, isoAsp was significantly elevated at these sites in MS patients. The generation of isoAsp could be responsible for eliciting an immune response to modified MBP and therefore be implicated in the etiology of MS

Human protein aging: modification and crosslinking through dehydroalanine and dehydrobutyrine intermediates

Nonenzymatic post-translational modification (PTM) of proteins is a fundamental molecular process of aging. The combination of various modifications and their accumulation with age not only affects function, but leads to crosslinking and protein aggregation. In this study, aged human lens proteins were examined using HPLC–tandem mass spectrometry and a blind PTM search strategy. Multiple thioether modifications of Ser and Thr residues by glutathione (GSH) and its metabolites were unambiguously identified. Thirty-four of 36 sites identified on 15 proteins were found on known phosphorylation sites, supporting a mechanism involving dehydroalanine (DHA) and dehydrobutyrine (DHB) formation through β-elimination of phosphoric acid from phosphoserine and phosphothreonine with subsequent nucleophilic attack by GSH. In vitro incubations of phosphopeptides demonstrated that this process can occur spontaneously under physiological conditions. Evidence that this mechanism can also lead to protein–protein crosslinks within cells is provided where five crosslinked peptides were detected in a human cataractous lens. Nondisulfide crosslinks were identified for the first time in lens tissue between βB2- & βB2-, βA4- & βA3-, γS- & βB1-, and βA4- & βA4-crystallins and provide detailed structural information on in vivo crystallin complexes. These data suggest that phosphoserine and phosphothreonine residues represent susceptible sites for spontaneous breakdown in long-lived proteins and that DHA- and DHB-mediated protein crosslinking may be the source of the long-sought after nondisulfide protein aggregates believed to scatter light in cataractous lenses. Furthermore, this mechanism may be a common aging process that occurs in long-lived proteins of other tissues leading to protein aggregation diseases

The etiology of human age-related cataract. Proteins don\u27t last forever

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

Can the Fact That Myelin Proteins Are Old and Break down Explain the Origin of Multiple Sclerosis in Some People?

Recent discoveries may change the way that multiple sclerosis (MS) is viewed, particularly with regard to the reasons for the untoward immune response. The fact that myelin proteins are long-lived, and that by the time we are adults, they are extensively degraded, alters our perspective on the reasons for the onset of autoimmunity and the origin of MS. For example, myelin basic protein (MBP) from every human brain past the age of 20 years, is so greatly modified, that it is effectively a different protein from the one that was laid down in childhood. Since only a subset of people with such degraded MBP develop MS, a focus on understanding the mechanism of immune responses to central nervous system (CNS) antigens and cerebral immune tolerance appear to be worthwhile avenues to explore. In accord with this, it will be productive to examine why all people, whose brains contain large quantities of a “foreign antigen”, do not develop MS. Importantly for the potential causation of MS, MBP from MS patients breaks down differently from the MBP in aged controls. If the novel structures formed in these MS-specific regions are particularly antigenic, it could help explain the origin of MS. If verified, these findings could provide an avenue for the rational synthesis of drugs to prevent and treat MS