Are ancient proteins responsible for the age-related decline in health and fitness?

Specific Post-Translational Modifications of VDAC3 in ALS-SOD1 Model Cells Identified by High-Resolution Mass Spectrometry

Damage induced by oxidative stress is a key driver of the selective motor neuron death in amyotrophic lateral sclerosis (ALS). Mitochondria are among the main producers of ROS, but they also suffer particularly from their harmful effects. Voltage-dependent anion-selective channels (VDACs) are the most represented proteins of the outer mitochondrial membrane where they form pores controlling the permeation of metabolites responsible for mitochondrial functions. For these reasons, VDACs contribute to mitochondrial quality control and the entire energy metabolism of the cell. In this work we assessed in an ALS cell model whether disease-related oxidative stress induces post-translational modifications (PTMs) in VDAC3, a member of the VDAC family of outer mitochondrial membrane channel proteins, known for its role in redox signaling. At this end, protein samples enriched in VDACs were prepared from mitochondria of an ALS model cell line, NSC34 expressing human SOD1G93A, and analyzed by nUHPLC/High-Resolution nESI-MS/MS. Specific over-oxidation, deamidation, succination events were found in VDAC3 from ALS-related NSC34-SOD1G93A but not in non-ALS cell lines. Additionally, we report evidence that some PTMs may affect VDAC3 functionality. In particular, deamidation of Asn215 alone alters single channel behavior in artificial membranes. Overall, our results suggest modifications of VDAC3 that can impact its protective role against ROS, which is particularly important in the ALS context. Data are available via ProteomeXchange with identifier PXD036728.

Recent progress in the analysis of protein deamidation using mass spectrometry

Deamidation is a nonenzymatic and spontaneous posttranslational modification (PTM) that introduces changes in both structure and charge of proteins, strongly associated with aging proteome instability and degenerative diseases. Deamidation is also a common PTM occurring in biopharmaceutical proteins, representing a major cause of degradation. Therefore, characterization of deamidation alongside its inter-related modifications, isomerization and racemization, is critically important to understand their roles in protein stability and diseases. Mass spectrometry (MS) has become an indispensable tool in site-specific identification of PTMs for proteomics and structural studies. In this review, we focus on the recent advances of MS analysis in protein deamidation. In particular, we provide an update on sample preparation, chromatographic separation, and MS technologies at multi-level scales, for accurate and reliable characterization of protein deamidation in both simple and complex biological samples, yielding important new insight on how deamidation together with isomerization and racemization occurs. These technological progresses will lead to a better understanding of how deamidation contributes to the pathology of aging and other degenerative diseases and the development of biopharmaceutical drugs.

Profiling the 'deamidome' of complex biosamples using mixed-mode chromatography-coupled tandem mass spectrometry

Deamidation is a spontaneous degenerative protein modification (DPM) that disrupts the structure and function of both endogenous proteins and various therapeutic agents. While deamidation has long been recognized as a critical event in human aging and multiple degenerative diseases, research progress in this field has been restricted by the technical challenges associated with studying this DPM in complex biological samples. Asparagine (Asn) deamidation generates L-aspartic acid (L-Asp), D-aspartic acid (D-Asp), L-isoaspartic acid (L-isoAsp) or D-isoaspartic acid (D-isoAsp) residues at the same position of Asn in the affected protein, but each of these amino acids displays similar hydrophobicity and cannot be effectively separated by reverse phase liquid chromatography. The Asp and isoAsp isoforms are also difficult to resolve using mass spectrometry since they have the same mass and fragmentation pattern in MS/MS. Moreover, the ¹³C peaks of the amidated peptide are often misassigned as monoisotopic peaks of the corresponding deamidated peptides in protein database searches. Furthermore, typical protein isolation and proteomic sample preparation methods induce artificial deamidation that cannot be distinguished from the physiological forms. To better understand the role of deamidation in biological aging and degenerative pathologies, new technologies are now being developed to address these analytical challenges, including mixed mode electrostatic-interaction modified hydrophilic interaction liquid chromatography (emHILIC). When coupled to high resolution, high accuracy tandem mass spectrometry this technology enables unprecedented, proteome-wide study of the 'deamidome' of complex samples. The current article therefore reviews recent advances in sample preparation methods, emHILIC-MS/MS technology, and MS instrumentation / data processing approaches to achieving accurate and reliable characterization of protein deamidation in complex biological and clinical samples.

Racemization of Serine Residues Catalyzed by Dihydrogen Phosphate Ion: A Computational Study

Spontaneous, nonenzymatic reactions in proteins are known to have relevance to aging and age-related diseases, such as cataract and Alzheimer’s disease. Among such reactions is the racemization of Ser residues, but its mechanism in vivo remains to be clarified. The most likely intermediate is an enol. Although being nonenzymatic, the enolization would need to be catalyzed to occur at a biologically relevant rate. In the present study, we computationally found plausible reaction pathways for the enolization of a Ser residue where a dihydrogen phosphate ion, H2PO4−, acts as a catalyst. The H2PO4− ion mediates the proton transfer required for the enolization by acting simultaneously as both a general base and a general acid. Using the B3LYP density functional theory method, reaction pathways were located in the gas phase and hydration effects were evaluated by single-point calculations using the SM8 continuum model. The activation barriers calculated for the reaction pathways found were around 100 kJ mol−1, which is consistent with spontaneous reactions occurring at physiological temperature. Our results are also consistent with experimental observations that Ser residue racemization occurs more readily in flexible regions in proteins.

Recent advances in mass spectrometric analysis of protein deamidation

Protein deamidation has been proposed to represent a "molecular clock" that progressively disrupts protein structure and function in human degenerative diseases and natural aging. Importantly, this spontaneous process can also modify therapeutic proteins by altering their purity, stability, bioactivity, and antigenicity during drug synthesis and storage. Deamidation occurs non-enzymatically in vivo, but can also take place spontaneously in vitro, hence artificial deamidation during proteomic sample preparation can hamper efforts to identify and quantify endogenous deamidation of complex proteomes. To overcome this, mass spectrometry (MS) can be used to conduct rigorous site-specific characterization of protein deamidation due to the high sensitivity, speed, and specificity offered by this technique. This article reviews recent progress in MS analysis of protein deamidation and discusses the strengths and limitations of common "top-down" and "bottom-up" approaches. Recent advances in sample preparation methods, chromatographic separation, MS technology, and data processing have for the first time enabled the accurate and reliable characterization of protein modifications in complex biological samples, yielding important new data on how deamidation occurs across the entire proteome of human cells and tissues. These technological advances will lead to a better understanding of how deamidation contributes to the pathology of biological aging and major degenerative diseases. © 2016 Wiley Periodicals, Inc. Mass Spec Rev 36:677-692, 2017.

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.

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

Acetic acid can catalyze succinimide formation from aspartic acid residues by a concerted bond reorganization mechanism: a computational study

Succinimide formation from aspartic acid (Asp) residues is a concern in the formulation of protein drugs. Based on density functional theory calculations using Ace-Asp-Nme (Ace = acetyl, Nme = NHMe) as a model compound, we propose the possibility that acetic acid (AA), which is often used in protein drug formulation for mildly acidic buffer solutions, catalyzes the succinimide formation from Asp residues by acting as a proton-transfer mediator. The proposed mechanism comprises two steps: cyclization (intramolecular addition) to form a gem-diol tetrahedral intermediate and dehydration of the intermediate. Both steps are catalyzed by an AA molecule, and the first step was predicted to be rate-determining. The cyclization results from a bond formation between the amide nitrogen on the C-terminal side and the side-chain carboxyl carbon, which is part of an extensive bond reorganization (formation and breaking of single bonds and the interchange of single and double bonds) occurring concertedly in a cyclic structure formed by the amide NH bond, the AA molecule and the side-chain C=O group and involving a double proton transfer. The second step also involves an AA-mediated bond reorganization. Carboxylic acids other than AA are also expected to catalyze the succinimide formation by a similar mechanism.

Identification of long-lived proteins retained in cells undergoing repeated asymmetric divisions

Long-lived proteins have been implicated in age-associated decline in metazoa, but they have only been identified in extracellular matrices or postmitotic cells. However, the aging process also occurs in dividing cells undergoing repeated asymmetric divisions. It was not clear whether long-lived proteins exist in asymmetrically dividing cells or whether they are involved in aging. Here we identify long-lived proteins in dividing cells during aging using the budding yeast, Saccharomyces cerevisiae. Yeast mother cells undergo a limited number of asymmetric divisions that define replicative lifespan. We used stable-isotope pulse-chase and total proteome mass-spectrometry to identify proteins that were both long-lived and retained in aging mother cells after ∼ 18 cells divisions. We identified ∼ 135 proteins that we designate as long-lived asymmetrically retained proteins (LARPS). Surprisingly, the majority of LARPs appeared to be stable fragments of their original full-length protein. However, 15% of LARPs were full-length proteins and we confirmed several candidates to be long-lived and retained in mother cells by time-lapse microscopy. Some LARPs localized to the plasma membrane and remained robustly in the mother cell upon cell division. Other full-length LARPs were assembled into large cytoplasmic structures that had a strong bias to remain in mother cells. We identified age-associated changes to LARPs that include an increase in their levels during aging because of their continued synthesis, which is not balanced by turnover. Additionally, several LARPs were posttranslationally modified during aging. We suggest that LARPs contribute to age-associated phenotypes and likely exist in other organisms.

Roles of intramolecular and intermolecular hydrogen bonding in a three-water-assisted mechanism of succinimide formation from aspartic acid residues

Aspartic acid (Asp) residues in peptides and proteins are prone to isomerization to the β-form and racemization via a five-membered succinimide intermediate. These nonenzymatic reactions have relevance to aging and age-related diseases. In this paper, we report a three water molecule-assisted, six-step mechanism for the formation of succinimide from Asp residues found by density functional theory calculations. The first two steps constitute a stepwise iminolization of the C-terminal amide group. This iminolization involves a quintuple proton transfer along intramolecular and intermolecular hydrogen bonds formed by the C-terminal amide group, the side-chain carboxyl group, and the three water molecules. After a conformational change (which breaks the intramolecular hydrogen bond involving the iminol nitrogen) and a reorganization of water molecules, the iminol nitrogen nucleophilically attacks the carboxyl carbon of the Asp side chain to form a five-membered ring. This cyclization is accompanied by a triple proton transfer involving two water molecules, so that a gem-diol tetrahedral intermediate is formed. The last step is dehydration of the gem-diol group catalyzed by one water molecule, and this is the rate-determining step. The calculated overall activation barrier (26.7 kcal mol⁻¹) agrees well with an experimental activation energy.

Separate mechanisms for age-related truncation and racemisation of peptide-bound serine

Some amino acids are particularly susceptible to degradation in long-lived proteins. Foremost among these are asparagine, aspartic acid and serine. In the case of serine residues, cleavage of the peptide bond on the N-terminal side, as well as racemisation, has been observed. To investigate the role of the hydroxyl group, and whether cleavage and racemisation are linked by a common mechanism, serine peptides with a free hydroxyl group were compared to analogous peptides where the serine hydroxyl group was methylated. Peptide bond cleavage adjacent to serine was increased when the hydroxyl group was present, and this was particularly noticeable when it was present as the hydroxide ion. Adjacent amino acid residues also had a pronounced affect on cleavage at basic pH, with the SerPro motif being especially susceptible to scission. Methylation of the serine hydroxyl group abolished truncation, as did insertion of a bulky amino acid on the N-terminal side of serine. By contrast, racemisation of serine occurred to a similar extent in both O-methylated and unmodified peptides. On the basis of these data, it appears that racemisation of Ser, and cleavage adjacent to serine, occur via separate mechanisms. Addition of water across the double bond of dehydroalanine was not detected, suggesting that this mechanism was unlikely to be responsible for conversion of L-serine to D-serine. Abstraction of the alpha proton may account for the majority of racemisation of serine in proteins.

Molecular identification and cellular localization of a potential transport system involved in cystine/cysteine uptake in human lenses

In this study we have sought to identify whether cystine uptake mechanisms previously identified in the rat lens are also found in the human lens. Using a combination of reverse transcriptase PCR, Western blotting and immunohistochemistry, we show that the light chain subunit of the cystine/glutamate exchanger (XC-), xCT, and members of the glutamate transporter family (XAG) which include the Excitatory Amino Acid Transporter 4 (EAAT4) and the Alanine Serine Cysteine Transporter 2 (ASCT2) are all present at the transcript and protein level in human lenses. We demonstrate that in young lenses xCT, EAAT4 and ASCT2 are expressed in all regions indicating that a potential cystine uptake pathway similar to that found in the rat might also exist in human lenses. However, with increasing age, the immunolabeling for all transporters decreases, with no xCT labelling detected in the centre of old donor lenses. Our results show that XC- and EAAT4/ASCT2 may work together to mediate cystine uptake in the lens core of young human lenses. This suggests that the lens contains uptake mechanisms that are capable of accumulating cystine/cysteine in the lens centre where cysteine can be used as an antioxidant or cystine utilised as a source for protein-S-S-cysteine (PSSC) formation to buffer against oxidative stress. With increasing age, transporters in the lens core undergo age dependent post translational modifications. However, despite processing of these transporters with age, our results indicate that this cystine uptake pathway could account for the increased PSSC levels previously observed in the nucleus of older human lenses.

Assessment of candidate ocular biomarkers of ageing in a South African adult population: relationship with chronological age and systemic biomarkers

Certain anatomic and functional parameters of the eye change with increasing chronological age. They may, therefore, serve as potential biomarkers of ageing. We investigated associations between four such ocular parameters (lens density, retinal vessel calibre, corneal endothelial cells and retinal nerve fibre layer thickness) and two 'cellular' biomarkers of ageing (leukocyte telomere length and CDKN2A expression), with frailty (a clinical correlate of biological ageing) in a population of South African adults. All ocular parameters revealed an association with either telomere length or CDKN2A expression. However, lens density was most strongly correlated with age, increased CDKN2A expression, and with frailty (p=0.05 and 0.03, respectively). Narrow retinal arteriolar diameter, associated with increased chronological age, was also associated with increased CDK2NA expression (0.42 vs. 0.31, p=0.02) but not with frailty. Ocular parameters may aid in determining biological age, warranting investigation in longitudinal studies.

The eye as a model of ageing in translational research--molecular, epigenetic and clinical aspects

The eye and visual system are valuable in many areas of translational research such as stem cell therapy, transplantation research and gene therapy. Changes in many ocular tissues can be measured directly, easily and objectively in vivo (e.g. lens transparency; retinal blood vessel calibre; corneal endothelial cell counts) and so the eye may also be a uniquely useful site as a model of ageing. This review details cellular, molecular and epigenetic mechanisms related to ageing within the eye, and describes ocular parameters that can be directly measured clinically and which might be of value in ageing research as the translational "window to the rest of the body". The eye is likely to provide a valuable model for validating biomarkers of ageing at molecular, epigenetic, cellular and clinical levels. A research agenda to definitively establish the relationship between biomarkers of ageing and ocular parameters is proposed.

Degradation of an old human protein: age-dependent cleavage of γS-crystallin generates a peptide that binds to cell membranes

Long-lived proteins exist in a number of tissues in the human body; however, little is known about the reactions involved in their degradation over time. Lens proteins, which do not turn over, provide a useful system to examine such processes. Using a combination of Western blotting and proteomic methodology, age-related changes to a major protein, γS-crystallin, were studied. By teenage years, insoluble intact γS-crystallin was detected, indicative of protein denaturation. This was not the only change, however, because blots revealed evidence of significant cross-linking as well as cleavage of γS-crystallin in all adult lenses. Cleavage at a serine residue near the C terminus was a major reaction that caused the release of a 12-residue peptide, SPAVQSFRRIVE, which bound tightly to lens cell membranes. Several other crystallin-derived peptides with double basic residues also lodged in the cell membrane fraction. Model studies showed that once cleaved from γS-crystallin, SPAVQSFRRIVE adopts a markedly different shape from that in the intact protein. Further, the acquired helical conformation may explain why the peptide seems to affect water permeability. This observation may help explain the changes to cell membranes known to be associated with aging in human lenses. Age-related cleavage of long-lived proteins may therefore yield peptides with untoward biological activity.

Racemization of two proteins over our lifespan: deamidation of asparagine 76 in γS crystallin is greater in cataract than in normal lenses across the age range

PURPOSE

Long-lived proteins are widespread in man, yet little is known about the processes that affect their function over time, or their role in age-related diseases.

METHODS

Racemization of two proteins from normal and cataract human lenses were compared with age using tryptic digestion and LC/mass spectrometry. Asp 151 in αA crystallin and Asn 76 in γS crystallin were studied.

RESULTS

Age-dependent profiles for the two proteins from normal lenses were different. In neither protein did the modifications increase linearly with age. For αA crystallin, racemization occurred most rapidly during the first 15 years of life, with approximately half of L-Asp 151 converted to D-isoAsp, L-isoAsp, and D-Asp in a ratio of 3:1:0.5. Values then changed little. By contrast, racemization of Asn 76 in γS crystallin was slow until age 15, with isoAsp accounting for only 5%. Values remained relatively constant until age 40 when a linear increase (1%/year) took place. When cataract lenses were compared with age-matched normal lenses, there were marked differences in the time courses of the two crystallins. For αA crystallin, there was no significant difference in Asp 151 racemization between cataract and normal lenses. By contrast, in γS crystallin the degree of conversion of Asn 76 to isoAsp in cataract lenses was approximately double that of normals at every age.

CONCLUSIONS

Modification of Asn and Asp over time may contribute to denaturation of proteins in the human lens. An accelerated rate of deamidation/racemization at selected sites in proteins, such as γS crystallin, may contribute to cataract formation.

Age-dependent deamidation of glutamine residues in human γS crystallin: deamidation and unstructured regions

Human aging is associated with the deterioration of long-lived proteins. Gradual cumulative modifications to the life-long proteins of the lens may ultimately be responsible for the pronounced alterations to the optical and physical properties that characterize lenses from older people. γS crystallin, a major human lens protein, is known to undergo several age-dependent changes. Using proteomic techniques, a site of deamidation involving glutamine 92 has been characterized and its time course established. The proportion of deamidation increased from birth to teen-age years and then plateaud. Deamidation at this site increased again in the eighth decade of life. There was no significant difference in the extent of deamidation between cataract and age-matched normal lenses. Gln92 is located in the linker region between the two domains, and the introduction of a negative charge at this site may alter the interaction between the two regions of the protein. Gln170, which is located in another unstructured part of γS crystallin, showed a similar deamidation profile to that of Gln92. As the other Gln residues in β-sheet regions of γS crystallin appear to remain as amides, modification of Gln92 and Gln170 thus conforms to a pattern whereby deamidation is localized to the unstructured regions of long-lived proteins.

Macromolecular deterioration as the ultimate constraint on human lifespan

A number of tissues and organs in the human body contain abundant proteins that are long-lived. This includes the heart, lung, brain, bone and connective tissues. It is proposed that the accumulation of modifications to such long-lived proteins over a period of decades alters the properties of the organs and tissues in which they reside. Such insidious processes may affect human health, fitness and ultimately may limit our lifespan. The human lens, which contains proteins that do not turnover, is used to illustrate the impact of these gradual deleterious modifications. On the basis of data derived from the lens, it is postulated that the intrinsic instability of certain amino acid residues, which leads to truncation, racemisation and deamidation, is primarily responsible for the age-related deterioration of such proteins. Since these post-translational modifications accumulate over a period of many years, they can only be studied using organisms that have lifespans measured in decades. One conclusion is that there may be important aspects of human aging that can be studied only using long-lived animals.

Energy metabolism and ageing regulation: metabolically driven deamidation of triosephosphate isomerase may contribute to proteostatic dysfunction

Research carried out up to 3 decades ago by Gracy and co-workers revealed that the activity of the glycolytic enzyme triosephosphate isomerase (TPI), which converts dihydroxyacetone phosphate (DHAP) to glyceraldehyde-3-phosphate (G3P), gradually declines whilst carrying out its catalytic function, primarily due to deamidation of certain asparagine residues. It is suggested here that excessive or continuous glycolysis increases TPI deamidation and thereby lowers TPI activity and causes accumulation of its substrate, DHAP, which in turn decomposes into methylglyoxal (MG), a well-recognised reactive bicarbonyl whose actions in cells and tissues, as well as at the whole organism level, mimic much age-relate dysfunction. The proposal helps to explain why suppression of glycolysis by caloric restriction, fasting and increased aerobic activity also suppresses generation of altered proteins which characterise the aged phenotype. It is proposed that these effects on TPI activity, though seemingly neglected in biogerontological contexts, reveal a mechanistic link between energy metabolism and age-related proteostatic dysfunction.

Racemisation and human cataract. D-Ser, D-Asp/Asn and D-Thr are higher in the lifelong proteins of cataract lenses than in age-matched normal lenses

ASTRACT: Several amino acids were found to undergo progressive age-dependent racemisation in the lifelong proteins of normal human lenses. The two most highly racemised were Ser and Asx. By age 70, 4.5% of all Ser residues had been racemised, along with >9% of Asx residues. Such a high level of inversion, equivalent to between 2 and 3 D- amino acids per polypeptide chain, is likely to induce significant denaturation of the crystallins in aged lenses. Thr, Glx and Phe underwent age-dependent racemisation to a smaller degree. In model experiments, D- amino acid content could be increased simply by exposing intact lenses to elevated temperature. In cataract lenses, the extent of racemisation of Ser, Asx and Thr residues was significantly greater than for age-matched normal lenses. This was true, even for cataract lenses removed from patients at the earliest ages where age-related cataract is observed clinically. Racemisation of amino acids in crystallins may arise due to prolonged exposure of these proteins to ocular temperatures and increased levels of racemisation may play a significant role in the opacification of human lenses.

Presbyopia and cataract: a question of heat and time

Not only are human lenses different in many ways from those of non-primates, they also undergo dramatic changes with age. These age-dependent alterations lead to perturbations in the properties of older lenses, and ultimately to disturbances in visual function, which typically become apparent at middle age. Recent data suggest that many, if not all, of these age-dependent features can be traced to the lack of macromolecular turnover in the lens and to the inexorable modifications to proteins and membrane components over a period of decades. Exposure of lenses to heat can reproduce many of these alterations, suggesting that long-term incubation at body temperature may be an important factor in aging the human lens. Two conclusions flow from this. Firstly, the human lens may be an ideal tissue for studying macromolecular aging in man. Secondly, it will be extremely challenging to examine the origin of human age-related conditions, such as presbyopia and nuclear cataract, using traditional laboratory animals. Characterising the unfolding and decomposition of long-lived macromolecules appears to provide the key to understanding the two most common human lens disorders: presbyopia and age-related nuclear cataract.