α- and β-crystallins modulate the head group order of human lens membranes during aging

Interaction of βL- and γ-Crystallin with Phospholipid Membrane Using Atomic Force Microscopy

Highly concentrated lens proteins, mostly β- and γ-crystallin, are responsible for maintaining the structure and refractivity of the eye lens. However, with aging and cataract formation, β- and γ-crystallin are associated with the lens membrane or other lens proteins forming high-molecular-weight proteins, which further associate with the lens membrane, leading to light scattering and cataract development. The mechanism by which β- and γ-crystallin are associated with the lens membrane is unknown. This work aims to study the interaction of β- and γ-crystallin with the phospholipid membrane with and without cholesterol (Chol) with the overall goal of understanding the role of phospholipid and Chol in β- and γ-crystallin association with the membrane. Small unilamellar vesicles made of Chol/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (Chol/POPC) membranes with varying Chol content were prepared using the rapid solvent exchange method followed by probe tip sonication and then dispensed on freshly cleaved mica disk to prepare a supported lipid membrane. The βL- and γ-crystallin from the cortex of the bovine lens was used to investigate the time-dependent association of βL- and γ-crystallin with the membrane by obtaining the topographical images using atomic force microscopy. Our study showed that βL-crystallin formed semi-transmembrane defects, whereas γ-crystallin formed transmembrane defects on the phospholipid membrane. The size of semi-transmembrane defects increases significantly with incubation time when βL-crystallin interacts with the membrane. In contrast, no significant increase in transmembrane defect size was observed in the case of γ-crystallin. Our result shows that Chol inhibits the formation of membrane defects when βL- and γ-crystallin interact with the Chol/POPC membrane, where the degree of inhibition depends upon the amount of Chol content in the membrane. At a Chol/POPC mixing ratio of 0.3, membrane defects were observed when both βL- and γ-crystallin interacted with the membrane. However, at a Chol/POPC mixing ratio of 1, no association of γ-crystallin with the membrane was observed, which resulted in a defect-free membrane, and the severity of the membrane defect was decreased when βL-crystallin interacted with the membrane. The semi-transmembrane or transmembrane defects formed by the interaction of βL- and γ-crystallin on phospholipid membrane might be responsible for light scattering and cataract formation. However, Chol suppressed the formation of such defects in the membrane, likely maintaining lens membrane homeostasis and protecting against cataract formation.

Binding of βL-Crystallin with Models of Animal and Human Eye Lens-Lipid Membrane

Several discoveries show that with age and cataract formation, β-crystallin binds with the lens membrane or associates with other lens proteins, which bind with the fiber cell plasma membrane, accompanied by light scattering and cataract formation. However, how lipids (phospholipids and sphingolipids) and cholesterol (Chol) influence β-crystallin binding to the membrane is unclear. This research aims to elucidate the role of lipids and Chol in the binding of β-crystallin to the membrane and the membrane's physical properties (mobility, order, and hydrophobicity) with β-crystallin binding. We used electron paramagnetic resonance (EPR) spin-labeling methods to investigate the binding of βL-crystallin with a model of porcine lens-lipid (MPLL), model of mouse lens-lipid (MMLL), and model of human lens-lipid (MHLL) membrane with and without Chol. Our results show that βL-crystallin binds with all of the investigated membranes in a saturation manner, and the maximum parentage of the membrane surface occupied (MMSO) by βL-crystallin and the binding affinity (Ka) of βL-crystallin to the membranes followed trends: MMSO (MPLL) > MMSO (MMLL) > MMSO (MHLL) and Ka (MHLL) > Ka (MMLL) ≈ Ka (MPLL), respectively, in which the presence of Chol reduces the MMSO and Ka for all membranes. The mobility near the headgroup regions of the membranes decreases with an increase in the binding of βL-crystallin; however, the decrease is more pronounced in the MPLL and MMLL membranes than the MHLL membrane. In the MPLL and MMLL membranes, the membranes become slightly ordered near the headgroup with an increase in βL-crystallin binding compared to the MHLL membrane. The hydrophobicity near the headgroup region of the membrane increases with βL-crystallin binding; however, the increase is more pronounced in the MPLL and MMLL membranes than the MHLL membrane, indicating that βL-crystallin binding creates a hydrophobic barrier for the passage of polar molecules, which supports the barrier hypothesis in cataract formation. However, in the presence of Chol, there is no significant increase in hydrophobicity with βL-crystallin binding, suggesting that Chol prevents the formation of a hydrophobic barrier, possibly protecting against cataract formation.

Native Ambient Mass Spectrometry of an Intact Membrane Protein Assembly and Soluble Protein Assemblies Directly from Lens Tissue

Membrane proteins constitute around two-thirds of therapeutic targets but present a significant challenge for structural analysis due to their low abundance and solubility. Existing methods for structural analysis rely on over-expression and/or purification of the membrane protein, thus removing any links back to actual physiological environment. Here, we demonstrate mass spectrometry analysis of an intact oligomeric membrane protein directly from tissue. Aquaporin-0 exists as a 113 kDa tetramer, with each subunit featuring six transmembrane helices. We report the characterisation of the intact assembly directly from a section of sheep eye lens without sample pre-treatment. Protein identity was confirmed by mass measurement of the tetramer and subunits, together with top-down mass spectrometry, and the spatial distribution was determined by mass spectrometry imaging. Our approach allows simultaneous analysis of soluble protein assemblies in the tissue.

Native Ambient Mass Spectrometry of an Intact Membrane Protein Assembly and Soluble Protein Assemblies Directly from Lens Tissue

Membrane proteins constitute around two-thirds of therapeutic targets but present a significant challenge for structural analysis due to their low abundance and solubility. Existing methods for structural analysis rely on over-expression and/or purification of the membrane protein, thus removing any links back to actual physiological environment. Here, we demonstrate mass spectrometry analysis of an intact oligomeric membrane protein directly from tissue. Aquaporin-0 exists as a 113 kDa tetramer, with each subunit featuring six transmembrane helices. We report the characterisation of the intact assembly directly from a section of sheep eye lens without sample pre-treatment. Protein identity was confirmed by mass measurement of the tetramer and subunits, together with top-down mass spectrometry, and the spatial distribution was determined by mass spectrometry imaging. Our approach allows simultaneous analysis of soluble protein assemblies in the tissue.

Alpha-Crystallin-Membrane Association Modulated by Phospholipid Acyl Chain Length and Degree of Unsaturation

α-crystallin-membrane association increases with age and cataracts, with the primary association site of α-crystallin being phospholipids. However, it is unclear if phospholipids’ acyl chain length and degree of unsaturation influence α-crystallin association. We used the electron paramagnetic resonance approach to investigate the association of α-crystallin with phosphatidylcholine (PC) membranes of different acyl chain lengths and degrees of unsaturation and with and without cholesterol (Chol). The association constant (Ka) of α-crystallin follows the trends, i.e., Ka (14:0−14:0 PC) > Ka (18:0−18:1 PC) > Ka (18:1−18:1 PC) ≈ Ka (16:0−20:4 PC) where the presence of Chol decreases Ka for all membranes. With an increase in α-crystallin concentration, the saturated and monounsaturated membranes rapidly become more immobilized near the headgroup regions than the polyunsaturated membranes. Our results directly correlate the mobility and order near the headgroup regions of the membrane with the Ka, with the less mobile and more ordered membrane having substantially higher Ka. Furthermore, our results show that the hydrophobicity near the headgroup regions of the membrane increases with the α-crystallin association, indicating that the α-crystallin-membrane association forms the hydrophobic barrier to the transport of polar and ionic molecules, supporting the barrier hypothesis in cataract development.

Cholesterol and cholesterol bilayer domains inhibit binding of alpha-crystallin to the membranes made of the major phospholipids of eye lens fiber cell plasma membranes

The concentration of α-crystallin decreases in the eye lens cytoplasm, with a corresponding increase in membrane-bound α-crystallin during cataract formation. The eye lens's fiber cell plasma membrane consists of extremely high cholesterol (Chol) content, forming cholesterol bilayer domains (CBDs) within the membrane. The role of high Chol content in the lens membrane is unclear. Here, we applied the continuous-wave electron paramagnetic resonance spin-labeling method to probe the role of Chol and CBDs on α-crystallin binding to membranes made of four major phospholipids (PLs) of the eye lens, i.e., phosphatidylcholine (PC), sphingomyelin (SM), phosphatidylserine (PS), and phosphatidylethanolamine (PE). Small unilamellar vesicles (SUVs) of PC, SM*, and PS with 0, 23, 33, 50, and 60 mol% Chol and PE* with 0, 9, and 33 mol% Chol were prepared using the rapid solvent exchange method followed by probe-tip sonication. The 1 mol% CSL spin-labels used during SUVs preparation distribute uniformly within the Chol/PL membrane, enabling the investigation of Chol and CBDs' role on α-crystallin binding to the membrane. For PC, SM*, and PS membranes, the binding affinity (Ka) and the maximum percentage of membrane surface occupied (MMSO) by α-crystallin decreased with an increase in Chol concentration. The Ka and MMSO became zero at 50 mol% Chol for PC and 60 mol% Chol for SM* membranes, representing that complete inhibition of α-crystallin binding was possible before the formation of CBDs within the PC membrane but only after the formation of CBDs within the SM* membrane. The Ka and MMSO did not reach zero even at 60 mol% Chol in the PS membrane, representing CBDs at this Chol concentration were not sufficient for complete inhibition of α-crystallin binding to the PS membrane. Both the Ka and MMSO were zero at 0, 9, and 33 mol% Chol in the PE* membrane, representing no binding of α-crystallin to the PE* membrane with and without Chol. The mobility parameter profiles decreased with an increase in α-crystallin binding to the membranes; however, the decrease was more pronounced for the membrane with lower Chol concentration. These results imply that the membranes become more immobilized near the headgroup regions with an increase in α-crystallin binding; however, the Chol antagonizes the capacity of α-crystallin to decrease the mobility near the headgroup regions of the membranes. The maximum splitting profiles remained the same with an increase in α-crystallin concentration, but there was an increase in the maximum splitting with an increase in the Chol concentration in the membranes. It implies that membrane order near the headgroup regions does not change with an increase in α-crystallin concentration but increases with an increase in Chol concentration in the membrane. Based on our data, we hypothesize that the Chol and CBDs decrease hydrophobicity (increase polarity) near the membrane surface, inhibiting the hydrophobic binding of α-crystallin to the membranes. Thus, our data suggest that Chol and CBDs play a positive physiological role by preventing α-crystallin binding to lens membranes and possibly protecting against cataract formation and progression.

Aberrant TGF-β1 signaling activation by MAF underlies pathological lens growth in high myopia

High myopia is a leading cause of blindness worldwide. Myopia progression may lead to pathological changes of lens and affect the outcome of lens surgery, but the underlying mechanism remains unclear. Here, we find an increased lens size in highly myopic eyes associated with up-regulation of β/γ-crystallin expressions. Similar findings are replicated in two independent mouse models of high myopia. Mechanistic studies show that the transcription factor MAF plays an essential role in up-regulating β/γ-crystallins in high myopia, by direct activation of the crystallin gene promoters and by activation of TGF-β1-Smad signaling. Our results establish lens morphological and molecular changes as a characteristic feature of high myopia, and point to the dysregulation of the MAF-TGF-β1-crystallin axis as an underlying mechanism, providing an insight for therapeutic interventions.

High myopia is associated with lens changes, but the underlying mechanisms are unclear. Here, the authors show increased equatorial diameter of the lens in subjects affected by high myopia, and find that these changes are associated with an increase in crystallin expression driven by the transcription factor MAF and TGF-β1 signaling.

Interaction of alpha-crystallin with four major phospholipids of eye lens membranes

It is well-studied that the significant factor in cataract formation is the association of α-crystallin, a major eye lens protein, with the fiber cell plasma membrane of the eye lens. The fiber cell plasma membrane of the eye lens consists of four major phospholipids (PLs), i.e., phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), and sphingomyelin (SM). Despite several attempts to study the interaction of α-crystallin with PLs of the eye lens membrane, the role of individual PL for the binding with α-crystallin is still unclear. We recently developed the electron paramagnetic resonance (EPR) spin-labeling method to study the binding of α-crystallin to the PC membrane (Mainali et al., 2020a). Here, we use the recently developed EPR method to explicitly measure the binding affinity (K_a) of α-crystallin to the individual (PE*, PS, and SM) and two-component mixtures (SM/PE, SM/PS, and SM/PC in 70:30 and 50:50 mol%) of PL membranes as well as the physical properties (mobility parameter and maximum splitting) of these membranes upon binding with α-crystallin. One of the key findings of this study was that the K_a of α-crystallin binding to individual PL membranes followed the trends: K_a(PC) > K_a(SM) > K_a(PS) > K_a(PE*), indicating PE* inhibits binding the most whereas PC inhibits binding the least. Also, the K_a of α-crystallin binding to two-component mixtures of PL membranes followed the trends: K_a(SM/PE) > K_a(SM/PS) > K_a(SM/PC), indicating SM/PC inhibits binding the most whereas SM/PE inhibits binding the least. Except for the PE* membrane, for which there was no binding of α-crystallin, the mobility parameter for all other membranes decreased with an increase in α-crystallin concentration. It represents that the membranes become more immobilized near the headgroup regions of the PLs when more and more α-crystallin binds to them. The maximum splitting increased only for the SM and the SM/PE (70:30 mol%) membranes, with an increase in the binding of α-crystallin. It represents that the PL headgroup regions of these membranes become more ordered after binding of α-crystallin to these membranes. Our results showed that α-crystallin binds to PL membranes in a saturable manner. Also, our data suggest that the binding of α-crystallin to PL membranes likely occurs through hydrophobic interaction between α-crystallin and the hydrophobic fatty acid core of the membranes, and such interaction is modulated by the PL headgroup's size and charge, hydrogen bonding between headgroups, and PL curvature. Thus, this study provides an in-depth understanding of α-crystallin interaction with the PL membranes made of individual and two-component mixtures of the four major PLs of the eye lens membranes.

The Proteome of Cataract Markers: Focus on Crystallins

Cataract is a major cause of blindness worldwide. It is characterized by lens opacification and is accompanied by extensive posttranslational modifications (PTMs) in various proteins. PTMs play an essential role in lens opacification. Several PTMs have been described in proteins isolated from relatively old human lenses, including phosphorylation, deamidation, racemization, truncation, acetylation, and methylation. An overwhelming majority of previous cataract proteomic studies have exclusively focused on crystallin proteins, which are the most abundant proteome components of the lens. To investigate the proteome of cataract markers, this chapter focuses on the proteomic research on the functional relevance of the major PTMs in crystallins of human cataractous lenses. Elucidating the role of these modifications in cataract formation has been a challenging task because they are among the most difficult PTMs to study analytically. The proteomic status of some amides presents similar properties in normal aged and cataractous lenses, whereas some may undergo greater PTMs in cataract. Therefore, it is of great importance to review the current proteomic research on crystallins, the major protein markers in different types of cataract, to elucidate the pathogenesis of this major human-blinding condition.

Effect of Mild Heating on Human Lens Epithelial Cells: A Possible Model of Lens Aging

This study aims to investigate the effect of mild heating on lens epithelial cells and to explore its possibility as an in vitro model for lens aging. Human lens epithelial cells (LECs) were heated at 50 °C for a cellular lens aging study. Analysis of the head group order of lens membranes was performed using Laurdan labeling. Immunofluorescence was performed to detect changes in α-crystallin expression and its cellular distribution. The chaperone-like activity of α-crystallin was also assessed. After mild heating, α-crystallin in LECs showed a tendency towards accumulation around the nucleus. The membrane head group environment of lens epithelial cells became more fluid with increasing time of exposure to mild heating, as indicated by increased water penetration. Furthermore, the chaperone activity of α-crystallin decreased, and suggests a relatively lower protective effect on other functional proteins in LECs. Thus, compared to the mild heating model based on lens tissue, this cellular model could provide a more convenient and accurate method for studying lens aging in vitro, including changes in membrane head group order in each cell, the real-time observation of crystallin distribution, and the monitoring of functional changes in the chaperone activity of crystallins as a result of aging.

Small molecules, both dietary and endogenous, influence the onset of lens cataracts

How the lens ages successfully is a lesson in biological adaption and the emergent properties of its complement of cells and proteins. This living tissue contains some of the oldest proteins in our bodies and yet they remain functional for decades, despite exposure to UV light, to reactive oxygen species and all the other hazards to protein function. This remarkable feat is achieved by a shrewd investment in very stable proteins as lens crystallins, by providing a reservoir of ATP-independent protein chaperones unequalled by any other tissue and by an oxidation-resistant environment. In addition, glutathione, a free radical scavenger, is present in mM concentrations and the plasma membranes contain oxidation-resistant sphingolipids what compromises lens function as it ages? In this review, we examine the role of small molecules in the prevention or causation of cataracts, including those associated with diet, metabolic pathways and drug therapy (steroids).

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.

αA-crystallin gene CpG islands hypermethylation in nuclear cataract after pars plana vitrectomy

PURPOSE

To investigate the DNA methylation status of αA-crystallin gene in cataract secondary to pars plana vitrectomy.

METHODS

Anterior capsular membranes of 40 eyes of 40 patients with cataract secondary to vitrectomy were collected. Another 20 eyes of 20 patients who received pars plana vitrectomy and phacoemulsification in the primary procedure, were recruited as control. Methylation status of the CpG islands of αA-crystallin gene was analyzed by pyrosequencing. Expression of αA-crystallin was evaluated by real-time polymerase chain reaction and western blot.

RESULTS

In the post vitrectomy group, five patients with posterior subcapsular opacity and four patients with cortical opacity were excluded from further analysis. The remaining 31 patients with nuclear cataract were assigned into two groups according to tamponade types: 19 of octafluoropropane (C3F8) and 12 of silicone oil (SiO). The average nuclear color grading was elevated both in C3F8 and SiO groups after vitrectomy. Compared to the control group, hypermethylation of the CpG islands in the αA-crystallin gene promoter was found in both post vitrectomy groups, accompanied by significantly reduced αA-crystallin expression. No statistically significant differences were found between the C3F8 and SiO groups either for DNA methylation status or αA-crystallin expression.

CONCLUSIONS

CpG islands hypermethylation of αA-crystallin gene may be involved in nuclear cataract formation after pars plana vitrectomy.

Old proteins and the Achilles heel of mass spectrometry. The role of proteomics in the etiology of human cataract

Proteomics may have enabled the root cause of a major human-blinding condition, age-related cataract, to be established. Cataract appears to result from the spontaneous decomposition of long-lived macromolecules in the human lens, and recent proteomic analysis has enabled both the particular crystallins, and the specific sites of amino acid modification within each polypeptide, to be identified. Analysis of proteins from cataract lenses has demonstrated that there are key sites on some structural proteins that show a consistently greater degree of deterioration than age-matched normal lenses. Proteomic analysis, using MS, revealed that the most abundant posttranslational modification of aged lens proteins is racemization. This is somewhat ironic, since structural isomers can be viewed as the "Achilles heel" of MS and there are typically few, if any, differences in the MS/MS spectra of tryptic peptides containing one d-amino acid. It is proposed that once a certain level of spontaneous PTM at key sites occurs, that protein-protein interactions are disrupted, and binding of complexes to cell membranes takes place that impairs cell-to-cell communication. These findings may apply more widely to age-related human diseases, in particular where the deterioration of long-lived proteins is a crucial component in the etiology.

Epigenetic regulation of αA-crystallin in high myopia-induced dark nuclear cataract

Purpose

To assess the etiology of early-onset dark nucleus in high-myopic patients and its relationship with the epigenetic regulation of αA-crystallin (CRYAA).

Methods

We reviewed clinical data from patients who underwent cataract surgery at our center in 2012. Lens epithelial samples were collected during capsulorhexis, whereas young lens epithelium was donated. Cataract type and severity were graded according to the Lens Opacity Classification System III (LOCS III). DNA methylation was analyzed by pyrosequencing the CpG islands of the CRYAA promoter in the following groups: Age-Related Cataract (ARC) Nuclear Color (NC) 2–3; High-Myopic Cataract (HMC) NC2–3; ARC NC5–6; HMC NC5–6; and in young lenses graded NC1. We analyzed CRYAA expression by real-time polymerase chain reaction (PCR), reverse transcription PCR, and immunohistochemistry.

Results

The odds ratio of dark nucleus in high-myopic patients was 5.16 (95% confidence interval: 3.98–6.69; p<0.001). CpG islands in lens epithelial CRYAA promoter in the HMC NC5–6 Group exhibited the highest methylation of all the groups, but no statistically significant differences were evident between the HMC NC2–3 and ARC NC2–3 Groups. Likewise, CRYAA mRNA and protein levels in the HMC NC5–6 Group were significantly lower than the ARC NC5–6 Group and high-myopic controls.

Conclusions

High myopia is a risk factor for dark nucleus. Downregulation of CRYAA via the hypermethylation of CpG islands in its promoter could underlie the earlier onset of dark nucleus in high-myopic patients.

Chaperones: needed for both the good times and the bad times

In this issue, we explore the assembly roles of protein chaperones, mainly through the portal of their associated human diseases (e.g. cardiomyopathy, cataract, neurodegeneration, cancer and neuropathy). There is a diversity to chaperone function that goes beyond the current emphasis in the scientific literature on their undoubted roles in protein folding and refolding. The focus on chaperone-mediated protein folding needs to be broadened by the original Laskey discovery that a chaperone assists the assembly of an oligomeric structure, the nucleosome, and the subsequent suggestion by Ellis that other chaperones may function in assembly processes, as well as in folding. There have been a number of recent discoveries that extend this relatively neglected aspect of chaperone biology to include proteostasis, maintenance of the cellular redox potential, genome stability, transcriptional regulation and cytoskeletal dynamics. So central are these processes that we propose that chaperones stand at the crossroads of life and death because they mediate essential functions, not only during the bad times, but also in the good times. We suggest that chaperones facilitate the success of a species, and hence the evolution of individuals within populations, because of their contributions to so many key cellular processes, of which protein folding is only one.

Carbon turnover in the water-soluble protein of the adult human lens

PURPOSE

Human eye lenses contain cells that persist from embryonic development. These unique, highly specialized fiber cells located at the core (nucleus) of the lens undergo pseudo-apoptosis to become devoid of cell nuclei and most organelles. Ostensibly lacking in protein transcriptional capabilities, it is currently believed that these nuclear fiber cells owe their extreme longevity to the perseverance of highly stable and densely packed crystallin proteins. Maintaining the structural and functional integrity of lenticular proteins is necessary to sustain cellular transparency and proper vision, yet the means by which the lens actually copes with a lifetime of oxidative stress, seemingly without any capacity for protein turnover and repair, is not completely understood. Although many years of research have been predicated upon the assumption that there is no protein turnover or renewal in nuclear fiber cells, we investigated whether or not different protein fractions possess protein of different ages by using the (14)C bomb pulse.

METHODS

Adult human lenses were concentrically dissected by gently removing the cell layers in water or shaving to the nucleus with a curved micrometer-controlled blade. The cells were lysed, and the proteins were separated into water-soluble and water-insoluble fractions. The small molecules were removed using 3 kDa spin filters. The (14)C/C was measured in paired protein fractions by accelerator mass spectrometry, and an average age for the material within the sample was assigned using the (14)C bomb pulse.

RESULTS

The water-insoluble fractions possessed (14)C/C ratios consistent with the age of the cells. In all cases, the water-soluble fractions contained carbon that was younger than the paired water-insoluble fraction.

CONCLUSIONS

As the first direct evidence of carbon turnover in protein from adult human nuclear fiber cells, this discovery supports the emerging view of the lens nucleus as a dynamic system capable of maintaining homeostasis in part due to intricate protein transport mechanisms and possibly protein repair. This finding implies that the lens plays an active role in the aversion of age-related nuclear (ARN) cataract.

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.

Instability of the cellular lipidome with age

The human lens nucleus is formed in utero, and from birth onwards, there appears to be no significant turnover of intracellular proteins or membrane components. Since, in adults, this region also lacks active enzymes, it offers the opportunity to examine the intrinsic stability of macromolecules under physiological conditions. Fifty seven human lenses, ranging in age from 12 to 82 years, were dissected into nucleus and cortex, and the nuclear lipids analyzed by electrospray ionization tandem mass spectrometry. In the first four decades of life, glycerophospholipids (with the exception of lysophosphatidylethanolamines) declined rapidly, such that by age 40, their content became negligible. In contrast the level of ceramides and dihydroceramides, which were undetectable prior to age 30, increased approximately 100-fold. The concentration of sphingomyelins and dihydrosphingomyelins remained unchanged over the whole life span. As a consequence of this marked alteration in composition, the properties of fiber cell membranes in the centre of young lenses are likely to be very different from those in older lenses. Interestingly, the identification of age 40 years as a time of transition in the lipid composition of the nucleus coincides with previously reported macroscopic changes in lens properties (e.g., a massive age-related increase in lens stiffness) and related pathologies such as presbyopia. The underlying reasons for the dramatic change in the lipid profile of the human lens with age are not known, but are most likely linked to the stability of some membrane lipids in a physiological environment.