Michael G. Friedrich

Presbyopia and heat: changes associated with aging of the human lens suggest a functional role for the small heat shock protein, α‐crystallin, in maintaining lens flexibility

Presbyopia, the inability to focus up close, affects everyone by age 50 and is the most common eye condition. It is thought to result from changes to the lens over time making it less flexible. We present evidence that presbyopia may be the result of age‐related changes to the proteins of the lens fibre cells. Specifically, we show that there is a progressive decrease in the concentration of the chaperone, α‐crystallin, in human lens nuclei with age, as it becomes incorporated into high molecular weight aggregates and insoluble protein. This is accompanied by a large increase in lens stiffness. Stiffness increases even more dramatically after middle age following the disappearance of free soluble α‐crystallin from the centre of the lens. These alterations in α‐crystallin and aggregated protein in human lenses can be reproduced simply by exposing intact pig lenses to elevated temperatures, for example, 50 °C. In this model system, the same protein changes are also associated with a progressive increase in lens stiffness. These data suggest a functional role for α‐crystallin in the human lens acting as a small heat shock protein and helping to maintain lens flexibility. Presbyopia may be the result of a loss of α‐crystallin coupled with progressive heat‐induced denaturation of structural proteins in the lens during the first five decades of life.

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.

Membrane Association of Proteins in the Aging Human Lens: Profound Changes Take Place in the Fifth Decade of Life

PURPOSE To characterize age-related changes to proteins in the center of the human lens. METHODS Human lenses of different ages were dissected using trephines. Sucrose density gradient centrifugation was used to separate the proteins from two defined nuclear regions. Densitometry of Coomassie-stained protein bands was compared with lipid analysis with the use of mass spectrometry. RESULTS A profound change in the density gradient profiles of lenses occurred at approximately age 40. As soluble crystallins decreased, four higher density bands appeared that were absent in younger lenses. These four bands contained crystallins, as well as membrane lipids, and appear to have resulted from the interaction of denatured crystallins with fiber cell membranes. CONCLUSIONS Changes in lens proteins and membranes can be detected in each decade of life; however, major changes to the lens crystallins of the nucleus take place between age 40 and 50, after the loss of free soluble alpha crystallin. These alterations are consistent with large-scale binding of crystallin aggregates to fiber cell membranes after middle age.

Free and Bound Water in Normal and Cataractous Human Lenses

PURPOSE To analyze free and total water in human normal and cataractous lenses. METHODS Thermogravimetric analysis was used to determine total water, and differential scanning calorimetry was used for free water. RESULTS In normal human lenses, the total water content of the nucleus remained unchanged with age, but the state of the water altered. The ratio of free to bound water increased steadily throughout adult life. In a 20-year-old person, there was approximately one bound water molecule for each free water molecule in the lens center, whereas in a 70- to 80-year-old person, there were two free water molecules for each bound water molecule. This conversion of bound to free water does not appear to be simply a consequence of the aggregation of soluble crystallins into high molecular weight aggregates because studies with intact pig lenses, in which such processes were facilitated by heat, did not show similar changes. The region of the lens in which the barrier to diffusion develops at middle age corresponds to a transition zone in which the protein concentration is intermediate between that of the cortex and the nucleus. In cataractous lenses, the free-to-bound water ratio was not significantly different from that of age-matched normal lenses; however, total water content in the center of advanced nuclear cataractous lenses was slightly lower than in normal lenses. CONCLUSIONS As the human lens ages, bound water is progressively changed to free water. Advanced nuclear cataract may be associated with lower total hydration of the lens nucleus.

Human prefrontal cortex phospholipids containing docosahexaenoic acid increase during normal adult aging, whereas those containing arachidonic acid decrease

Membrane phospholipids make up a substantial portion of the human brain, and changes in their amount and composition are thought to play a role in the pathogenesis of age-related neurodegenerative disease. Nevertheless, little is known about the changes that phospholipids undergo during normal adult aging. This study examined changes in phospholipid composition in the mitochondrial and microsomal membranes of human dorsolateral prefrontal cortex over the adult life span. The largest age-related changes were an increase in the abundance of both mitochondrial and microsomal phosphatidylserine 18:0_22:6 by approximately one-third from age 20 to 100 years and a 25% decrease in mitochondrial phosphatidylethanolamine 18:0_20:4. Generally, increases were seen with age in phospholipids containing docosahexaenoic acid across both membrane fractions, whereas phospholipids containing either arachidonic or adrenic acid decreased with age. These findings suggest a gradual change in membrane lipid composition over the adult life span.

Molecular signatures of long‐lived proteins: autolytic cleavage adjacent to serine residues

The centre of the human lens, which is composed of proteins that were synthesized prior to birth, is an ideal model for the evaluation of long‐term protein stability and processes responsible for the degradation of macromolecules. By analysing the sequences of peptides present in human lens nuclei, characteristic features of intrinsic protein instability were determined. Prominent was the cleavage on the N‐terminal side of serine residues. Despite accounting for just 9% of the amino acid composition of crystallins, peptides with N‐terminal Ser represented one‐quarter of all peptides. Nonenzymatic cleavage at Ser could be reproduced by incubating peptides at elevated temperatures. Serine residues may thus represent susceptible sites for autolysis in polypeptides exposed to physiological conditions over a period of years. Once these sites are cleaved, other chemical processes result in progressive removal or ‘laddering’ of amino acid residues from newly exposed N‐ and C‐termini. As N‐terminal Ser peptides originated from several crystallins with unrelated sequences, this may represent a general feature of long‐lived proteins.

Is protein methylation in the human lens a result of non-enzymatic methylation by S-adenosylmethionine?

Since crystallins in the human lens do not turnover, they are susceptible to modification by reactive molecules over time. Methylation is a major post-translational lens modification, however the source of the methyl group is not known and the extent of modification across all crystallins has yet to be determined. Sites of methylation in human lens proteins were determined using HPLC/mass spectrometry following digestion with trypsin. The overall extent of protein methylation increased with age, and there was little difference in the extent of modification between soluble and insoluble crystallins. Several different cysteine and histidine residues in crystallins from adult lenses were found to be methylated with one cysteine (Cys 110 in γD crystallin) at a level approaching 70%, however, methylation of crystallins was not detected in fetal or newborn lenses. S-adenosylmethionine (SAM) was quantified at significant (10-50 μM) levels in lenses, and in model experiments SAM reacted readily with N-α-tBoc-cysteine and N-α-tBoc-histidine, as well as βA3-crystallin. The pattern of lens protein methylation seen in the human lens was consistent with non-enzymatic alkylation. The in vitro data shows that SAM can act directly to methylate lens proteins and SAM was present in significant concentrations in human lens. Thus, non-enzymatic methylation of crystallins by SAM offers a possible explanation for this major human lens modification.

Old Proteins in Man: A Field in its Infancy

It has only recently been appreciated that the human body contains many long-lived proteins (LLPs). Their gradual degradation over time contributes to human aging and probably also to a range of age-related disorders. Indeed, the role of progressive damage of proteins in aging may be indicated by the fact that many neurological diseases do not appear until after middle age. A major factor responsible for the deterioration of old proteins is the spontaneous breakdown of susceptible amino acid residues resulting in racemization, truncation, deamidation, and crosslinking. When proteins decompose in this way, their structures and functions may be altered and novel epitopes can be formed that can induce an autoimmune response.

Degradation of an Old Human Protein AGE-DEPENDENT CLEAVAGE OF γS-CRYSTALLIN GENERATES A PEPTIDE THAT BINDS TO CELL MEMBRANES

Background: Long-lived proteins degrade over time. Results: Lens γS-crystallin is extensively modified with age, and truncation near the C terminus generates a peptide that binds tightly to cell membranes. Conclusion: The peptide released from γS-crystallin can adopt an α-helical conformation and may alter permeability by interacting with cell membranes. Significance: Peptide binding may explain why human lens membranes change with age. 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.

Large-Scale Binding of α-Crystallin to Cell Membranes of Aged Normal Human Lenses: A Phenomenon That Can Be Induced by Mild Thermal Stress

PURPOSE With age, large amounts of crystallins become associated with fiber cell membranes in the human lens nucleus, and it has been proposed that this binding of protein may lead to the obstruction of membrane pores and the onset of a barrier to diffusion. This study focused on membrane binding within the barrier region and the outermost lens cortex. METHODS Human lenses across the age range were used, and the interaction of crystallins with membranes was examined using sucrose density gradient centrifugation, two-dimensional gel electrophoresis, and amine-reactive isobaric tagging technology. Lipids were quantified using shotgun lipidemics. RESULTS Binding of proteins to cell membranes in the barrier region was found to be different from that in the lens nucleus because in the barrier and outer cortical regions, only one high-density band formed. Most of the membrane-associated protein in this high-density band was α-crystallin. Mild thermal stress of intact young lenses led to pronounced membrane binding of proteins and yielded a sucrose density pattern in all lens regions that appeared to be identical with that from older lenses. CONCLUSIONS α-Crystallin is the major protein that binds to cell membranes in the barrier region of lenses after middle age. Exposure of young human lenses to mild thermal stress results in large-scale binding of α-crystallin to cell membranes. The density gradient profiles of such heated lenses appear to be indistinguishable from those of older normal lenses. The data support the hypothesis that temperature may be a factor responsible for age-related changes to the human lens.