Senescence and the accumulation of abnormal proteins

Mass spectrometry-based multi-attribute method for mutation analysis in the early development of therapeutic proteins

The early intervention is essential, and later development cannot compensate for this initial generation of an antibody drug. Especially for sequence variants (SVs), should cause concern during the early bioprocess development. The advancement of bioprocess development is paralleled by development of state-of-the-art analytical methods that will provide further information. In the present study, a mass spectrometry (MS)-based multi-attribute method (MAM) was used to simultaneously monitor the SVs and other quality attributes in the early bioprocess development of ofatumumab, and a sequence variant (SV) was detected by a subunit-based MAM. Subsequently, the variant was further identified by MS/MS and confirmed by adding a synthetic peptide. Furthermore, the content of the SV was detected via DNA sequencing. The levels of the variant (T175A mutant) in the light chain were demonstrate to be nearly consistent at the DNA and protein levels, suggesting that the mutation may have negligible effect on both the transcriptional and translational levels. Collectively, these results indicate that broad-spectrum, rapid, and accurate platform such as MS-based MAM should be implemented to quality control for the early development of therapeutic proteins, it will also be important to establish an effective and integrated MAM to control SVs during therapeutic proteins development.

Random errors in protein synthesis activate an age-dependent program of muscle atrophy in mice

Random errors in protein synthesis are prevalent and ubiquitous, yet their effect on organismal health has remained enigmatic for over five decades. Here, we studied whether mice carrying the ribosomal ambiguity (ram) mutation Rps2-A226Y, recently shown to increase the inborn error rate of mammalian translation, if at all viable, present any specific, possibly aging-related, phenotype. We introduced Rps2-A226Y using a Cre/loxP strategy. Resulting transgenic mice were mosaic and showed a muscle-related phenotype with reduced grip strength. Analysis of gene expression in skeletal muscle using RNA-Seq revealed transcriptomic changes occurring in an age-dependent manner, involving an interplay of PGC1α, FOXO3, mTOR, and glucocorticoids as key signaling pathways, and finally resulting in activation of a muscle atrophy program. Our results highlight the relevance of translation accuracy, and show how disturbances thereof may contribute to age-related pathologies.

By introducing a ribosomal ambiguity mutation into mice, Moore et al. establish an in-vivo model to investigate how age-related diseases are related to decreasing accuracy in protein synthesis. Their findings potentially offer new insights into the pathological changes observed in age-related diseases, such as muscle atrophy

Structure, function, aging and turnover of aggrecan in the intervertebral disc

BACKGROUND

Aggrecan is the major non-collagenous component of the intervertebral disc. It is a large proteoglycan possessing numerous glycosaminoglycan chains and the ability to form aggregates in association with hyaluronan. Its abundance and unique molecular features provide the disc with its osmotic properties and ability to withstand compressive loads. Degradation and loss of aggrecan result in impairment of disc function and the onset of degeneration.

SCOPE OF REVIEW

This review summarizes current knowledge concerning the structure and function of aggrecan in the normal intervertebral disc and how and why these change in aging and degenerative disc disease. It also outlines how supplementation with aggrecan or a biomimetic may be of therapeutic value in treating the degenerate disc.

MAJOR CONCLUSIONS

Aggrecan abundance reaches a plateau in the early twenties, declining thereafter due to proteolysis, mainly by matrix metalloproteinases and aggrecanases, though degradation of hyaluronan and non-enzymic glycation may also participate. Aggrecan loss is an early event in disc degeneration, although it is a lengthy process as degradation products may accumulate in the disc for decades. The low turnover rate of the remaining aggrecan is an additional contributing factor, preventing protein renewal. It may be possible to retard the degenerative process by restoring the aggrecan content of the disc, or by supplementing with a bioimimetic possessing similar osmotic properties.

GENERAL SIGNIFICANCE

This review provides a basis for scientists and clinicians to understand and appreciate the central role of aggrecan in the function, degeneration and repair of the intervertebral disc.

Oxidative stress in aging: advances in proteomic approaches

Aging is a gradual, complex process in which cells, tissues, organs, and the whole organism itself deteriorate in a progressive and irreversible manner that, in the majority of cases, implies pathological conditions that affect the individual's Quality of Life (QOL). Although extensive research efforts in recent years have been made, the anticipation of aging and prophylactic or treatment strategies continue to experience major limitations. In this review, the focus is essentially on the compilation of the advances generated by cellular expression profile analysis through proteomics studies (two-dimensional [2D] electrophoresis and mass spectrometry [MS]), which are currently used as an integral approach to study the aging process. Additionally, the relevance of the oxidative stress factors is discussed. Emphasis is placed on postmitotic tissues, such as neuronal, muscular, and red blood cells, which appear to be those most frequently studied with respect to aging. Additionally, models for the study of aging are discussed in a number of organisms, such as Caenorhabditis elegans, senescence-accelerated probe-8 mice (SAMP8), naked mole-rat (Heterocephalus glaber), and the beagle canine. Proteomic studies in specific tissues and organisms have revealed the extensive involvement of reactive oxygen species (ROS) and oxidative stress in aging.

Age-related changes in the mitochondrial proteome of the fungus Podospora anserina analyzed by 2D-DIGE and LC-MS/MS

Many questions concerning the molecular processes during biological aging remain unanswered. Since mitochondria are central players in aging, we applied quantitative two-dimensional difference gel electrophoresis (2D-DIGE) coupled to protein identification by mass spectrometry to study the age-dependent changes in the mitochondrial proteome of the fungus Podospora anserina - a well-established aging model. 67 gel spots exhibited significant, but remarkably moderate intensity changes. While typically the observed changes in protein abundance occurred progressively with age, for several proteins a pronounced change was observed at late age, sometimes inverting the trend observed at younger age. The identified proteins were assigned to a wide range of metabolic pathways including several implicated previously in biological aging. An overall decrease for subunits of complexes I and V of oxidative phosphorylation was confirmed by Western blot analysis and blue-native electrophoresis. Changes in several groups of proteins suggested a general increase in protein biosynthesis possibly reflecting a compensatory mechanism for increased quality control-related protein degradation at later age. Age-related augmentation in abundance of proteins involved in biosynthesis, folding, and protein degradation pathways sustain these observations. Furthermore, a significant decrease of two enzymes involved in the degradation of γ-aminobutyrate (GABA) supported its previously suggested involvement in biological aging.

BIOLOGICAL SIGNIFICANCE

We have followed the time course of changes in protein abundance during aging of the fungus P. anserina. The observed moderate but significant changes provide insight into the molecular adaptations to biological aging and highlight the metabolic pathways involved, thereby offering new leads for future research.

The 2010 ESPEN Sir David Cuthbertson Lecture: new and old proteins: clinical implications

The past century had witnessed vast advances in biomedical research, particularly in the fields of genomics and proteomics, yet the translation of these discoveries into clinical practice has been hindered by gaps in mechanistic understanding of variability governing disease susceptibility and pathogenesis. Among the greatest challenges are the dynamic nature of the proteome and the imperfect methodologies currently available to study it. Here, we review key recently developed proteomic techniques that have allowed for dynamic characterization of protein quality, as well as quantity, and discuss their potential applications in understanding aging and metabolic disorders including diabetes. These methodologies revealed that senescence is characterized, in part, by decreased rates of de novo protein synthesis and potentially also degradation, in addition to concomitantly increased levels of oxidative stress, ultimately resulting in excessive accumulation of damaged and dysfunctional proteins. Insulin may be a key mediator in these pathologies, as hyperinsulinemia has been shown to hinder protein degradation while transient insulin deficiency may accelerate oxidative damage. We also discuss two interventions that have been proposed to delay, and possibly reverse, senescence by augmenting protein degradation: chronic caloric restriction and aerobic exercise.

Longevity of elastin in human intervertebral disc as probed by the racemization of aspartic acid

BACKGROUND

Aging and degeneration of human intervertebral disc (IVD) are associated with biochemical changes, including racemization and glycation. These changes can only be counteracted by protein turnover. Little is known about the longevity of IVD elastin in health or disease. Yet, such knowledge is important for a quantitative understanding of tissue synthesis and degradation.

METHODS

We have measured the accumulation of d-Asp and pentosidine in IVD elastin. Samples representing a broad range of ages (28-82years) and degeneration grades (1-5) were analyzed.

RESULTS

d/l-Asp for elastin increased linearly with age from 3.2% (early 30s) to 14.8% (early 80s) for normal tissue (grades 1-2) and from 1.7% (late 20s) to 6.0% (until the mid 50s) for degenerate tissue (grades 3-5) with accumulation rates of 16.2±3.1×10(-4) and 11.7±3.8×10(-4)year(-1), respectively; no significant difference was found between these values (p<0.05). Above the mid 50s, a decrease in d-Asp accumulation was recorded in the degenerate tissue. d-Asp accumulation correlated well with pentosidine content for elastin from healthy and degenerate tissues combined. We conclude that IVD elastin is metabolically-stable and long-lived in both healthy and degenerate human IVDs, with signs of new synthesis in the latter. The correlation of d-Asp with pentosidine content suggests that both these agents may be used as markers in the overall aging process of IVD.

GENERAL SIGNIFICANCE

Accumulation of modified IVD elastin argues for its longevity and may have a negative impact on its role in disc function. Weak signs of newly synthesized molecules may act to counteract this effect in degenerate tissue.

Cellular stress responses, hormetic phytochemicals and vitagenes in aging and longevity

Modulation of endogenous cellular defense mechanisms represents an innovative approach to therapeutic intervention in diseases causing chronic tissue damage, such as in neurodegeneration. This paper introduces the emerging role of exogenous molecules in hormetic-based neuroprotection and the mitochondrial redox signaling concept of hormesis and its applications to the field of neuroprotection and longevity. Maintenance of optimal long-term health conditions is accomplished by a complex network of longevity assurance processes that are controlled by vitagenes, a group of genes involved in preserving cellular homeostasis during stressful conditions. Vitagenes encode for heat shock proteins (Hsp) Hsp32, Hsp70, the thioredoxin and the sirtuin protein systems. Dietary antioxidants, such as polyphenols and L-carnitine/acetyl-L-carnitine, have recently been demonstrated to be neuroprotective through the activation of hormetic pathways, including vitagenes. Hormesis provides the central underpinning of neuroprotective responses, providing a framework for explaining the common quantitative features of their dose response relationships, their mechanistic foundations, their relationship to the concept of biological plasticity as well as providing a key insight for improving the accuracy of the therapeutic dose of pharmaceutical agents within the highly heterogeneous human population. This paper describes in mechanistic detail how hormetic dose responses are mediated for endogenous cellular defense pathways including sirtuin, Nrfs and related pathways that integrate adaptive stress responses in the prevention of neurodegenerative diseases. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.

Functional interplay between mitochondrial and proteasome activity in skin aging

According to the mitochondrial theory of aging, reactive oxygen species (ROS) derived primarily from mitochondria cause cumulative oxidative damage to various cellular molecules and thereby contribute to the aging process. On the other hand, a pivotal role of the proteasome, as a main proteolytic system implicated in the degradation of oxidized proteins during aging, is suggested. In this study, we analyzed mitochondrial function in dermal fibroblasts derived from biopsies obtained from healthy young, middle-aged, and old donors. We also determined proteasome activity in these cells, using a degron-destabilized green fluorescent protein (GFP)-based reporter protein. We found a significant decrease in mitochondrial membrane potential in samples from aged donors, accompanied by a significant increase in ROS levels. Respiratory activity was not significantly altered with donor age, probably reflecting genetic variation. Proteasome activity was significantly decreased in fibroblasts from middle-aged donors compared with young donors; fibroblasts derived from the oldest donors displayed a high heterogeneity in this assay. We also found intraindividual coregulation of mitochondrial and proteasomal activities in all human fibroblast strains tested, suggesting that both systems are interdependent. Accordingly, pharmacological inhibition of the proteasome led to decreased mitochondrial function, whereas inhibition of mitochondrial function in turn reduced proteasome activity.

Discovery and investigation of misincorporation of serine at asparagine positions in recombinant proteins expressed in Chinese hamster ovary cells

Misincorporation of amino acids in proteins expressed in Escherichia coli has been well documented but not in proteins expressed in mammalian cells under normal recombinant protein production conditions. Here we report for the first time that Ser can be incorporated at Asn positions in proteins expressed in Chinese hamster ovary cells. This misincorporation was discovered as a result of intact mass measurement, peptide mapping analysis, and tandem mass spectroscopy sequencing. Our analyses showed that the substitution was not related to specific protein molecules or DNA codons and was not site-specific. We believe that the incorporation of Ser at sites coded for Asn was due to mischarging of tRNA(Asn) rather than to codon misreading. The rationale for substitution of Asn by Ser and not by other amino acids is also discussed. Further investigation indicated that the substitution was due to the starvation for Asn in the cell culture medium and that the substitution could be limited by using the Asn-rich feed. These observations demonstrate that the quality of expressed proteins should be closely monitored when altering cell culture conditions.

On methionine restriction, suppression of mitochondrial dysfunction and aging

Rats and mice, when subjected to methionine restriction (MetR), may live longer with beneficial changes to their mitochondria. Most explanations of these observations have centered on MetR somehow suppressing the effects of oxygen free radicals. It is suggested here that MetR's effects on protein metabolism should also be considered when attempting to explain its apparent anti-aging actions. Methionine is the initiating amino acid in mRNA translation. It is proposed that MetR decreases the protein biosynthesis rate due to methionine limitation, which correspondingly decreases generation of ribosomal-mediated error proteins, which then lowers the total abnormal protein load that cellular proteases and chaperone proteins (mitochondrial and cytoplasmic) must deal with. This will increase protease availability for elimination of proteins damaged postsynthetically and help delay abnormal protein accumulation, the major molecular symptom of aging. The slowed rate of protein synthesis may also alter protein folding, which could also alter polypeptide susceptibility to oxidative attack. MetR will also increase lysosomal proteolysis, including autophagy of dysfunctional mitochondria, and promote mitogenesis. MetR may decrease synthesis of S-adenosyl-methionine (SAM), which could decrease spontaneous O(6)-methylguanine formation in DNA. However decreased SAM may compromise repair of protein isoaspartate residues by protein-isoaspartate methyltransferase (PIMT). Changes in SAM levels may also affect gene silencing. All the above may help explain, at least in part, the beneficial effects of MetR.

Understanding ageing from an evolutionary perspective

There is clear heritability of human longevity. However, the genetics of ageing is likely to be complex. Evolution theory tells us not to expect genes that have been selected to promote ageing. Ageing is not programmed but results from accumulation of somatic damage, owing to limited investments in maintenance and repair. Genes controlling the levels of activities, such as DNA repair and antioxidant defence, thus regulate longevity. In addition, there may be contributions either from late-acting deleterious genes that escape the force of natural selection or that trade benefit at an early age against harm at older ages. In some species, there is evidence that genes have evolved to detect and respond to changes in the environment, e.g. food supply. Evolutionary understanding can also help to understand important features of the human life history such as menopause.

On why decreasing protein synthesis can increase lifespan

An explanation is offered for the increased lifespan of Caenorhabditis elegans when mRNA translation is inhibited due to loss of the initiation factor IFE-2 [Hansen, M., Taubert, T., Crawford, D., Libina, N., Lee, S.-J., Kenyon, C., 2007. Lifespan extension by conditions that inhibit translation in Caenorhabditis elegans. Ageing Cell 6, 95-110; Pan, K.Z., Palter, J.E., Rogers, A.N., Olsen, A., Chen, D., Lithgow, G.J., Kapahi, P., 2007. Inhibition of mRNA translation extends lifespan in Caenorhabditis elegans. Ageing Cell 6, 111-119; Syntichaki, P., Troulinaki, K., Tavernarakis, N., 2007. eIF4E function in somatic cells modulates ageing in Caenorhabditis elegans. Nature 445, 922-926]. It is suggested that the general reduction of protein synthesis, due to the decreased frequency of mRNA translation, also lowers the cellular load of erroneously synthesized polypeptides which the constitutive protein homeostatic apparatus (proteases and chaperones proteins) normally eliminates. This situation results in "spare" proteolytic and chaperone function which can then deal with those proteins modified post-synthetically, e.g. by oxidation and/or glycation, which are thought to contribute to the senescent phenotype. This increased availability of proteolytic and chaperone functions may thereby contribute to the observed increase in organism stress resistance and lifespan.

Pathomolecular effects of homocysteine on the aging process: A new theory of aging

Homocysteine has been associated with the most common age-related diseases but never associated with the acceleration of the aging process. This theoretical paper will try to demonstrate the pro-aging effects of homocysteine at the molecular, cellular, and organ level. High homocysteine levels in homocystinuria are associated with premature disease of the cardiovascular, skeletal, neurological, and other systems. These observations are similar to those noted in the aging process and should be considered as a progeroid syndrome. There is enough scientific evidence to support that homocysteine accelerates the aging process at the cellular and at the organism level. Most importantly, decreasing homocysteine levels by dietary or pharmacological interventions could prolong maximum life span in humans and/or delay the onset of the most common age-related diseases.

The involvement of macroautophagy in aging and anti-aging interventions

Macroautophagy is a process that sequesters and degrades organelles and macromolecular constituents of cytoplasm for cellular restructuring and repair, and as a source of nutrients for metabolic use in early starvation. Extensive evidence has been reported that macroautophagy process declines with increasing age. This impairment, probably due to ad libitum feeding, may cause accumulation of altered structures leading to the age-related decline in cell functions. It has been suggested that caloric restriction (CR) and disruption of insulin-like signals contrast the process of aging by prolonged stimulation of macroautophagy. According to this hypothesis, it is shown that life-long weekly administration of an anti-lipolytic drug decreases glucose and insulin levels, stimulates autophagy and intensifies anti-aging effects of submaximal CR.

What accounts for the wide variation in life span of genetically identical organisms reared in a constant environment?

Individual organisms show marked variability in life span, even when they are of the same genotype and are raised in a common environment protected from extrinsic hazards. This intrinsic variability of life span is thought to arise from the stochastic nature of the cellular and molecular mechanisms controlling development and ageing. In this article we review what is currently understood about the factors underlying the variability of life span and consider the implications for research that aims to improve the predictability of health in old age.

Calcium-dependent and aspartyl proteases in neurodegeneration and ageing in C. elegans

Proteolytic mechanisms have been implicated in the process of ageing, and in many neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's diseases, which are most prevalent in old age. Simple model organisms such as the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster, which offer the prowess of sophisticated genetic approaches, have contributed to our understanding of ageing and neurodegeneration. Intensive research in these systems has resulted in detailed models of the ageing process, and also of several neurodegenerative diseases, which recapitulate same aspects of the human pathologies. Inappropriate cell death is a major component of these and other devastating conditions such as stroke. The dissection of the molecular mechanisms underlying the process of cell degeneration in ageing is of utmost importance. Evidence from investigations in C. elegans implicates deregulated proteolysis as one major determinant of cellular destruction in neurodegeneration and ageing, and suggests that the process depends critically on the activation of calcium-dependent, calpain proteases and lysosomal aspartyl proteases. Apart from shedding light on important but inadequately understood facets of such phenomena, these discoveries hold promise for developing novel, effective intervention strategies aiming to ameliorate or even counter inappropriate cell death.

Protein turnover plays a key role in aging

Although the molecular mechanism of aging is unknown, a progressive increase with age in the concentration of damaged macromolecules, especially proteins, is likely to play a central role in senescent decline. In this paper, we discuss evidence that the progressive decrease in protein synthesis and turnover can be the primary cause of the increase in the concentration of damaged proteins with age. Conversely, protein damage itself is likely to be the cause of the decrease in protein turnover. This could establish a positive feedback loop where the increase in protein damage decreases the protein turnover rate, leading to a further increase in the concentration of damaged proteins. The establishment of such a feedback loop should result in an exponential increase in the amount of protein damage-a protein damage catastrophe-that could be the basis of the general deterioration observed in senescent organisms.

Caloric restriction and lifespan: a role for protein turnover?

Oxidative damage to cellular macromolecules has been postulated to be a major contributor to the ageing of diverse organisms. Oxidative damage can be limited by maintaining high anti-oxidant defenses and by clearing/repairing damage efficiently. Protein turnover is one of the main routes by which functional proteins are maintained and damaged proteins are removed. Protein turnover rates decline with age, which might contribute to the accumulation of damaged proteins in ageing cells. Interestingly, protein turnover rates are maintained at high levels in caloric restricted animals. Whether changes in protein turnover are a cause or a consequence of ageing is not clear, and this question has not been a focal point of modern ageing research. Here we survey work on protein turnover and ageing and suggest that powerful genetic models such as the nematode Caenorhabditis elegans are well suited for a thorough investigation of this long-standing question.

Nutrition, oxidative damage, telomere shortening, and cellular senescence: individual or connected agents of aging?

There is substantial and long-standing literature linking the level of general nutrition to longevity. Reducing nutrition below the amount needed to sustain maximum growth increases longevity in a wide range of organisms. Oxidative damage has been shown to be a major feature of the aging process. Telomere shortening is now well established as a key process regulating cell senescence in vitro. There is some evidence that the same process may be important for aging in vivo. Very recently it has been found that oxidative damage accelerates telomere shortening. It is therefore possible for us to propose as an outline hypothesis that the level of nutrition determines oxidative damage which in turn determines telomere shortening and cell senescence and that this pathway is important in determining aging and longevity in vivo. We also propose that telomeres in addition to their well-recognized role in "counting" cell divisions are also, through their GGG sequence, important monitors of oxidative damage over the life span of a cell. This may explain the evolutionary conservations of this triplet in the repeat telomere sequence unit.

Molecular gerontology. Bridging the simple and the complex

It is clear, both empirically and theoretically, that the mechanisms of aging are multiple and complex. Nevertheless, single gene mutations and simple interventions such as calorie restriction have broad effects on the senescent phenotype. The major challenge is to unite highly reductionist analysis of molecular components with integrative model systems that can "put it all together." Two themes are developed. In the first, biochemical models are described that show how the network concept of cellular aging can be used to integrate multiple biochemical mechanisms that contribute to cellular instability. In the second theme, the role of intrinsic developmental chance is examined as a major factor contributing, in addition to genes and environment, to the divergence of the senescent phenotype. The implications of these themes for research strategies in molecular gerontology are discussed.

Aging and longevity. A paradigm of complementation between homeostatic mechanisms and genetic control?

Aging is a universal and inevitable phenomenon that affects nearly all animal species. It can be considered the product of an interaction between genetic, environmental, and lifestyle factors, which in turn influence longevity that varies between and within species. It has been proposed not only that the aging process is under genetic control, but that it can also be considered a result of the failure of homeostasis due to the accumulation of damage. This review article discusses these issues, focusing on the function of genes that associate with aging and longevity, as well as on the molecular mechanisms that control cell survival and maintenance during aging.

Protein oxidation in response to increased transcriptional or translational errors

In this study, we show a correlation between synthesis of aberrant proteins and their oxidative modification. The level of aberrant proteins was elevated in Escherichia coli cultures by decreasing transcriptional or translational fidelity using specific mutations or drugs. Protein carbonylation, an oxidative modification, increased in parallel to the induction of the heat shock chaperone GroEL. As the protein turnover rates and level of intracellular oxidative stress remained unchanged, it appears that carbonylation results from the increased susceptibility of the misfolded proteins. These studies show that the cellular protein oxidation is not limited only by available reactive oxygen species, but by the levels of aberrant proteins. Thus, protein oxidation seen in aging cells may be the consequence also of reduced transcriptional/translational fidelity, and protein structures appear to have evolved to minimize oxidative damage. In addition, we discuss the possibility that carbonylation, being an unrepairable protein modification, may serve as a tagging system to shunt misfolded proteins between pathways of refolding by chaperones or the proteolytic apparatus.

Carnosine reacts with a glycated protein

Oxidation and glycation induce formation of carbonyl (CO) groups in proteins, a characteristic of cellular aging. The dipeptide carnosine (beta-alanyl-L-histidine) is often found in long-lived mammalian tissues at relatively high concentrations (up to 20 mM). Previous studies show that carnosine reacts with low-molecular-weight aldehydes and ketones. We examine here the ability of carnosine to react with ovalbumin CO groups generated by treatment of the protein with methylglyoxal (MG). Incubation of MG-treated protein with carnosine accelerated a slow decline in CO groups as measured by dinitrophenylhydrazine reactivity. Incubation of [(14)C]-carnosine with MG-treated ovalbumin resulted in a radiolabeled precipitate on addition of trichloroacetic acid (TCA); this was not observed with control, untreated protein. The presence of lysine or N-(alpha)-acetylglycyl-lysine methyl ester caused a decrease in the TCA-precipitable radiolabel. Carnosine also inhibited cross-linking of the MG-treated ovalbumin to lysine and normal, untreated alpha-crystallin. We conclude that carnosine can react with protein CO groups (termed "carnosinylation") and thereby modulate their deleterious interaction with other polypeptides. It is proposed that, should similar reactions occur intracellularly, then carnosine's known "anti-aging" actions might, at least partially, be explained by the dipeptide facilitating the inactivation/removal of deleterious proteins bearing carbonyl groups.

Ovarian ageing and the general biology of senescence

Ovarian ageing is not only of major importance in its own right but is also of interest for its relationship with the general biology of senescence. A key feature of ageing is the distinction in higher animals between the immortality of the germ-line and the mortality of somatic cells and tissues. The ovary contains the female germ cells, and it is through these cells that the female contribution to germ-line immortality is effected. It is abundantly clear that individual oocytes can and do age and that the ageing of the ovary plays a major role in initiating or accelerating a series of other senescent changes. To understand how ovarian ageing fits within the general biology of senescence, it is necessary to explain why ageing occurs at all, to examine the likely mechanisms of general ageing, and to ask whether there is anything special about ovarian ageing and its relationship with the human menopause. Research on ovarian ageing interacts with the our emerging understanding of the general biology of senescence at many levels, ranging from the evolution of the human life history to the biochemical and cellular mechanisms of ageing and longevity.

Aging, calorie restriction and ubiquitin-dependent proteolysis in the livers of Emory mice

Calorie restriction (R), the only known method to delay the aging process and extend mean and maximal lifespan, has been shown to delay the age-related decline in protein degradation. There are several proteolytic pathways. The ubiquitin- and ATP-dependent proteolytic pathway (UPP) is frequently associated with degradation of damaged abnormal and/or regulatory proteins. We examined the effect of aging and R on supernatants of livers taken from young (4.5 months) and old (23 months) Emory mice. Aging was associated with increased levels of endogenous ubiquitin conjugates, enhanced ability to form high molecular weight conjugates and ubiquitin activating (E1) and ubiquitin conjugating (E2) activity in the control (C) liver supernatants. The age-related increase in levels of endogenous ubiquitin conjugates in liver appears to be primarily due to increased E1 and E2 activities. R prevented the age-related increase in E1 and E2 activity, and thus prevented the age-related increase in levels of ubiquitin conjugates. In spite of the age-related increase in ubiquitin conjugates, no age-related changes in ubiquitin-dependent proteolytic pathway were observed in the C animals. R was associated with an enhanced ability (130%) to degrade beta-lactoglobulin by the ubiquitin-dependent proteolytic pathway in livers from 4.5-month-old animals relative to age-matched C livers. However, rates of the ubiquitin-dependent degradation of beta-lactoglobulin in the 23-month-old C and R animals were indistinguishable. There were no age- or diet-related differences in the ability to degrade another substrate, oxidized ribonuclease (RNase).

The origins of human ageing

The origins of human ageing are to be found in the origins and evolution of senescence as a general feature in the life histories of higher animals. Ageing is an intriguing problem in evolutionary biology because a trait that limits the duration of life, including the fertile period, has a negative impact on Darwinian fitness. Current theory suggests that senescence occurs because the force of natural selection declines with age and because longevity is only acquired at some metabolic cost. In effect, organisms may trade late survival for enhanced reproductive investments in earlier life. The comparative study of ageing supports the general evolutionary theory and reveals that human senescence, while broadly similar to senescence in other mammalian species, has distinct features, such as menopause, that may derive from the interplay of biological and social evolution.

What is the relationship between osteoarthritis and ageing?

The relationship between osteoarthritis and ageing raises important questions about what exactly defines 'normal' ageing and whether the pathogenesis of osteoarthritis shares common pathways with other age-associated dysfunctions, or whether osteoarthritis is a time-dependent disorder distinct from normal ageing with a separate causative mechanism at work. Theories of ageing now emphasize the stochastic nature of the ageing process, that is the role played by accumulation of essentially random cell and tissue damage, such as somatic mutations, oxidative damage and the formation of aberrant proteins. The role of genetic factors in determining longevity and predisposition to age-associated diseases is probably in programming the efficiency of somatic maintenance functions and in influencing the development of a durable soma. Gene-environment interactions, for example through lifestyle, can also be important. Many of the risk factors and mechanisms that are thought to contribute to osteoarthritis can be accommodated within this framework.

Network theory of aging

Evolution theory indicates that investment in mechanisms of somatic maintenance and repair is likely to be limited, suggesting that aging may result from the accumulation of unrepaired somatic defects. An important corollary of this hypothesis is that multiple mechanisms of aging operate in parallel. We describe a recently developed "network theory of aging" that integrates the contributions of defective mitochondria, aberrant proteins, and free radicals in the aging process and that includes the protective effects of antioxidant enzymes and proteolytic scavengers. Possibilities for further extension of the theory and its role in prediction and simulation of experimental results are discussed.

Demonstration of cellular aging and senescence in serially passaged long-term cultures of human trabecular osteoblasts

The proliferative capacity and cellular and biochemical characteristics of human trabecular bone osteoblasts were analysed throughout their replicative lifespan in vitro. Like several other cell types, human osteoblasts demonstrated a typical Hayflick phenomenon of cellular aging comprising a period of rapid proliferation until cumulative population doubling level (CPDL) 22 to 24, followed by a phase of slow growth and the final cessation of cell division at CPDL 32 to 34. Comparing young cells (less than 20% lifespan completed) and old cells (more than 90% lifespan completed) revealed a progressive increase in population doubling (PD) time, a decrease in attachment frequency, a decrease in the number of S-phase positive cells, a decrease in the rates of DNA, RNA and protein synthesis, an increase in the protein content per cell and an increased proportion of senescence-specific beta-galactosidase positive cells. While osteoblastic production of collagen type I decreased progressively during aging, alkaline phosphatase activity dropped rapidly after the first few passages and then remained constant during the rest of the proliferative lifespan, Significant morphological changes from thin and spindle-shaped early passage young cells to large, flattened and irregularly shaped late passage old cells full of intracellular debris were observed. In comparison, osteoblasts established from an osteoporotic bone sample showed a maximum CPDL of less than 5, had a longer PD time and exhibited abnormal senescent morphology. Thus, we have demonstrated for the first time that human osteoblasts, like several other diploid cell types, have a limited proliferative capacity in vitro and undergo aging and senescence as measured by various cellular and biochemical markers. In addition, preliminary studies show that cells from osteoporotic bone have a severely reduced proliferative capacity. This model of bone cell aging facilitates study of the molecular mechanisms of osteoblast senescence as well as factors related to osteoblast dysfunction in patients with osteoporosis.

In vitro study of gingival fibroblasts from normal and inflamed tissue: age-related responsiveness

The aim of this study was to characterize some phenotypic expressions of fibroblasts from the human oral mucosa. Gingival and lower forearm fibroblasts from young (20-30 years) and elderly (> 60 years) subjects were analyzed. Gingival fibroblasts were taken from donors with (P) and without (NP) periodontal disease, while skin biopsies were taken from healthy subjects. Cell proliferation was assessed by evaluating the cell multiplication coefficient (C.M.C.). The proliferation potential of gingival fibroblasts from elderly individuals with and without periodontopathy did not differ from that of young subjects in the same condition but differed significantly in the skin samples. Enzyme neutral endopeptidase (EC 3.4.24.11) (NEP) activity, studied as a possible marker of cell ageing, showed an age-related increase in human skin fibroblasts but not consistently in gingival fibroblasts from individuals with or without periodontal disease. Cell area and substrate adhesion were evaluated by morphometric analysis. There were no significant differences between elderly P and NP subjects, while significant differences were observed between young and elderly P subjects. In conclusion, proliferative capacity and NEP activity in gingival fibroblasts did not appear to be age-related, probably because their microenvironment is continually moistened by saliva, which continues to contain growth factors, notably EGF, even into senescence. Tissue reaction and repair are important clinical and therapeutic implications.

The initiation of senescence and its relationship to embryonic cell differentiation

Mouse embryonic stem cells have an unlimited lifespan in cultures if they are prevented from differentiating. After differentiating, they produce cells which divide only a limited number of times. These changes seen in cultures parallel events that occur in the developing embryo, where immortal embryonic cells differentiate and produce mortal somatic ones. The data strongly suggest that differentiation initiates senescence, but this view entails additional assumptions in order to explain how the highly differentiated sexual gametes manage to remain potentially immortal. Cells differentiate by blocking expression from large parts of their genome and it is suggested that losses or gains of genetic totipotency determine cellular lifespans. Cells destined to be somatic do not regain totipotency and senesce, while germ-line cells regain complete genome expression and immortality after meiosis and gamete fusions. Losses of genetic totipotency could induce senescence by lowering the levels of repair and maintenance enzymes.

A re-examination of the effects of ionizing radiation on lifespan and transformation of human diploid fibroblasts

Human diploid fibroblasts, strain MRC-5, were sequentially irradiated with 60Co gamma rays at intervals during their in vitro lifespan. The results indicate that 3 or 6 doses of 1 Gy can increase lifespan, and the same was true for cells treated with 3 doses of 3 Gy. Higher doses (5 x 3 Gy) did reduce growth potential, suggesting either that mid-late passage cells become more sensitive to radiation, or that doses beyond a given threshold reduce population lifespan by multiple cellular hits. The life extension induced by gamma rays might be due to an induced hypermethylation of DNA. Alternatively, oxygen radicals produced by irradiation might trigger an adaptive stress response which would remove damaged macromolecules and thereby increase the cells' growth potential. Whichever explanation is correct, the results show that the human fibroblast system is not appropriate for the study of the well known effect of ionizing radiation in shortening the lifespan of experimental animals. Contrary to earlier published results, populations of cells treated with cumulative doses of 15 Gy or 18 Gy and held for nearly 3 months after they had reached senescence (Phase III), produced no foci of transformed cells.