Repetition of molecular-genetic information as a possible factor in evolutionary changes of life span

Ageing: definitions, mechanisms and the magnitude of the problem

All multi-cellular organisms undergo change with time. Conception heralds the onset of growth and development, leading to reproductive competence and propagation of the species. With time, organisms age, leading to death as a final end-point. Whilst our knowledge and definitions of growth and reproduction are firmly established, the concept of ageing remains less well understood. One of the reasons for the lack of a singular definition of ageing is that it can be considered in many different ways, according to social, behavioural, physiological, morphological, cellular and molecular changes. Research has led to a number of theories being proposed that may explain the ageing process. In this chapter, we will review some of these theories and address some of the following fundamental questions: What is ageing? How can ageing be measured? When does ageing begin? When is an organism defined as old?

How and why we age

After performing the miracles that takes us from conception to birth, and then to sexual maturation and adulthood, natural selection was unable to favor the development of a more elementary mechanism that would simply maintain those earlier miracles forever. The manifestations of this failure are called aging. Because few feral animals age, evolution could not have favored a genetic program for age changes. Natural selection favors animals that are most likely to become reproductively successful by developing better survival strategies and greater reserve capacity in vital systems to better escape predation, disease, accidents, and environmental extremes. Natural selection diminishes after reproductive success because the species will not benefit from members favored for greater longevity. The level of physiological reserve remaining after reproductive maturity determines longevity and evolves incidental to the selection process that acts on earlier developmental events. Physiological reserve does not renew at the same rate that it incurs losses because molecular disorder increases at a rate greater than the capacity for repair. These are age changes, and they increase vulnerability to predation, accidents, or disease. Failure to distinguish aging from disease has not only blurred our efforts to understand the fundamental biology of aging, but it has profound political and economic consequences that compromise the field of biogerontology. Changes attributable to disease, or pathological change, can be distinguished from age changes for at least four important reasons. Unlike any known disease, (1) age changes occur in every human given sufficient time, (2) age changes cross virtually all species barriers, (3) no disease afflicts all members of a species only after the age of reproductive success, and (4) aging occurs in all feral animals subsequently protected by humans, even when that species probably has not experienced aging for thousands or millions of years. The resolution of age-associated diseases will not advance our knowledge of aging, just as the resolution of the diseases of childhood did not advance our knowledge of childhood development. We have failed to convey that greater support must be given to a question that is rarely posed. It is a question that is applicable to all age-associated diseases, and its resolution will also advance our fundamental knowledge of aging: "Why are old cells more vulnerable to pathology and disease than are young cells?" During the first half of this century it was believed that because cultured normal cells were immortal, aging must be caused by extra-cellular events. Thirty-five years ago we overthrough this dogma when we found that normal cells do have a limited capacity to divide, and that age changes can occur intracellularly. We also observed that only abnormal or cancer cells are immortal. Normal cells are mortal because telomeres shorten at each division. Immortal cancer cells express the enzyme telomerase that prevents shortening. Recently, it was discovered that when the catalytic subunit of the telomerase gene is inserted into normal cells they become immortal.

Aging and mortality: manifestations of increasing informational entropy of the genome?

Information theory has provided insight to the means by which organisms maintain life against the inevitable influence of the second law of thermodynamics. The low entropy content of genetic information is used to harness the energy that an organism in an open system must utilize to maintain its low entropy and life. Redundancy lowers informational entropy, enhances meaning and promotes complexity. The genome is characterized by large quantities of 'redundant' DNA. Longitudinal Gompertzian analysis has shown that 'the rate of loss of vitality' (from the Strehler-Mildvan modification of the Gompertz model of aging and mortality) has remained constant for general and disease-specific mortality. Applying the concepts of information theory to aging and mortality suggests that 'the rate of loss of vitality' may be synonymous with the rate of degradation of redundant genetic information (or alternatively, the rate of increase in informational entropy of the genome). This interpretation is consistent with the hypothesis that genetic instability of repeated DNA sequences may be fundamentally related to the process of aging. Consequently, there may be no specific aging or 'death' genes per se, but rather aging and mortality are the inevitable natural consequence of increasing informational entropy (decreasing redundancy) contained within the genome.

Age-related decreases in muscarinic cholinergic receptor binding in the human brain measured with positron emission tomography (PET)

Muscarinic cholinergic M1 and M2 receptors in young and aged adult male volunteers were studied using [N-11C-methyl]-benztropine, a specific muscarinic cholinergic receptor ligand, and high resolution positron emission tomography (PET). A regionally specific pattern of decreased binding was observed in aged volunteers. Using two separate methods of data analysis, thalamic, hippocampal and cerebellar regions showed no decreases in the apparent specific binding of [N-11C-methyl]-benztropine while frontal, parietal, temporal and occipital cortices as well as the corpus striatum showed age related changes in binding that declined (in 82 yrs old subject) to about 50% of the value obtained from the youngest volunteer (19 yrs). These data suggest that regions high in muscarinic receptor density, the corpus striatum and the cortical mantle, show a greater rate of decline than those areas that have a relatively low number of muscarinic receptors. Furthermore, this study demonstrates the usefulness of PET and [N-11C-methyl]-benztropine for assessing age related regional changes in muscarinic cholinergic receptor binding in the living human brain.

Age related variations of hepatocarcinogenic effect of azo dye (3'-MDAB) as linked to the level of hepatocyte polyploidization

Four-week-old CBA mice fed a diet containing the hepatocarcinogenic azo dye 3'-MDAB showed a rapid polyploidization of hepatocytes, a sharp increase of two liver-specific acid soluble non-histone proteins (LSP 1 and 2) and induction of hepatomas between 44 and 52 weeks of the regimen. More mature 18-week-old mice fed the same diet did not develop induced hepatocarcinogenesis after 55 weeks of the regimen. Interruption of the azo dye regimen showed that the increase of LSP 1 and 2 was reversible, whereas the carcinogenic effect and polyploidization were irreversible. Sprague-Dawley rats were more sensitive to the carcinogenic effect of the azo dye regimen. It is suggested that the higher resistance of older mice to the carcinogenic effect could be linked to the higher level of hepatocyte polyploidization and that the increase of LSP 1 and 2 is relevant to the toxic effect of the azo dye.

Biological mechanisms of ageing

In a short review of the theories or proposed biological mechanisms of ageing it is necessary to be very selective in the ideas which are included or rejected. Although many theories of ageing are advanced in the literature a high proportion can be considered to be special cases of a previous hypothesis. No one individual theory satisfactorily explains the biological mechanism of ageing, indeed elements of many theories may be required to understand such a complex phenomena. Such a view has been formally expressed by Wright and Davison who consider that all theories represent too simplistic an approach to the understanding of ageing, a biochemical complexity which can only be resolved using system analysis techniques.

Aging and the immune system

Information is presented on changes in the aging immune system. An analysis of separate streams of cellular aging in the inbred mouse is presented. It is demonstrated that there is extensive polymorphism in the aging of different types of executive and regulatory cell lines.

Developmental switches in reiterated genes may reduce the rate of age changes in DNA

The possible role of the redundancy of genetic information in the regulation of the ageing rate has been discussed in several works. However, it was shown recently that the gene reiteration in most cases is represented by families of similar, but not identical genes. Their expression usually related to the different stages of development and when "early" embryonic or fetal genes are active, the "late" or adult genes are repressed. It is known that the DNA repair needs double stranded structure of DNA which is usual for inactive genes. Genes which are being transcribed and active are repressed by unwound, relaxed DNA which is less protected by the DNA repair enzymes. Aging of genetic information in somatic cells can be, therefore, considered as stage specific and alterations of "early" embryonic and fetal genes do not constitute the genetic load which influences the ageing rate of differentiated cells.

Changes in DNA polymerases alpha, beta, and gamma during the replicative life span of cultured human fibroblasts

DNA polymerases from IMR-90 human diploid fibroblasts at various passage levels and from HeLa cells were purified and fractionated into alpha 1, alpha 2, alpha 3, beta, and gamma species and subspecies, and than the accuracy with which each one copied synthetic template-primers was measured in the presence of Mn2+ or Mg2+. All activities from fibroblasts of later population doubling levels incorporated noncomplementary triphosphates more frequently than did the same polymerase type from earlier population doubling levels. HeLa polymerase activities copied several different templates in the presence of Mn2+ with greater fidelity than enzymes from fibroblasts of population doubling level 27 or greater. The total DNA polymerase activity extracted from IMR-90 cells decreased with increasing population doubling levels. The alpha-polymerase activity generally declined with increasing population doubling levels, while beta-polymerase activity remained relatively constant, except at the very end of the cellular replicative life span. In addition, the amounts of alpha 2 and alpha 3 became progressively lower relative to alpha 1, and a new alpha-type polymerase activity, alpha 0, appeared upon diethylaminoethylcellulose chromatography. HeLa cells also contained three alpha species, though two of them eluted from diethylaminoethylcellulose at higher phosphate concentrations than alpha species from fibroblasts. Postconfluent IMR-90 cells of population doubling level 21 had a decreased level of alpha-polymerase relative to that recovered from rapidly growing cells. This polymerase activity had some chromatographic properties similar to enzyme from late-passage cells. In addition, the alpha-, beta-, and gamma-polymerases from these cells had decreased fidelities relative to those isolated from subconfluent cells.

Evidence for an increased level of DNA damage in high doubling level human diploid cells culture

Excision repair after ultraviolet (UV) irradiation was estimated in human diploid fibroblast-like cells (HDF) by treating DNA with a crude extract from Micrococcus luteus that contained pyrimidine dimer-specific endonucleases. The assays were done before and after the cells had been arrested in an essentially nonmitotic state. When low and high population doubling level (PDL) cells were assayed under these conditions, no age-related difference in the number of UV-induced endonuclease sensitive sites was observed, but the removal of these sites was more rapid in high PDL arrested cells. There was a difference between the calculated number average molecular weights (Mn) of DNA from unirradiated low and high PDL arrested cells. The lower Mn of the DNA from high PDL cells indicated that other enzymes present in the M. luteus extract were acting upon non-UV-induced DNA distortions and that these were present to a greater extent in the chromatin associated regions of older cells. These results support the hypothesis that DNA damage accumulates as HDF progress through their in vitro life span.

Senescence of cultured human diploid fibroblasts. Are mutations responsible?

Two predictions of the error/mutation hypothesis of cellular senescence (Orgel, '73) namely,a) exponential accumulation of somatic mutations during the replicative lifespan and b) shortening of culture lifespan upon treatment with mutagens have been examined experimentally in a strain of cultured human diploid fibroblasts. Our studies show that as cells traverse the replicative lifespan (from 10 to 75 mean population doublings (MPD); total lifespan congruent to 95 MPD), no rapid and exponential increase occurs in the accumulation of mutations measured by the frequencies of Thgr(thioguanine resistance) and Dipr (diphtheria toxin resistance) mutants. Furthermore, repeated cycles of treatment (from 1- to 14-times) of human fibroblasts with two mutagens, ethyl methane sulfonate (EMS) and N-methyl-N' nitro-nitrosoguanidine, which led to a marked increase in the mutation frequency for the Dipr marker (congruent to 100-fold), failed to shorten the lifespan of cultured fibroblasts. On the contrary, repeated mutagen treatment (12 times with EMS) prolonged the lifespan of one replicative culture (110 MPD versus 94--98 MPD). These results strongly indicate that mutations are unlikely to be the primary event in cellular senescence and suggest instead that senescence is probably controlled by one or more (specific) gene(s) whose expression can be modified by mutations.

Age related alterations in the chromosomal proteins from human diploid fibroblasts

Histone and non-histone chromosomal proteins from human diploid fibroblasts of different in vitro ages were extracted and subjected to SDS--polycrylamide gel electrophoresis. The proteins were taken from cells maintained in three distinct culture states: preconfluent (log phase of growth), confluent (stationary phase of growth), and arrested (presumptive G0 phase). Age associated alterations in the incorporation of radioactive amino acids were detected in the fractionated proteins in every culture state but were less pronounced during the arrested state. Age related difference detected under growth conditions may reflect variations in the proliferative nature of the populations. Differences seen during the arrested state may be indicative of basic changes in the chromosomal protein complement of different age populations.

Spontaneous mutations in ageing human cells: studies using a Herpesvirus probe

Several fundamental theories of ageing postulate that spontaneous mutations increase in somatic cells as they age. This has not until recently been directly tested in mammalian cells, because of the problem of estimating mutation rate in declining cell populations. One possible approach is to infect cells with lytic virus and measure the mutation frequency in virus made by the cellular machinery. Three temperature-sensitive mutants of Herpesvirus were used to infect human fibroblasts in vitro. The reversion frequency was examined in virus harvested from young and old cells. It was found that with tsE there was a hundredfold increase in reversion rate in old compared to young cells, with tsD the rates were roughly similar and with tsG a fortyfold decrease in old cells. These differences are not due to differential rates of virus production. It is proposed that errors in specific functions in old cells influence the spontaneous mutation rate of the three viral mutants in opposite directions. It is pointed out that certain unknown factors limit the usefulness of viral probes in ageing research and cast doubt on previous work of this type.

Age-related changes in B and T lymphocytes and decline of humoral immune responsiveness in aged mice

The activity of the immune system declines in an age-related fashion, after reaching a peak in young-adult animals. Although this pattern has been well documented, the mechanisms responsible for the decline of immune potential in senescence have not been fully elucidated. At the same time, it has become apparent that the immune response depends on complex interactions among different cell types. We review the results of the analyses of B- and T-lymphocyte function and activity in the humoral responsiveness of aged mice that have been performed in a number of laboratories, with a variety of experimental techniques. We conclude that an increase in T-cell suppressor function is the first immunologic lesion of aging in mice. This is followed by a decrease in T-cell helper function and finally by a loss of B-cell function.

The biology of aging: 1976, an overview

This article is a comprehensive overview of the biology of aging. The largely unproductive history of research on aging is presented, to lay the groundwork for a discussion of why such research is performed and what aging levels and human models are employed. The anatomic, biochemical, physiologic and behavioral measures used in such studies are delineated. Some of the most promising theories of aging (genetic, autoimmune, viral, free radical) are discussed, as are factors that could prolong the lifespan if optimally employed. Philosophic considerations are emphasized.

Ribosomal RNA gene dosage as a function of tissue and age for mouse and human

The average number of rRNA genes per haploid genome (rRNA gene dosage) of the cells present in liver and brain was determined throughout the lifespan of the inbred C57BL/6J mouse strain and of human. Ribosomal RNA gene dosage was determined using the RNA-excess DNA - RNA hybridization technique. DNA was extracted and purified using a CsCl/chloroform method with a high percent yield (over 90%) to minimize any possible effects of tissue and age-dependent selective loss or gain of rRNA genes. Radioactive rRNA was from the liver of the youngest age group for either mouse or human in all hybridization experiments, with DNA from the different tissues and age groups being the only variable. In the young mouse (35-49 days), the rRNA gene dosage was 36% higher in brain (114 genes), as compared to liver (84 genes). The rRNA gene dosage remained essentially constant as a function of age for mouse brain; but between the age of about 220 to 440 days, it increased in liver, attaining approximately an equal value to that of brain. No significant difference was found in the rRNA gene dosage of brain or liver between different mice of the same age. In contrast to this result, a significant difference was found between human tissues of similar age. The rRNA gene dosage ranged about 2-fold (148-289) between 2 months to 75 years of age. An age-dependent trend, similar to that for mouse liver, was found when the averages of four different age groups totaling 20 individuals were compared. However, this was not statistically significant. No difference in the rRNA gene dosage as a function of sex or tissue was apparent. Several models are discussed to account for these results.

Percent satellite DNA as a function of tissue and age of mice

A selective loss of satellite DNA was found to occur to different extents as a function of tissue and age of mice using several common DNA extraction and purification procedures. This result emphasizes a serious problem that may be encountered in comparative studies of DNA structure and composition if selective loss of specific DNA sequences occurs. We have developed a DNA extraction and purification procedure that is simple and reliable and gives a high percent DNA yield, which substantially reduces the selective loss of heterochromatin DNA sequences. The method features a centrifugation step of a proteolytic digest of chromatin in 2.4 M CsCl. Percent DNA yield of 82-98% are routinely obtained with no apparent loss of satellite DNA sequences from different tissues or ages of mice. Utilizing this method, percent satellite DNA was found to remain essentially constant at 11 +/- 1% for spleen, kidney, and brain tissues obtained from mice of 10-780 days of age. However, for liver, percent satellite DNA remained at about 7-8% from 10 to 300 days of age and then increased to about 12-13% from 300 to 600 days of age. During this latter time interval (300-600 days), an increase of DNA per nucleus of about 3-fold occurred, due to the formation of tetra- and octaploid cell types. A steady loss in the total number of nuclei per gram of liver as a function of age was also found. These two opposing effects resulted in a nearly constant amount of DNA per gram and per organ for liver throughout the lifespan of the mouse.

The age-dependent decrease in creatine kinase and aldolase activities in human striated muscle is not caused by an accumulation of faulty proteins

In human striated muscle obtained in surgery, an age-dependent decrease in aldolase and creatine kinase specific activities and an increase in DNA content per wet weight was found. In the group of the elderly (64-84 years), the enzymes decreased by 40-60% when compared with a group between 24 and 47 years old, while DNA content rose by a factor of 1.53 indicating loss of tissue water. Titration of aldolase and creatine kinase molecules by specific antibodies against aldolase A and creatine kinase MM isozymes, respectively, revealed very little accumulation of aldolase cross-reacting materials in the old age group (1.13 fold), and no accumulation of inactive creatine kinase molecules. Similar conclusions can be drawn from thermostability analyses of these two enzymes. The data do not support the view that accumulation of modified proteins due to random errors or to post-translational alternations is a general or causative phenomenon of aging in human muscle tissue.