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.

Human senescence

Human life expectancy has increased dramatically through improvements in public health, housing, nutrition and general living standards. Lifespan is now limited chiefly by intrinsic senescence and its associated frailty and diseases. Understanding the biological basis of the ageing process is a major scientific challenge that will require integration of molecular, cellular, genetic and physiological approaches. This article reviews progress that has been made to date, particularly with regard to the genetic contribution to senescence and longevity, and assesses the scale of the task that remains.

A network theory of ageing: the interactions of defective mitochondria, aberrant proteins, free radicals and scavengers in the ageing process

Evolution theory indicates that ageing is caused by progressive accumulation of defects, since the evolutionary optimal level of maintenance is always below the minimum required for indefinite survival. Evolutionary theories also suggest that multiple processes are operating in parallel, but unfortunately they make no predictions about specific mechanisms. To understand and evaluate the many different mechanistic theories of ageing which have been proposed, it is therefore important to understand and study the network of maintenance processes which control cellular homeostasis. In this paper we describe a Network Theory of Ageing which integrates the contributions of defective mitochondria, aberrant proteins, and free radicals to the ageing process, and which includes the protective effects of antioxidant enzymes and proteolytic scavengers. The model simulations not only confirm and explain many experimental, age related findings like an increase in the fraction of inactive proteins, a significant rise in protein half-life, an increase in the amount of damaged mitochondria, and a drop in the energy generation per mitochondrion, but they also show interactions between the different theories which could not have been observed without the network approach. In some simulations, for example, the mechanism of the final breakdown seems to be a consequence of the cooperation of mitochondrial and cytoplasmic reactions, the mitochondria being responsible for a long term, gradual change which eventually triggers a short lived cytoplasmic error loop.

Cycles, chaos, and evolution in virus cultures: a model of defective interfering particles

Defective interfering particles (DIP) are spontaneous deletion mutants of viruses that replicate at the expense of the parent virus. DIP have complex effects on the growth of viruses in vitro, including the establishment of persistent infection, cyclical variation in virus titer, eradication of replicating virus, and rapid evolution of the virus. We show here that a simple mathematical model, based only on experimental observations, can explain all of the major effects of DIP on the population dynamics of virus growth. The variation in virus titer caused by DIP has many features that are characteristic of deterministic chaos: it follows that the quantitative effects of DIP are intrinsically unpredictable beyond a short time. We conclude (i) that other factors, such as temperature-sensitive virus mutants or interferons, need not be invoked to explain the complex effects of DIP; and (ii) that dominantly interfering viruses should only be used with great caution for therapeutic purposes, since their effects are, in principle, unpredictable.

Towards a network theory of ageing: a model combining the free radical theory and the protein error theory

Many different theories of ageing have been proposed, based often on highly specific molecular causes. Recent advances in evolutionary theory support the idea that ageing is caused by progressive accumulation of defects, but indicate that multiple processes are likely to operate in parallel. This calls for an understanding of ageing and longevity in terms of a network of maintenance processes that controls the capability of the system to preserve homeostasis. Here we develop a theoretical model which begins the task of implementing a Network Theory of Ageing. To do this the model integrates the ideas of the Free Radical Theory, describing the reactions of free radicals, antioxidants and proteolytic enzymes, with the Protein Error Theory, describing the error propagation loops within the cellular translation machinery. The simulations show that an increased radical production and/or insufficient radical protection can destabilize an otherwise stable translation system. The model supports the idea that caloric restriction prolongs life via a reduction of the generation of radicals. Another result of the model is that protein half-life increases with time as a natural consequence of the interaction between proteolytic enzymes and radicals. Finally the model strengthens certain evolutionary ageing theories by showing that there is a positive correlation between maintenance related energy consumption and lifespan.

Defective interfering particles and virus evolution

Almost all viruses produce replication-defective mutants that have complex effects on the growth and evolution of the virus in culture. These effects can be explained qualitatively by a simple mathematical model. However, the model shows that the quantitative effects of these mutants are intrinsically unpredictable.

Mitochondrial mutations, cellular instability and ageing: modelling the population dynamics of mitochondria

All eukaryotic cells rely on mitochondrial respiration as their major source of metabolic energy (ATP). However, the mitochondria are also the main cellular source of oxygen radicals and the mutation rate of mtDNA is much higher than for chromosomal DNA. Damage to mtDNA is of great importance because it will often impair cellular energy production. However, damaged mitochondria can still replicate because the enzymes for mitochondrial replication are encoded entirely in the cell nucleus. For these reasons, it has been suggested that accumulation of defective mitochondria may be an important contributor to loss of cellular homoeostasis underlying the ageing process. We describe a mathematical model which treats the dynamics of a population of mitochondria subject to radical-induced DNA mutations. The model confirms the existence of an upper threshold level for mutations beyond which the mitochondrial population collapses. This threshold depends strongly on the division rate of the mitochondria. The model also reproduces and explains (i) the decrease in mitochondrial population with age, (ii) the increase in the fraction of damaged mitochondria in old cells, (iii) the increase in radical production per mitochondrion, and (iv) the decrease in ATP production per mitochondrion.

Accuracy of tRNA charging and codon: anticodon recognition; relative importance for cellular stability

Cellular homeostasis and the mechanisms which control homeostasis are important for understanding such fundamental processes as ageing and the origin of life. Several models have studied the importance of accurate protein synthesis for cellular stability, but these models have not considered the complexities of the translation process in any detail. Here we develop a new model which describes the interplay between aminoacyl-tRNA (aatRNA) synthetases, the cellular pool of charged tRNAs and the process of codon: anticodon recognition. We also take the processive character of the ribosomes into account. In common with previous work, our model predicts that the cellular translation apparatus can either be stable or deteriorate progressively with time. However, because our model explicitly describes different subreactions of the overall translation process, we are also able to assess the relative importance of accurate tRNA charging and codon: anticodon recognition for cellular stability. It appears that the tRNA charging by the aatRNA synthetases plays the key role in controlling the long-term stability of the cell. Ribosomal errors are less important because error-prone ribosomes, being processive, produce mainly inactive proteins which do not contribute to error propagation within the translation machinery.

Comparative life spans of species: why do species have the life spans they do?

Proximate answers to questions about species longevity are to be found in the physiological processes that regulate duration of life. But what are these processes, and how are they themselves controlled? This leads to ultimate, evolutionary questions about longevity. What are the selection forces that favor one life span instead of another for a given species? To understand the evolution of life span we need also to understand the evolution of aging. A plausible hypothesis is that because of the requirement for reproduction, natural selection favors a strategy that invests fewer resources in maintenance of somatic cells and tissues than are necessary for indefinite survival. This "disposable soma" theory predicts that aging is due to the accumulation of unrepaired somatic defects and the primary genetic control of longevity operates through selection to raise or lower the investment in basic cellular maintenance systems in relation to the level of environmental hazard.

Neuronal precursor cells in the rat hippocampal formation contribute to more than one cytoarchitectonic area

We have tested the hypothesis that cell lineage restriction boundaries define the borders between cytoarchitectonic areas in the cerebral cortex. Clonally related cells were identified using a retroviral marking technique, and the dispersion of neuronal clones was examined with respect to the transitions between cortical areas. We chose to study the hippocampal formation because we found that clones of hippocampal neurons, unlike those in neocortex, are compact and readily identifiable in the adult and that transitions between areas in the hippocampus are sharp relative to the spread of a typical clone. We conclude, contrary to the hypothesis, that clones of neurons transgress the boundaries between areas in the hippocampal formation, that border-crossing clones are observed as frequently as would be expected if clones spread freely over the hippocampus with no constraint imposed by area borders, and that different types of pyramidal neurons, characteristic of different areas, may appear to a single clone. different areas, may appear in a single clone.

The genetic contribution to human T-cell receptor repertoire

Recent studies in mice have highlighted the importance of polymorphic genetic loci such as the major histocompatibility complex (MHC) or minor lymphocyte-stimulating antigen (Mls) in determining the nature of the peripheral T-cell receptor (TcR) population. As our knowledge of the equivalent process in humans is incomplete, we have utilized a modification of the polymerase chain reaction (PCR) to determine the overall genetic contribution to the normal human TcR variable V beta gene repertoire. These data demonstrate that the normal human T-cell population contains members of all the major TcR V beta families and that there is considerable variation in the relative amounts of specific TcR V beta transcripts between individuals. We have established that the normal peripheral TcR V beta repertoire is more concordant in identical twins than in unrelated individuals. The relative importance of genetic factors in determining the peripheral TcR repertoire is emphasized by these results and suggests that, in humans, the genetic control of immune responsiveness is mediated in part by the peripheral TcR repertoire.

Evolution of senescence: late survival sacrificed for reproduction

In so far as it is associated with declining fertility and increasing mortality, senescence is directly detrimental to reproductive success. Natural selection should therefore act in the direction of postponing or eliminating senescence from the life history. The widespread occurrence of senescence is explained by observing that (i) the force of natural selection is generally weaker at late ages than at early ages, and (ii) the acquisition of greater longevity usually involves some cost. Two convergent theories are the 'antagonistic pleiotropy' theory, based in population genetics, and the 'disposable soma' theory, based in physiological ecology. The antagonistic pleiotropy theory proposes that certain alleles that are favoured because of beneficial early effects also have deleterious later effects. The disposable soma theory suggests that because of the competing demands of reproduction less effort is invested in the maintenance of somatic tissues than is necessary for indefinite survival.

Genetic basis of limited cell proliferation

Knowledge about the changes that occur as cells traverse their replicative lifespans grows apace, as evidenced by the articles in this issue. Controversy over the interpretation of this knowledge continues, however, and is indeed fuelled by new discoveries (e.g., see Cristofalo, 1990; Holliday, 1990; Smith, 1990). This paper makes a brief commentary on the problems of cellular ageing, with particular emphasis on the unfolding picture of the genetic control of ageing and longevity which derives from evolutionary theory (Kirkwood and Rose, 1991). The case is argued for a synthetic view which recognizes that the immediate causes of limited cell proliferation probably involve some form of active genetic control, but that the ultimate reason for cell ageing is found in evolutionary theories which suggest that the ageing process is not actively programmed and that senescence may be due to the accumulation of damage.

Tests for the statistical significance of protein sequence similarities in data-bank searches

A suite of tests to evaluate the statistical significance of protein sequence similarities is developed for use in data bank searches. The tests are based on the Wilbur-Lipman word-search algorithm, and take into account the sequence lengths and compositions, and optionally the weighting of amino acid matches. The method is extended to allow for the existence of a sequence insertion/deletion within the region of similarity. The accuracy of statistical distributions underlying the tests is validated using randomly generated sequences and real sequences selected at random from the data banks. A computer program to perform the tests is briefly described.