The role of limited cell replicative capacity in pathological age change. A review

Age-related cellular changes in the long-lived bivalve A. islandica

One of the biggest challenges to studying causes and effects of aging is identifying changes in cells that are related to senescence instead of simply the passing of chronological time. We investigated two populations of the longest living non-colonial metazoan, Arctica islandica, with lifespans that differed sixfolds. Of four investigated parameters (nucleic acid oxidation, protein oxidation, lipid oxidation, and protein instability), only nucleic acid oxidation increased with age and correlated with relative lifespan. Nucleic acid oxidation levels increased significantly faster and were significantly higher in the shorter-lived than the longer-lived population. In contrast, neither protein oxidation, lipid oxidation, nor protein stability changed over time. Protein resistance to unfolding stress when treated with urea was significantly lower overall in the shorter-lived population, and lipid peroxidation levels were higher in the longer-lived population. With the exception of nucleic acid oxidation, damage levels of A. islandica do not change with age, indicating excellent cellular maintenance in both populations. Since correlations between nucleic acid oxidation and age have also been shown previously in other organisms, and nucleic acid oxidation accumulation rate correlates with relative age in both investigated populations, nucleic acid oxidation may reflect intrinsic aging mechanisms.

The role of protein conformational switches in pharmacology: its implications in metabolic reprogramming and protein evolution

Besides pharmacogenomics and drug dynamics, pharmacological properties of a drug could also arise from protein conformational switches. These switches would arise from the following mechanisms: (a) slight shifts away from a protein's native conformation induced by mutation, (b) changes in the protein's environment allowing for structural rearrangements to form hitherto unknown conformations, (c) parsing the protein into foldable polypeptide fragment(s) by either proteolysis of the native structure or (d) perturbation of the native conformation to generate polypeptide fragment(s). These switches are modulated by changes in the protein's matrix properties such as pH, temperature, ligands-their nature, concentration and complexes; micronutrients, oxidant/antioxidant status and metabolic products within the functional environment of the protein. The pharmacological implications of these are discussed in light of polypharmacology arising from protein isomerism, cross-pharmacology, possible decreases in both the expressible and expressed protein population and metabolic reprogramming-and ultimately, how these factors relate to diseases. Further implications include variational drug toxicity and drug response idiosyncrasies. Another important consequence is that the "whole life" history of the individual would play an active role in that individual's response to disease severity and drug response up to that very moment and is prone to variations with changes in pre-disposing factors.

Cognitive deterioration and associated pathology induced by chronic low-level aluminum ingestion in a translational rat model provides an explanation of Alzheimer's disease, tests for susceptibility and avenues for treatment

A translational aging rat model for chronic aluminum (Al) neurotoxicity mimics human Al exposure by ingesting Al, throughout middle age and old age, in equivalent amounts to those ingested by Americans from their food, water, and Al additives. Most rats that consumed Al in an amount equivalent to the high end of the human total dietary Al range developed severe cognitive deterioration in old age. High-stage Al accumulation occurred in the entorhinal cortical cells of origin for the perforant pathway and hippocampal CA1 cells, resulting in microtubule depletion and dendritic dieback. Analogous pathological change in humans leads to destruction of the perforant pathway and Alzheimer's disease dementia. The hippocampus is thereby isolated from neocortical input and output normally mediated by the entorhinal cortex. Additional evidence is presented that Al is involved in the formation of neurofibrillary tangles, amyloid plaques, granulovacuolar degeneration, and other pathological changes of Alzheimer's disease (AD). The shared characteristics indicate that AD is a human form of chronic Al neurotoxicity. This translational animal model provides fresh strategies for the prevention, diagnosis, and treatment of AD.

Biology and Evolution of Aging: Implications for Basic Gerontological Health Research

The maximum lifespan of different animal species is genetically determined. Many biochemical and physiological systems which influence longevity have apparently evolved to regulate growth and development in a way which maximizes fitness given the ecological niche and constraints on the species. The diversity of individual genetic effects on aging makes it unlikely that either extrinsic factors such as nutrition and medicine or genetic intervention will have dramatic effects on the maximum lifespan of a species, in spite of significant qualitative effects on individuals. However, understanding the fundamental genetic determinants of senescence may be of particular importance to the treatment and or prevention of age associated problems involving tissue degeneration and/or cancer. Considerable investment in basic biological research on both aging and developmental processes is needed before this understanding can be achieved.

Cataloging altered gene expression in young and senescent cells using enhanced differential display

Recently, a novel PCR-based technique, differential display (DD), has facilitated the study of differentially expressed genes at the mRNA level. We report here an improved version of DD, which we call Enhanced Differential Display (EDD). We have modified the technique to enhance reproducibility and to facilitate sequencing and cloning. Using EDD, we have generated and verified a catalog of genes that are differentially expressed between young and senescent human diploid fibroblasts (HDF). From 168 genetags that were identified initially, 84 could be sequenced directly from PCR amplified bands. These sequences represent 27 known genes and 37 novel genes. By Northern blot analysis we have confirmed the differential expression of a total of 23 genes (12 known, 11 novel), while 19 (seven known, 12 novel) did not show differential expression. Several of the known genes were previously observed by others to be differentially expressed between young and senescent fibroblasts, thereby validating the technique.

Telomere length predicts replicative capacity of human fibroblasts

When human fibroblasts from different donors are grown in vitro, only a small fraction of the variation in their finite replicative capacity is explained by the chronological age of the donor. Because we had previously shown that telomeres, the terminal guanine-rich sequences of chromosomes, shorten throughout the life-span of cultured cells, we wished to determine whether variation in initial telomere length would account for the unexplained variation in replicative capacity. Analysis of cells from 31 donors (aged 0-93 yr) indicated relatively weak correlations between proliferative ability and donor age (m = -0.2 doubling per yr; r = -0.42; P = 0.02) and between telomeric DNA and donor age (m = -15 base pairs per yr; r = -0.43; P = 0.02). However, there was a striking correlation, valid over the entire age range of the donors, between replicative capacity and initial telomere length (m = 10 doublings per kilobase pair; r = 0.76; P = 0.004), indicating that cell strains with shorter telomeres underwent significantly fewer doublings than those with longer telomeres. These observations suggest that telomere length is a biomarker of somatic cell aging in humans and are consistent with a causal role for telomere loss in this process. We also found that fibroblasts from Hutchinson-Gilford progeria donors had short telomeres, consistent with their reduced division potential in vitro. In contrast, telomeres from sperm DNA did not decrease with age of the donor, suggesting that a mechanism for maintaining telomere length, such as telomerase expression, may be active in germ-line tissue.

Telomere loss: mitotic clock or genetic time bomb?

The Holy Grail of gerontologists investigating cellular senescence is the mechanism responsible for the finite proliferative capacity of somatic cells. In 1973, Olovnikov proposed that cells lose a small amount of DNA following each round of replication due to the inability of DNA polymerase to fully replicate chromosome ends (telomeres) and that eventually a critical deletion causes cell death. Recent observations showing that telomeres of human somatic cells act as a mitotic clock, shortening with age both in vitro and in vivo in a replication dependent manner, support this theory's premise. In addition, since telomeres stabilize chromosome ends against recombination, their loss could explain the increased frequency of dicentric chromosomes observed in late passage (senescent) fibroblasts and provide a checkpoint for regulated cell cycle exit. Sperm telomeres are longer than somatic telomeres and are maintained with age, suggesting that germ line cells may express telomerase, the ribonucleoprotein enzyme known to maintain telomere length in immortal unicellular eukaryotes. As predicted, telomerase activity has been found in immortal, transformed human cells and tumour cell lines, but not in normal somatic cells. Telomerase activation may be a late, obligate event in immortalization since many transformed cells and tumour tissues have critically short telomeres. Thus, telomere length and telomerase activity appear to be markers of the replicative history and proliferative potential of cells; the intriguing possibility remains that telomere loss is a genetic time bomb and hence causally involved in cell senescence and immortalization.

Telomeres, telomerase and senescence

Eukaryotic chromosomes end with tandem repeats of simple sequences. These GC rich repeats allow telomere replication and stabilize chromosome ends. Telomere replication involves an equilibrium of sequence loss and addition at the ends of chromosomes. Repeats are added de novo by telomerase, an unusual DNA polymerase. Telomerase is an RNP in which an essential RNA component provides the template for the added telomere repeats. Telomere length maintenance plays an essential role in cell viability.

Phospholipase activity and plasma membrane homeostasis

Cyclic changes in the physiochemical state of the plasma membrane appear to be necessary for the normal functioning of the cell, especially with respect to division and differentiation. Such changes require a flexible membrane lipid composition to permit the necessary sequence of physicochemical changes to occur during these cellular events. This flexibility can be lost as a result of peroxidation-induced cross linking of membrane constituents, which prevents the normal physico-chemical membrane changes from occurring, resulting in abnormal cellular function. It is proposed that phospholipase A2 and C form a mutually regulatory enzyme system playing an important role with respect to the maintenance of membrane composition and flexibility.

Low tryptophan diets delay reproductive aging

Newly weaned female rats fed diets severely deficient in the essential amino acid tryptophan show marked delays in reproductive aging, with conception and delivery occurring as late as 36 months. The rate of aging in these rats seems inversely related to both their early growth rates and the accessibility of brain tryptophan. The subsequent age retardation may depend on a reduction in both early cell loss and rate of brain maturation.