Mistranslation and ageing in Neurospora

The msh1 gene is responsible for short life span mutant natural death and functions to maintain mitochondrial DNA integrity

Wild-type filamentous fungus Neurospora crassa continues to grow its hyphae for a very lengthy period of time (>2 years), whereas mutations at the natural death (nd) locus shorten life span (approximately 20 days). By positional cloning based on heat augmented mutagen sensitivity of the nd strain, we identified a nonsense mutation in the msh1 gene, an eukaryotic homolog of bacterial MutS, and this mutation resulted in encoding non-functional polypeptide. By tagging with GFP, subcellular localization of the MSH1 protein in the mitochondria was observed, and knock out of the msh1 gene caused severe growth deficiency accompanying mitochondrial DNA (mtDNA) aberrations such as large-scale mtDNA deletions and rearrangements as seen in the nd strain. These results suggested that MSH1 may maintain mtDNA integrity. Thus, loss of function compromises mtDNA, leading to the acceleration of cellular aging.

Causes of aging are likely to be many: robin holliday and changing molecular approaches to cell aging, 1963-1988

Causal complexities involved in biological phenomena often generate ambiguous experimental results that may create epistemic niches for new approaches and interpretations. The exploration for new approaches may foment momentum of larger epistemological shifts, and thereby introduce the possibilities of adopting new technologies. This paper describes British molecular biologist Robin Holliday's cell aging research from 1963 to the 1980s that transformed from simple hypothesis testing to working on various alternative and integrative approaches designed to deal with complex data. In the 1960s, hoping to use biochemical investigations of cells to settle a debate about whether DNA mutations or protein errors caused aging, Holliday carried out a series of experiments with fruit flies, fungi, and human fibroblast cells. The results seemed to demonstrate that cytoplasmic protein errors caused cell aging. However, other scientists obtained contradictory results and raised issues about potential flaws in Holliday's experiments. In the 1970s, working as the director of the Genetics Division of the National Institute for Medical Research in Mill Hill, United Kingdom, Holliday relied on available talents of his associates, including computational expertise, to explore alternative hypotheses and approaches. By the early 1980s, they had worked out an epigenetic explanation and had established integrative, evolutionary models of cell aging that incorporated both DNA mutations and protein errors as critical factors. By delineating Holliday's research path from simply testing hypotheses to integrating multiple factors involved in aging, this paper offers an account of the difficulties in targeting molecular cause in cell aging around the 1970s, whose failures nevertheless opened up an epistemic niche for integration.

Twenty years of ageing research at the Mill Hill laboratories

Research on ageing was carried out in the Genetics Division laboratories, Mill Hill, London, from 1970 to 1990, resulting in more than 100 publications. The work centred around the in vitro ageing of human diploid fibroblasts, but there was also research on transformed cells, rat and mouse tissues, human lymphocytes, chick cells, mice and a microbial model system. The major conclusion from all this research, together with a broad overview of the whole field of gerontology, is that ageing has multiple causes, and that adult animals become senescent through the eventual failure of several important maintenance mechanisms.

N(2)-(1-Carboxyethyl)deoxyguanosine, a nonenzymatic glycation adduct of DNA, induces single-strand breaks and increases mutation frequencies

N(2)-(1-Carboxyethyl)deoxyguanosine (CEdG) is a major nonenzymatic glycation product of DNA. The effect of CEdG modification, which was specifically prepared by incubation with dihydroxyacetone, on plasmid DNA topology was evaluated by gel electrophoresis. A time-dependent decrease of supercoiled plasmid-DNA was observed in parallel to the increase of CEdG adducts; the half-life time of the supercoiled plasmid-DNA was estimated to be approximately 16-18 h. CEdG-modified plasmid DNA showed a 25-fold reduced transformation efficiency. When modified DNA was used to transform Escherichia coli cells, a 6-fold increase in mutation frequency was determined by measuring loss of alpha-complementation. For the mutator strain BMH71-18mutS, an 8-fold increase in mutation frequency was observed. Although the exact mechanism of DNA damage is unclear, the occurrence of spontaneous depurination is likely. These findings suggest that a defined DNA glycation reaction can lead to DNA damage in vivo.

Cell death and longevity: implications of Fas-mediated apoptosis in T-cell senescence

Two prominent features of immune senescence are altered T-cell phenotype and reduced T-cell response. We have previously shown that T-cell senescence is greatly reduced in CD2-fas transgenic mice, in which the Fas apoptosis molecule is constantly expressed on T cells. Using a different experimental approach, the relationship between T-cell senescence and apoptosis was analyzed on human peripheral blood mononuclear cells. The results indicate that there was increased apoptosis of CD45RO- (CD45RA+) T cells upon activation. We propose that this could account for the increase in CD45RO+ "memory" T cells with aging in humans. T-cell responsiveness remained high in CD2-fas transgenic aged mice, but there was no increase in overall life span of these mice. Increased T-cell responsiveness was associated with an increased acute-phase response and serum amyloid A deposition in the glomerulus of aged CD2-fas transgenic mice. Therefore, restoration of the T-cell immune function using a CD2-fas transgene produced undesirable side-effects to aged transgenic mice. In addition to its role in activation-induced cell death, Fas-mediated apoptosis may be important in deleting T cells in response to DNA damage. It may also inhibit cell-cycle progression by cleaving various kinases and DNA repair enzymes. We observed that cell lines derived from human premature aging diseases have a higher sensitivity to Fas-mediated apoptosis. The implications of these observations are discussed.

Hyperactive recombination in the mitochondrial DNA of the natural death nuclear mutant of Neurospora crassa

In Neurospora crassa, a recessive mutant allele of a nuclear gene, nd (natural death), causes rapid degeneration of the mitochondrial DNA, a process that is manifested phenotypically as an accelerated form of senescence in growing and stationary mycelia. To examine the mechanisms that are involved in the degradation of the mitochondrial chromosome, several mitochondrial DNA restriction fragments unique to the natural-death mutant were cloned and characterized through restriction, hybridization, and nucleotide sequence analyses. All of the cloned DNA pieces contained one to four rearrangements that were generated by unequal crossing-over between direct repeats of several different nucleotide sequences that occur in pairs and are dispersed throughout the mitochondrial chromosome of wild-type Neurospora strains. The most abundant repeats, a family of GC-rich sequences that includes the so-called PstI palindromes, were not involved in the generation of deletions in the nd mutant. The implication of these results is that the nd allele hyperactivates a general system for homologous recombination in the mitochondria of N. crassa. Therefore, the nd+ allele either codes for a component of the complex of proteins that catalyzes recombination, and possibly repair and replication, of the mitochondrial chromosome or specifies a regulatory factor that controls the synthesis or activity of at least one enzyme or ancillary factor that is affiliated with mitochondrial DNA metabolism.

Hyperactive recombination in the mitochondrial DNA of the natural death nuclear mutant of Neurospora crassa

In Neurospora crassa, a recessive mutant allele of a nuclear gene, nd (natural death), causes rapid degeneration of the mitochondrial DNA, a process that is manifested phenotypically as an accelerated form of senescence in growing and stationary mycelia. To examine the mechanisms that are involved in the degradation of the mitochondrial chromosome, several mitochondrial DNA restriction fragments unique to the natural-death mutant were cloned and characterized through restriction, hybridization, and nucleotide sequence analyses. All of the cloned DNA pieces contained one to four rearrangements that were generated by unequal crossing-over between direct repeats of several different nucleotide sequences that occur in pairs and are dispersed throughout the mitochondrial chromosome of wild-type Neurospora strains. The most abundant repeats, a family of GC-rich sequences that includes the so-called PstI palindromes, were not involved in the generation of deletions in the nd mutant. The implication of these results is that the nd allele hyperactivates a general system for homologous recombination in the mitochondria of N. crassa. Therefore, the nd+ allele either codes for a component of the complex of proteins that catalyzes recombination, and possibly repair and replication, of the mitochondrial chromosome or specifies a regulatory factor that controls the synthesis or activity of at least one enzyme or ancillary factor that is affiliated with mitochondrial DNA metabolism.

Nuclear gene for mitochondrial leucyl-tRNA synthetase of Neurospora crassa: isolation, sequence, chromosomal mapping, and evidence that the leu-5 locus specifies structural information

We have isolated and characterized the nuclear gene for the mitochondrial leucyl-tRNA synthetase (LeuRS) of Neurospora crassa and have established that a defect in this structural gene is responsible for the leu-5 phenotype. We have purified mitochondrial LeuRS protein, determined its N-terminal sequence, and used this sequence information to identify and isolate a full-length genomic DNA clone. The 3.7-kilobase-pair region representing the structural gene and flanking regions has been sequenced. The 5' ends of the mRNA were mapped by S1 nuclease protection, and the 3' ends were determined from the sequence of cDNA clones. The gene contains a single short intron, 60 base pairs long. The methionine-initiated open reading frame specifies a 52-amino-acid mitochondrial targeting sequence followed by a 942-amino-acid protein. Restriction fragment length polymorphism analyses mapped the mitochondrial LeuRS structural gene to linkage group V, exactly where the leu-5 mutation had been mapped before. We show that the leu-5 strain has a defect in the structural gene for mitochondrial LeuRS by restoring growth under restrictive conditions for this strain after transformation with a wild-type copy of the mitochondrial LeuRS gene. We have cloned the mutant allele present in the leu-5 strain and identified the defect as being due to a Thr-to-Pro change in mitochondrial LeuRS. Finally, we have used immunoblotting to show that despite the apparent lack of mitochondrial LeuRS activity in leu-5 extracts, the leu-5 strain contains levels of mitochondrial LeuRS protein to similar to those of the wild-type strain.

Unstable mitochondrial DNA in natural-death nuclear mutants of Neurospora crassa

The natural-death mutant of Neurospora crassa has an accelerated senescence phenotype caused by a recessive mutation, nd, in a nuclear gene that is located in linkage group I. An examination of mitochondrial functions, however, revealed that the mutant has phenotypic and molecular defects similar to those commonly associated with maternally transmitted fungal senescence syndromes, including (i) deficiencies in cytochromes aa3 and b; (ii) a deficit in small subunits of mitochondrial ribosomes, and hence defective mitochondrial protein synthesis; and (iii) accumulation of gross rearrangements, including large deletions, in the mitochondrial chromosome of vegetatively propagated cells. These traits indicate that the nd+ allele codes for a function that is essential for stable maintenance of the mitochondrial chromosome, possibly a protein involved in replication, repair, or recombination.

Nuclear gene for mitochondrial leucyl-tRNA synthetase of Neurospora crassa: isolation, sequence, chromosomal mapping, and evidence that the leu-5 locus specifies structural information

We have isolated and characterized the nuclear gene for the mitochondrial leucyl-tRNA synthetase (LeuRS) of Neurospora crassa and have established that a defect in this structural gene is responsible for the leu-5 phenotype. We have purified mitochondrial LeuRS protein, determined its N-terminal sequence, and used this sequence information to identify and isolate a full-length genomic DNA clone. The 3.7-kilobase-pair region representing the structural gene and flanking regions has been sequenced. The 5' ends of the mRNA were mapped by S1 nuclease protection, and the 3' ends were determined from the sequence of cDNA clones. The gene contains a single short intron, 60 base pairs long. The methionine-initiated open reading frame specifies a 52-amino-acid mitochondrial targeting sequence followed by a 942-amino-acid protein. Restriction fragment length polymorphism analyses mapped the mitochondrial LeuRS structural gene to linkage group V, exactly where the leu-5 mutation had been mapped before. We show that the leu-5 strain has a defect in the structural gene for mitochondrial LeuRS by restoring growth under restrictive conditions for this strain after transformation with a wild-type copy of the mitochondrial LeuRS gene. We have cloned the mutant allele present in the leu-5 strain and identified the defect as being due to a Thr-to-Pro change in mitochondrial LeuRS. Finally, we have used immunoblotting to show that despite the apparent lack of mitochondrial LeuRS activity in leu-5 extracts, the leu-5 strain contains levels of mitochondrial LeuRS protein to similar to those of the wild-type strain.

Unstable mitochondrial DNA in natural-death nuclear mutants of Neurospora crassa

The natural-death mutant of Neurospora crassa has an accelerated senescence phenotype caused by a recessive mutation, nd, in a nuclear gene that is located in linkage group I. An examination of mitochondrial functions, however, revealed that the mutant has phenotypic and molecular defects similar to those commonly associated with maternally transmitted fungal senescence syndromes, including (i) deficiencies in cytochromes aa3 and b; (ii) a deficit in small subunits of mitochondrial ribosomes, and hence defective mitochondrial protein synthesis; and (iii) accumulation of gross rearrangements, including large deletions, in the mitochondrial chromosome of vegetatively propagated cells. These traits indicate that the nd+ allele codes for a function that is essential for stable maintenance of the mitochondrial chromosome, possibly a protein involved in replication, repair, or recombination.

Biochemical comparison of the Neurospora crassa wild-type and the temperature-sensitive leucine-auxotroph mutant leu-5. Detailed kinetic comparison of the leucyl-tRNA synthetases

The cytoplasmic leucyl-tRNA synthetases were purified from a wild-type Neurospora crassa and from a temperature-sensitive leucine-auxotroph (leu-5) mutant. A detailed steady-state kinetic study of the aminoacylation of the tRNALeu from N. crassa by the purified synthetases was carried out. These enzymes need preincubation with dithioerythritol and spermine before the assay in order to become fully active. The Kappm value for leucine was lowered by high ATP concentrations and correspondingly the Kappm,ATP was lowered by high leucine concentrations. The Kappm,Leu was lowered by high pH, a pK value of 6.7 (at 30 degrees C) was calculated for the ionizable group affecting the Km. At the concentrations of 2 mM ATP, 20 microM leucine, 0.3 microM tRNALeu, and pH 7 the apparent Km values were Kappm,ATP = 1.3 mM, Kappm,Leu = 49 microM and Kappm,tRNA = 0.15 microM. No essentially altered cytoplasmic leucyl-tRNA synthetase was produced by the temperature-sensitive mutant strain when kept at 37 degrees C. In none of these experiments could we find any difference between the wild-type enzyme and the enzyme from the mutant strain (whether grown at permissive temperature, 28 degrees C, or grown at permissive temperature for 24 h followed by growth at 37 degrees C). We therefore think that the small difference in the Km value for leucine of the wild-type and mutant enzyme, established in some earlier investigations, is not due to a difference in the kinetic properties of the enzyme molecules but to an external influence. The almost total lack of the mitochondrial leucyl-tRNA synthetase in the mutant strain besides the leucine autotrophy remains the only difference between the wild-type and mutant strains.

Biochemical comparison of the Neurospora crassa wild type and the temperature-sensitive and leucine-auxotroph mutant leu-5. Purification of the cytoplasmic and mitochondrial leucyl-tRNA synthetases and comparison of the enzymatic activities and the degradation patterns

The cytoplasmic leucyl-tRNA synthetases of Neurospora crassa wild type (grown at 37 degrees C) and mutant (grown at 28 degrees C) were purified approximately 1770-fold and 1440-fold respectively. Additional enzyme preparations were carried out with mutant cells grown for 24 h at 28 degrees C and transferred then to 37 degrees C for 10-70 h of growth. The mitochondrial leucyl-tRNA synthetase of the wild type was purified approximately 722-fold. The mitochondrial mutant enzyme was found only in traces. The cytoplasmic leucyl-tRNA synthetase from the mutant (grown at 37 degrees C) in vivo is subject of a proteolytic degradation. This leads to an increased pyrophosphate exchange, without altering aminoacylation. Proteolysis in vitro by trypsin or subtilisin of isolated cytoplasmic wild-type and mutant leucyl-tRNA synthetases, however, did not establish and difference in the degradation products and in their catalytic properties. Comparing the cytoplasmic wild-type and mutant enzymes (grown at 28 degrees C) via steady-state kinetics did not show significant differences between these synthetases either. The rate-determining step appears to be after the transfer of the aminoacyl group to the tRNA, e.g. a conformational change or the release of the product. Besides leucine only isoleucine is activated by the enzymes with a discrimination of approximately 1:600; however, no Ile-tRNALeu is released. Similarly these enzymes, when tested with eight ATP analogs, cannot be distinguished. For both enzymes six ATP analogs are neither substrates nor inhibitors. Two analogs are substrates with identical kinetic parameters. The mitochondrial wild-type leucyl-tRNA synthetase is different from the cytoplasmic enzyme, as particularly exhibited by aminoacylating Escherichia coli tRNALeu but not N. crassa cytoplasmic tRNALeu. The presence of traces of the analogous mitochondrial mutant enzyme could be demonstrated. Therefore, the difference between wild-type and mutant leu-5 does not rest in the catalytic properties of the cytoplasmic leucyl-tRNA synthetases. Differences in other properties of these enzymes are not excluded. In contrast the activity of the mitochondrial leucyl-tRNA synthetase of the mutant is approximately 1% of that of the wild-type enzyme.

Levels of uracil DNA glycosylase and AP endonuclease in murine B- and T-lymphocytes do not change with age

Two DNA repair enzyme activities, uracil DNA glycosylase and AP endonuclease, were measured in extracts of T- and B-lymphocytes isolated from mice ranging in age from 3 to 24 months. T- and B-lymphocytes had roughly equal levels of AP endonuclease which did not change appreciably with age. T-lymphocytes had roughly twice as high a level of uracil DNA glycosylase as B-lymphocytes; these levels were not affected by age either. This constancy with age contrasts dramatically with increases in both enzymes--roughly 3-fold on a protein basis or 50-fold on a per cell basis--in a transformed line (MPC-11) derived from a carcinogen-induced lymphocytoma. These results are similar to those obtained with cultured murine fibroblasts, wherein a relative constancy was noted with passage of non-transformed cells, followed by dramatic changes upon transformation (La Belle, M & Linn, S, Mutat res 132 (1984) 51). Hence these enzyme assays do not support the notion of a drop in base excision DNA repair capacity as being a causative factor in aging, but suggest instead that DNA repair properties might differ dramatically in transformed vs non-transformed cells.

Isoleucyl-tRNA synthetase from bakers' yeast: variable discrimination between tRNAIle and tRNAVal and different pathways of cognate and noncognate aminoacylation under standard conditions, in the presence of pyrophosphatase, elongation factor Tu-GTP complex, and spermine

Error rates in discrimination between cognate tRNAIle and noncognate tRNAVal in the aminoacylation reaction with isoleucine catalyzed by isoleucyl-tRNA synthetase from yeast have been investigated in three sets of experiments under different assay conditions. The overall discrimination factor was first determined by isoleucylation of tRNAVal/tRNAIle mixtures. In the second set of experiments, the number of AMP molecules formed per Ile-tRNA in the cognate and noncognate reactions was measured. The higher AMP formation in the noncognate aminoacylation is assigned to a proofreading reaction step. The calculated proofreading factors and an estimated initial discrimination factor yield overall discriminations that are consistent with those obtained from the first set of experiments. In the third series of studies, the orders of substrate addition and product release of cognate and noncognate isoleucylation reactions were investigated by initial rate kinetic methods. From kcat and Km values, the overall discrimination factors were calculated and showed again a good coincidence with those observed in the preceding sets of experiments. Besides under standard assay conditions, aminoacylation reactions were studied in the presence of pyrophosphatase or elongation factor Tu-GTP complex, under addition of both these proteins, in presence of these two additional proteins and spermine at high and low magnesium concentrations, and under special conditions that favor misacylations. Furthermore, isoleucylation of tRNAIle was tested at increased and decreased pH in the standard enzyme assay. Variation of the assay conditions results in changing discrimination factors, which differ by a factor of about 10. Substitution of tRNAIle by tRNAVal in the isoleucylation reaction causes changes in substrate addition and product release orders and thus of the whole catalytic cycle. For aminoacylation of tRNAIle, four different orders of substrate addition and product release appear: the sequential ordered ter-ter, the rapid equilibrium sequential random ter-ter, the random bi-uni uni-bi ping-pong, and a bi-bi uni-uni ping-pong mechanism with a rapid equilibrium segment. tRNAVal is aminoacylated in rapid equilibrium random ter-ter order, in a bi-bi uni-uni ping-pong mechanism with a rapid equilibrium segment, and in two bi-uni uni-bi ping-pong mechanisms. It is assumed that the different assay conditions can be regarded as a stepwise approximation to physiological conditions and that considerable changes in error rates may be also possible in vivo up to 1 order of magnitude.

Altered degradation of intracellular proteins in aging human fibroblasts

Confluent cultures of fibroblasts at different population doubling levels were incubated with [14C]leucine for 2 days and with [3H]leucine for 2 h to label long-lived and short-lived proteins, respectively. Proteolysis was then measured in the presence of excess unlabeled leucine to prevent reutilization of the isotope. Catabolism of long-lived proteins was reduced in senescent cells when measured in media without fetal bovine serum, insulin, fibroblast growth factor, or dexamethasone. In contrast, degradation of short-lived proteins was increased in senescent cells but only when measured in the presence of serum, hormones, and growth factors. Further experiments with cells of varying ages indicate that in unsupplemented medium half-lives of long-lived proteins lengthened by as much as 20 min per population doubling and in supplemented media half-lives of short-lived proteins decreased by 4 min per population doubling. The reduced catabolism of long-lived proteins in senescent cells cannot be explained by age-related changes in protein secretion or cell death during degradation measurements. These alterations in proteolysis may have major effects on protein content and composition in senescent cells.

Mistranslation in bacteriophage-infected anucleate minicells of Escherichia coli: a test for error propagation

The theory of error propagation proposes that errors occurring during expression of the genetic code lead to increased levels of error occurrence in successive generations. A model system for testing error propagation in bacteriophage T7 infected anucleate minicells of Escherichia coli is described. Errors in translation were were stimulated by addition of gentamicin to phage infected minicells, and the error frequency based on the illegitimate incorporation of L-[35S] cysteine into the T7 0.3 gene protein calculated to be on the order of 1 error per 10 000 codons translated. Errors in the synthesis of T7 early gene products have also been detected as suppression of a UAG nonsense codon in gene 1 of the T7 DNA-dependent RNA polymerase, and as increased charge heterogeneity in the gene 1.3 product (DNA ligase). The question of error propagation has been addressed by infecting minicells with a mutant of T7 containing nonsense mutations in the early gene 1 and late gene 16. Results demonstrate that a T7 DNA-dependent RNA polymerase containing misincorporated amino acids is unable, by mistranscription, to suppress a UAG nonsense codon located in the late T7 gene 16.

DNA repair in cultured mouse cells of increasing population doubling level

Cultures of mouse cells of various population doubling levels (PDL) were examined for DNA-repair capabilities as estimated by (i) the excision of pyrimidine dimers; (ii) unscheduled DNA synthesis (UDS) in response to UV-irradiation or N-methyl-N'-nitrosoguanidine (MNNG) treatment; (iii) the levels of two DNA-repair enzyme activities, uracil DNA glycosylase and AP endonuclease. The responses to ultraviolet light and MNNG decreased rapidly within the first two PDL and more slowly thereafter until essentially no repair was detected by PDL 12. A continuous cell line which emerged from the cultured cells after a crises period had some restoration of repair capability. The amount of uracil DNA glycosylase activity decreased by approximately 40% before the crises period then decreased by 90% in the continuous cell line. In contrast, the amount of AP endonuclease activity present in the precrises cells showed no significant change until PDL 12, then increased 6-7-fold in the continuous cell line.

Human lymphocytes resistant to 6-thioguanine increase with age

Using an autoradiographic technique we determined the number of circulating lymphocytes that were resistant to 6-thioguanine and which were presumably mutants at the hypoxanthine--guanine phosphoribosyl transferase locus. The number in normal individuals was found to increase exponentially with age. The data suggest a relationship between mutagenesis and ageing, perhaps by way of a decline with age in the fidelity of DNA replication or repair.

Error propagation in Escherichia coli and its relation to cellular ageing

The mistranslation of alkaline phosphatase may not provide a definitive measure of errors in Escherichia coli protein synthesis. beta-Galactosidase which, unlike alkaline phosphatase, is an intracellular enzyme exhibits different mistranslation kinetics. Previous conclusions based on alkaline phosphatase data and showing no relation between error propagation and ageing may require re-evaluation.

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.

Analysis of the translational capacity of the male accessory gland during aging in Acheta domesticus

The protein synthesizing capacity of the male cricket accessory gland was inspected for changes associated with aging by analysis of organs from young (2 weeks post-imaginal molt), middle-aged (4 and 6 weeks) and senescent (8 weeks) animals. Total RNA content and the percentage of ribosomes in polysomes show an increase up to 4 weeks and then a steady decline. The rates of protein synthesis by young and old glands incubated in vitro were comparable, although the old glands were significantly less efficient in precursor uptake and therefore showed lower incorporation levels. The overall picture appears to be one of a steady but unspectacular decline in protein synthesizing capacity in aging animals. The quality of the secretory proteins formed by young and old glands was inspected by SDS--polyacrylamide gel electrophoresis and by isoelectric focusing, using both wide and narrow pH-range gels. While some differences in the banding patterns of isoelectric focusing gels were detected, they were eliminated by the inclusion of Triton X-100 in the gels. No other evidence for age-associated qualitative or quantitative changes was detected, strongly suggesting that proteins from young and old glands are identical.

Error propagation in viable cells

Error propagation is the process, predicted by theoretical models, whereby errors in translating the genetic code will beget fresh errors in successive generations. It has been postulated that error propagation may underly the mortality of cells which display clonal senescence. We have demonstrated the occurrence of error propagation in viable cells of E. coli during growth in a low concentration of streptomycin, a drug which promotes ribosomal ambiguity. We monitored error propagation by measuring mistranslation of a specific UAA codon, and measured viability by direct enumeration of both live and dead cells through a sensitive microscopic technique. We find that the error frequency may be artificially increased by at least an order of magnitude without generating any detectable increase in the proportion of dead cells or of cells whose descendents are doomed to clonal senescence. The error frequency increases gradually over the course of a few generations, in qualitative agreement with the notion of error propagation, and eventually stabilizes at a constant value much higher than normal. The kinetics of this increase agree quantitatively with the Hoffman-Kirkwood and Holliday formulation of error propagation, for parameter values which dictate convergence to a stable error frequency. This convergent behaviour, under conditions of enhanced mistranslation, demonstrates that the normal parameters are well removed from the region of instability in error propagation; even an order of magnitude increase in mistranslation does not tip the translation system into the unstable mode which has been postulated to underly cell senescence. Thus, the error catastrophe theory of cell senescence cannot apply to the translation system of bacteria. We have reviewed experimental data on the fidelity of translation in somatic cells of higher organisms which militate against the notion that the translation system in these cell types could be much closer to the region of instability than in bacteria. These considerations controvert the error catastrophe theory of cell senescence.

Evidence for an increase in presumed somatic mutation during the ageing of human cells in culture

A method has been developed for the direct measurement of genetic variants in mammalian cells in culture which does not require cell division and is therefore suitable for ageing studies. Previous evidence suggests that these variants are mutations. The variant frequency was measured during the lifespan of populations of MRC-5 human embryo lung fibroblasts. The frequency increased substantially during the lifespan of the culture and regression analysis shows that the increase is exponential. No increase invariant frequency was found in a immortal transformed line cultured for over 100 passages. Evidence is presented that the variants are regulatory mutations. This is the first direct evidence for the involvement of presumed somatic mutations in the ageing of mammalian cells. 5-Fluorouracil but not p-fluorophenylalanine shortened the life of the cells and increase the variant frequency. The data lend support to a model in which mutations are an expected consequence of errors in protein synthesis.

Mutation selection and tumour progression

It is proposed that tumour progression results from the increased mutability of tumour cells and that this mutability leads to the acquisition of a large number of 'neutral' mutations having no significant effects under physiological conditions but capable of lethal expression under different conditions. High temperatures often lead to expression of neutral mutations and this could provide a mechanism for the observed thermosensitivity of tumour cells, and a rational basis for the use of hyperthermia in cancer treatment.

Altered nuclear deoxyribonucleic acid alpha-polymerases in senescent cultured chick embryo fibroblasts

DNA alpha-polymerase has been partially purified from nuclei of cultured chic, fibroblasts and separated on phosphocellulose columns into two distinct activities designated DNA polymerases alpha(a) and alpha(b), respectively. The enzyme preparations were devoid of activities of DNA beta,gamma-polymerases terminal deoxyribonucleoside transferase, DNase, DNA-dependent RNA polymerase, and phosphatase. DNA polymerases alpha(a) and alpha(b) both having molecular weights of 160 000, constitute 35-50 and 65-50%, respectively, of the activity of alpha-polymerase in the nucleus. These enzymes differ in their requirements for maximal activity, their relative ability to copy oligo(dG)-poly(dC), their response to ribonucleoside triphosphates, and their kinetics of heat inactivation. When the properties of alpha polymerases derived from early or late passage cultures have been compared, no difference could be detected as a function of cell age in the specific activities of the polymerases in crude cell extracts, their chromatographic behavior on diethylaminoethylcellulose and phosphocellulose columns, and their relative abilities to utilize single deoxyribonucleoside triphosphates with activated DNA template. On the other hand, both enzymes become partially heat labile in aging cells. Also, the activity of DNA polymerase alpha(a) from young cells was stimulated by 2--10 mM adenosine or cytidine triphosphates, whereas the same enzyme from old cultures was inhibited by these agents. Conversely, these ribonucleoside triphosphates inhibited the activity of polymerase alpha(b) in young cells but slightly stimulated this enzyme derived from senescent fibroblasts. In addition, the relative ability of DNA polymerase alpha(a) to copy oligo(dG)-poly(dC) decreased in aged cells, whereas that of DNA polymerase alpha(b) increased. We have also observed significant differences in the effects of potassium chloride and N-ethylmaleimide on the activity of DNA polymerase alpha(a) from old cells as compared to young cells. These age-related alterations in the properties of the two avian DNA polymerases may reflect structural or conformational changes in these enzymes.

Decreased fidelity of DNA polymerase activity isolated from aging human fibroblasts

DNA polymerase (deoxynucleosidetriphosphate: DNA nucleotidyltransferase, EC 2.7.7.7 or DNA nucleotidyltransferase) activity, isolated from late and early passage cells of the diploid human fibroblast line, MRC-5, was compared. The level of activity dropped with increasing passage. In addition, when the fidelity of polymerization was monitored with four synthetic templates under a variety of conditions, it was observed that the enzyme from late passage cells was more error-prone. The possible relation of these observations to "senescence" of the fibroblasts is discussed.

Ageing of Neurospora crassa. IV. Induction of senescence in wild type by dietary amino acid analogs and reversal by antioxidants and membrane stabilizers

The extensional growth rate of wild-type 74A8 N. crassa in the presence of various concentrations of 19 amino acid analogs was measured. Seven analogs were not inhibitory at concentrations in the range of one to 10mM. Of the remaining 12 analogs, nine inhibited growth in a novel way. The kinetics of growth in the presence of these analogs at 30 degrees were characterized by seven sequential phases: (1)lag; (2) acceleration of growth rate; (3) steady-state growth rate; (4) exponential rate of decline of growth rate; (5) no growth or growth rate less than or equal to 0.1 mm h-1; (6) accleration of growth rate; and (7) steady state. At 33 degrees, phases 6 and 7 did not occur and irreparable death of the clones occurred. The mechanism by which the clones acquired resistance at 30 degrees appeared to involve a combination of physiological adaptation and cellular selection. Dietary application of either free radical scavengers or surface-active membrane 'stabilizers' alleviated or prevented the inhibition and deterioration of growth rate which occurred in the presence of the nine amino acid analogs. Culture with either 4-fluorophenylalanine or ethionine led to an increase of the activities of antioxygenic enzymes glutathione peroxidase, glutathione reductase and superoxide dismutase. The amino acid analogs that cause senescence and death of growing cells are known to be incorporated into proteins and such proteins are generally abnormal. Because a substantial fraction of cellular protein occurs in membranes and the proteins synthesized by mitochondria are exclusively intrinsic membrane proteins, we suggest that a primary consequence of errors in protein synthesis is the production of faulty membranes. The deterioration of such membranes with associated lipid autoxidation and free radical production proceeding as a chain reaction at an exponential rate may in itself contribute to the exponential rate of cellular deterioration which is characteristic of the ageing process. According to this hypothesis, dietary membrane stabilizers, free radical scavengers and antioxygenic enzymes protect cells from error catastrophy arising from the chain of events leading from membrane deterioration.

Ageing of Neurospora crassa. I. Evidence for the free radical theory of ageing from studies of a natural-death mutant

A recessive mutant of Neurospora crassa, called natural-death, is characterized by a decreasing clonal growth potential under all nutritional conditions and the irreversible cessation of growth. The primary molecular defect of this mutant is not known. Evidence presented here, based upon measurements of the activities and thermolabilities of several enzymes, suggests that faulty protein synthesis is probably not a cause of the senescence and death of the mutant, as suggested by Lewis and Holliday (Nature, 228 (1970) 877). Three lines of evidence indicate that lipid autoxidation and associated free radical reactions contribute to the senescence and death of this mutant: (1) The relative times before the onset of senescence and death of mutant clones in the last 40% of their chronological life-span were prolonged 2 to 3-fold by either dietary antioxidants or selenite and the total life-span was increased by 40% to 80%. These compounds also alleviated the senescent morphology and enhanced biomass production; (2) Senescing clones accumulated a green fluorescent pigment in situ, but dietary antioxidant nordihydroguaiaretic acid prevented this accumulation. The fluorescent pigment exhibited the spectral properties of lipofuscin, an end product of lipid autoxidation; (3) Relative to wild type, mycelial extracts of the mutant exhibited a 2 to 4-fold excess of activities of the antioxygenic enzymes superoxide dismutase, glutathione peroxidase and glutathione reductase. We briefly review: (1) the roles of antioxygenic enzymes and antioxidants in their protection against cellular damage from lipid autoxidation and free radical reactions; and (2) the major lines of evidence which appear to support a form of the free radical theory of ageing, encompassing the interrelated processes of membrane deterioration, lipid autoxidation and deleterious free radical reactions as the major causes of cellular deterioration.

Evidence contrary to the protein error hypothesis for in vitro senescence

A strain of diploid fibroblasts, obtained from the skin of a male infant, was cultured in vitro and cells were tested throughout their lifespan for the appearance of altered glucose-6-phosphate dehydrogenase (G-6-PD) detected either by thermostability studies or by immunotitration. No significant difference was found in the proportion of thermolabile enzyme in 31 young cultures (4.8 +/- 1%, S.E.), in comparison with that in 19 old cultures (4.9 +/- 1%, S.E.). Old cultures had ceased active cell division (49-60 doublings); DNA replication, measured by [3H]thymidine uptake over a period of 24 hours, was limited to less than 5% of these cells. Young cells (5-22 doublings) had a [3H]thymidine labeling index of 75-85%. Titration of G-6-PD activity in extracts of young and old cells with neutralizing antibody directes specifically against G-6-PD failed to detect an increment of enzymatically defective G-6-PD in old cells. The thermostability studies were capable of detecting altered G-6-PD in skin fibroblasts from a female heterozygous for a thermolabile mutant of G-6-PD, and in fibroblasts treated with a proline analogue, azetidine carboxylic acid. The immunotitration technique was also capable of detecting catalytically altered G-6-PD from the thermolabile mutant and G-6-PD inactivated with N-ethylameimide. These findings argue against a protein error catastrophe as the cause of in vitro clonal senescence.

A rapid rise in cell variants during the senescence of populations of human fibroblasts

An attempt has been made to measure the frequency of mutations throughout the lifespan of human fibroblast strain MRC-5. A novel procedure has been used which involves staining individual cells for high levels of glucose-6-phosphate dehydrogenase. Evidence is presented that this phenotype is due to mutation. The frequency of variants was scored from passage 16 until the final phase of senescence (passage 60). There is an exponential increase of stained cells throughout this period. The results are in agreement with the general error theory, which proposes that aging is due to a breakdown in the fidelity of information transfer between macromolecules.

Age-related changes associated with the induction of o-pyrocatechuic acid carboxylase in Aspergillus ornatus

Mycelial mats of Aspergillus ornatus grown on cellulose xanthate membranes placed on a defined agar medium showed o-pyrocatechuic acid carboxyalse activity which could be induced to over six times its basal level by the addition of 0.1% L-tryptophan to the medium. Exposure of mature mycelial mats to cyclohexamide prior to induction indicated that de novo protein synthesis was required for this induction to occur. The effect of age of this enzyme adaptation process was investigated by transferring membranes and mats of variuos ages from defined medium to fresh defined medium plus 0.1% L-tryptophan With increasing mycelial age, the basal enzyme activity level remained relatively unchanged while the level to which the enzyme could be induced decreased significantly. This decrease was linear when plotted against the mean cell age of mycelium, conforming to a general model of age-dependent modifications of the enzyme adaptation process proposed by Adelman. Several possible explanations of this phenomenon are discussed.