Cell-free protein synthesis by kidney from the aging female Fischer F344 rat

The effect of aging on the genetic expression of renin by mouse kidney

The effect of aging on the genetic expression of renin was studied in kidney tissue by measuring renin mRNA levels. RNA was isolated from the kidneys of 5- to 37-month-old male C57BL/6J mice. The relative levels of the renin mRNA were measured by dot blot hybridization using a cDNA probe to renin. The size of renin mRNA as determined by northern blot analysis was found to be 1.6 Kb. The size of renin mRNA did not change with increasing age. However, the levels of renin mRNA in kidney RNA decreased 70% after 12 months of age.

Protein synthesis and the components of protein synthetic machinery during cellular aging

The slowing down of protein synthesis is a change widely observed during the aging of organisms. It has also been claimed that a decline in the rate of protein synthesis occurs during cellular aging. However, the evidence in favour of this view is not clear-cut, and reliable estimates of rates of protein synthesis during cellular aging have yet to be made. Studies on various components of the protein synthetic machinery during cellular aging have revealed a decline in the efficiency and accuracy of ribosomes, an increase in the levels of rRNA and tRNA, and a decrease in the amounts and activities of elongation factors. Detailed studies on the structure and function of ribosomes, tRNA isoacceptor profiles, activities of aminoacyl-tRNA synthetases, levels and activities of initiation factors, rates of protein elongation, and the accuracy of protein synthesis will be needed before the molecular mechanisms of the regulation of protein synthesis during cellular aging can be understood.

Alteration of aminoacyl-tRNA synthetase with age: heat-labilization of the enzyme by oxidative damage

Active oxygens have been suggested to be involved in age-related alterations of organelles and molecules. In this study we investigated the influence of active oxygen on aminoacyl-tRNA synthetases partially purified from rat liver. Treatment of leucyl-tRNA synthetase with Fe3(+)-ascorbate resulted in the increased heat-lability of the enzyme. The inactivation was inhibited by radical scavengers such as mannitol and benzoate, suggesting that hydroxyl radicals are responsible for heat-labilization of the enzyme. On the other hand, a considerable part of tyrosyl-tRNA synthetase was converted to heat-labile forms without added iron and ascorbate under aerobic conditions but not under anaerobic conditions. These and other findings suggested that the heat-labilization of this enzyme is caused by active oxygens probably generated by the reaction of dioxygen and transition metal ions present in the enzyme preparations. Heat-inactivation curves of the enzymes modified as described above were similar to those observed for the enzymes from aged animals in that these enzymes exhibited higher percentages of heat-labile forms than the unmodified enzymes from young animals [Takahashi and Goto, 1987, Arch. Gerontol. Geriatr. 6, 73-82; Takahashi and Goto, 1987, Arch. Biochem. Biophys. 257, 200-206]. The present findings are consistent with the theory that active oxygens are involved in the age-related alterations of enzymes.

Fidelity of aminoacylation by rat-liver tyrosyl-tRNA synthetase. Effect of age

The possibility of change in the rate of misrecognition of amino acids by rat liver tyrosyl-tRNA synthetase during aging was investigated. Frequency of misrecognition of phenylalanine vs tyrosine was determined at two temperatures by competitive assay using partially purified enzymes. At 25 degrees C, the error frequencies were 5.17 x 10(-8) and 8.24 x 10(-8) in young and old animals, respectively. These values are much below the reported error frequencies for the prokaryotic enzymes: i.e. approximately 5 x 10(-6). Although the fidelity of tyrosyl-tRNA synthetase from old animals appeared to be slightly lower, the difference was not statistically significant. At 37 degrees C, the error frequencies were increased 1.3-1.5-fold, but again the difference between young and old animals was not significant. To our knowledge, this is the first report in which fidelity of aminoacyl-tRNA synthetase of animals of various ages has been compared using natural amino acids.

Protective effect of sodium molybdate on the acute toxicity of mercuric chloride. V. Enhancement of renal regeneration after exposure to HgCl2

Pretreatment with Na2MoO4 protected rats from HgCl2-induced decreases in the renal concentration of amino acids, RNA, DNA, ATP and dry matter. It also reduced the mercury-induced increases in renal water, Ca and serum creatinine. Ma2MoO4 considerably elevated the RNA/DNA ratio in the renal cortex after treatment with HgCl2. In addition, subcellular distribution of mercury was markedly altered by pretreatment with Na2MoO4, specifically Na2MoO4 pretreatment decreased the mercury content in the particulate fractions such as the nuclei and mitochondria while increasing the mercury content of the cytosol. Sephadex G-75 gel filtration showed that the increase in mercury content in the cytosol of Na2MoO4-pretreated rats is due to an increase in the metal content of a metallothionein-like fraction. These results suggest that Na2MoO4-pretreatment protects against HgCl2 renal toxicity by stimulating mercury-mediated metallothionein induction in the renal cortex and renal regenerative processes.

Aging in the AXC/SSh rat: characterization of moderately abundant ventral prostate proteins showing age-dependent diminution and one protein exhibiting age-invariant content

To determine whether prior demonstrations of age-related decrements in prostate content of minor, androgen regulated proteins represent a generalized phenomenon, we validated a denaturing polyacrylamide gel electrophoretic protocol for separation and quantification of moderately abundant ventral prostate cytoplasmic proteins. We established age-related, progressive 3- to 3.5-fold decreases in prostate content of proteins of 90, 79, 63, and 58 kDa and found that content of a 46 kDa protein was age-invariant. The amount of 90 and 46 kDa proteins was not significantly altered, whereas the level of 79, 63 and 58 kDa proteins decreased during 72 h post-orchiectomy of 3-month-old rats. Testosterone injection of intact 26-month-old rats caused an average 2-fold increase in 90, 79, 63, and 58 kDa protein content and did not affect 46 kDa protein level. Because we demonstrated the 46 kDa protein is not a secretory protein, absence of an affect of aging or testosterone on prostate content is not due to secretion mediated inaccessibility to intracellular processing. The apparent relation between age and prostate content of these proteins is not a consequence of potential age-related changes in ventral prostate cell content or distribution because biochemical and histologic analyses show this does not significantly occur. Our studies establish age-related decreases in ventral prostate content of moderately abundant, androgen responsive proteins and show that content of at least one protein is age- and androgen-independent. It remains to be determined whether these findings reflect direct effects of gene regulation.

Protein synthesis by liver ribosomes from aged rats

The age-related decrease in protein synthesis by cell-free systems has been traced to a factor which can be obtained by high salt extraction of young polysomes. Such extracts, when added to old ribosomes in young post-ribosomal supernate, stimulate the level of Poly(U)-directed protein synthesis. Extracts of old polysomes have essentially no effect. The deficient factor is not EF-2 and is highly unlikely to be EF-1, as this component resides almost entirely in the post-ribosomal supernates used in the reaction mixture. Since initiation factors are not necessary for Poly(U)-directed protein synthesis and EF-1 and EF-2 do not appear to be involved, the nature of the soluble factor which is deficient in old ribosomes appears to lie outside of proteins which are commonly implicated in the age-related slowing of protein synthesis.

Functional modification of rat liver ribosomes by the in vitro action of N-methyl-N'-nitro-N-nitrosoguanidine

The mechanism by which N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) inhibits protein synthesis has been studied in a rat liver cell free system. Using preformed aminoacyl-tRNA it was observed that incorporation of amino acid into polyribosomal protein was inhibited in the presence of low concentration of MNNG. This inhibition was not reversed by increasing the concentration of soluble factors. Transfer RNAs modified previously by treatment with MNNG and subsequently esterified with amino acids were transferred to polyribosomes with the same efficiency as those species which were not modified. Polyribosomes, on the other hand, lost activity to incorporate amino acids after pretreatment with MNNG. This inactivation was dependent on the concentration of MNNG with which polyribosomes were treated. When poly(U) was used with MNNG-treated polyribosomes, its translation, after correction for endogenous translation, was also found to be significantly low as compared to the case with untreated polyribosomes. Purified ribosomes stripped of endogenous mRNA when treated with increasing concentrations of MNNG progressively lost ability to support polyphenylalanine synthesis programmed by poly(U). The treated ribosomes, however, neither inhibited the activity of control ribosomes nor induced any loss of fidelity of translation by poly(U). It is concluded that MNNG inhibits protein synthesis through functional inactivation of ribosomes resulting from direct modification of ribosomal proteins possibly involving nitroguanidination of lysine residues.

Effect of age and dietary restriction on protein synthesis by isolated kidney cells

The influence of age and food restriction on kidney protein synthesis was studied in Fischer F344 rats. The rate of total protein synthesis by suspensions of kidney cells declined 60% between 4 and 31 months of age. The rate of protein synthesis by kidney cells isolated from 19-month old rats fed a restricted diet (60% of diet consumed by rats fed ad libitum) was 45% higher than the rate of protein synthesis by kidney cells isolated from 19-month old rats fed ad libitum. The excretion of protein in the urine was measured to assess the effect of the age related decline in protein synthesis on kidney function. A dramatic increase in proteinuria was observed with increasing age, and rats fed the restricted diet excreted significantly less protein in the urine than rats fed ad libitum.

Alteration of aminoacyl tRNA synthetases with age: accumulation of heat-labile enzyme molecules in rat liver, kidney and brain

Age-related changes of aminoacyl tRNA synthetases were investigated in tissues from 3- to 24-month-old rats. The proportions of heat-labile enzymes in the liver, kidney and brain were 0-20% in young animals, but 15-40% in old animals. The proportions of heat-labile enzymes increase from about 15 months of age when the probability of death also increases greatly. These findings suggest that decrease in functional activities of various tissues in old animals is in some way related to the accumulation of these altered proteins.

Age-related changes in turnover and concentration of a subset of thorax polypeptides from Drosophila melanogaster

Six polypeptides from Drosophila melanogaster were analyzed by two-dimensional polyacrylamide gel electrophoresis for age-related changes in protein turnover and protein concentration. The protein samples, from thoraces of 6 day (young), 26 day (middle) and 45 day (old) flies, were compared using autoradiography for [35S]methionine labeled proteins and silverstaining for unlabeled preparations. The combination of these two techniques has permitted determination of the relative turnover rate as well as the steady-state concentration of these proteins as a function of age. The autoradiographs reveal that all of the analyzed proteins decrease in turnover rate whereas the absolute protein concentrations, as determined from the silverstained gels, show a decrease for three proteins, no change for two, and an increase in one protein with age. These findings show a remarkable quantitative heterogeneity of ageing changes in proteins, but absence of alterations in the fidelity of the translation of these polypeptides.

Translation fidelity in the aging mammal: studies with an accurate in vitro system on aged rats

The accuracy of poly(U) translation was measured in the post-mitochondrial supernatant from whole brain of 7- and 33-month-old Fischer 344 rats. Measurements were made: under in vitro conditions in which translation fidelity was similar to what is known about the accuracy of translation in vivo; and under stresses of varying Mg2+ concentrations (3-12 mM), pH (6.6-8.4), temperature (26-42 degrees C) and in the presence or absence of 2.4% ethanol. No significant difference could be detected between the responses of old and young extracts, the activities of their Phe- and Leu-tRNA synthetases, and their endogenous amounts of Phe-tRNA and Leu-tRNA, despite the fact that the rats studied corresponded in age (by actuarial criteria) to 90-year-old human beings. The accuracy of poly(U) translation was also studied: in liver and hippocampus extracts from 7- and 33-month-old rats; and in brain extracts from 3- and 29-month-old rats. The results were similar to those obtained in brain extracts from 7-month-old rats. Explanations are provided for the inconsistencies which exist in the literature regarding the effect of aging on the accuracy of protein synthesis. It is shown that the inconsistencies are likely to reflect inadequate methodology in three previous studies rather than biological diversity in the control of translation fidelity in aged animals.

Effect of molybdenum on the acute toxicity of mercuric chloride. IV. Effect of molybdenum on mercury-mediated metallothionein mRNA induction

In order to elucidate the mechanism of the stimulative effect of molybdenum on mercury-mediated renal metallothionein induction, the levels of translatable metallothionein mRNA (MT mRNA) in the kidneys of rats treated with saline or Na2MoO4 or HgCl2 or Na2MoO4 and HgCl2 were measured by translation experiments in cell-free protein synthesizing systems. The time course of accumulation of mercury in renal nuclei of rats given HgCl2 with or without Na2MoO4-pretreatment was also investigated. Molybdenum, itself, did not elevate levels of MT mRNA compared to saline controls at all time points (0, 6 and 14 h after exposure to HgCl2) but rapidly elevated the levels of the mRNA more than Hg-dosed rats when HgCl2 was also administered. On the other hand, the time course study in renal nuclei showed that the mercury content of nuclei was consistently lower in Mo-Hg-dosed rats than in Hg-dosed rats at all time points (4, 8 and 24 h after exposure to HgCl2). These results suggest that the stimulative effect of molybdenum on mercury-mediated metallothionein induction is coupled with an increase of the mRNA coding for the low molecular weight protein and that such an increase in the levels of translatable MT mRNA is not due to the difference in uptake of mercury into renal nuclei.

Functional deterioration of mouse liver ribosomes during aging: translational activity and activity for formation of the 47 S initiation complex

The translational activities of ribosomes from young and old mouse livers were examined in an assay system dependent on rabbit globin mRNA. Old ribosomes showed 30-40% lower activity than young ribosomes. This observation, together with our previous findings (N. Mori, D. Mizuno and S. Goto, Mech. Ageing Dev., 10 (1979) 379-398), suggests functional deterioration of ribosomes of old animals. To examine the mechanism of the deterioration, the activities for formation of the initiation complex of 40 S ribosomal subunits in the livers of young and old mice were examined in vitro. The activity was found to be 15-20% lower in old mice (21 months old or more) than in young ones. This fact partly explains the decreased activity for ribosomal protein synthesis in old mice.

mRNA population in the liver, kidney and brain of young and senescent mice: analysis of in vitro translation products

Possible alterations in the population of poly(A)(+)mRNA during ageing were investigated by translation in vitro of poly(A)(+)mRNA from the liver, kidney and brain of male ddY mice of different ages. [35S]Methionine-labeled translation products were analysed by two-dimensional polyacrylamide gel electrophoresis followed by fluorography. A protein product with a molecular weight of 30 000 and isoelectric point of 6.5 was reproducibly observed only in the fluorograms of translation products of poly(A)(+)mRNA derived from the livers of senescent mice (24.5 months old). However, no age-related change was detected in the translation products of the kidney and brain. These results suggest that gene expression in liver cells changes at the level of the population of cytoplasmic poly(A)(+)mRNA during ageing.

Age-related changes in different steps of protein synthesis of liver and kidney of rats

Protein synthesis in cell-free systems of rat liver and kidney decreases markedly with age. Examination of activity changes of the different steps revealed for both types of organs that reduced binding of aminoacyl-tRNA to ribosomes and reduced peptidyl transfer might be of major importance for the decrease in overall protein synthesis whereas ageing has only little effect on translocation as well as on initiation and termination.

Codon recognition fidelity of ribosomes at the first and second positions does not decrease during aging

We have developed a novel method to examine the translational fidelity of mammalian ribosomes in vitro, where protamine mRNA was used as a template. This method enabled us to determine frequency of misrecognition of purine bases at the second position of arginine codons (AGR/AAR) in the mRNA. Using this method the fidelity of translation of ribosomes derived from mouse livers was found to remain unchanged from 2 to 29 months, the maximum life span of the animal. This conclusion is not consistent with the "error catastrophe" theory of aging. This is the first report in which translational fidelity of ribosomes of animals of various ages has been compared by an in vitro translation of a natural mRNA.

Decreased rates of protein synthesis by cell-free preparations from different organs of aging mice

Protein synthetic activity was determined in postmitochondrial preparations from heart, brain, kidney, liver and skeletal muscle of 5-26-month-old female C57B1/6J mice. An age-dependent decrease in the rate of protein synthesis was exhibited by all preparations except heart muscle. A 65% decrease in translational rate was found in liver, with the greatest decrease appearing after 21 months. Translation in the brain preparation declined little during the first 20 months, but dropped 33% between 20 and 26 months. The kidney preparation decreased 30% during the first 16 months and 70% by the end of 26 months of age. Skeletal muscle showed an overall decrease of 85% in translation rate. In contrast, heart muscle decreased no more than 10% over the life-span of the mice. From these results, it appears that aging has a differential effect on protein synthesis in different kinds of cells.

The complement of cytoplasmic tRNAs, including queuosine-containing tRNAs, in adult and senescent Wistar rat liver and their levels of aminoacylation

Our previous studies showed that both total cytoplasmic tRNAs and aminoacyl-tRNA synthetases isolated from senescent (24-30 month) female Wistar rat liver were less capable of supporting cell-free protein synthesis than were the same fractions isolated from adult (10-13 month) rat liver. The present study investigates the molecular basis for this age-related result. No significant age-related differences were found in the extent of aminoacylation of the liver cytoplasmic tRNA population, the total tRNA synthetase activity, the rate of aminoacylation of individual tRNAs, or in the overall complement of tRNA species as detected by two-dimensional gel electrophoresis. In homologous senescent aminoacylation assays, consisting of tRNAs and tRNA synthetases from senescent animals, alanine, arginine and aspartic acid were charged to a greater extent and methionine to a lesser extent compared to homologous adult assays. In heterologous assays, adult synthetases were significantly more active than senescent synthetases when charging isoleucine, methionine, phenylalanine, proline and glutamic acid, and less active when charging alanine, aspartic acid and serine. Also, senescent synthetases charged both adult and senescent tRNAs with methionine to a lesser extent than did adult synthetases. In homologous senescent assays with queuosine-containing tRNAs, asparagine, aspartic acid and histidine were charged to a greater extent and tyrosine to a lesser extent compared to homologous adult assays. Results with queuosine-tRNAs are discussed in terms of their potential ability to lower the efficiency of translation in senescent liver.

Organ pattern of age-related changes in the aminoacyl-tRNA synthetase activities of the mouse

The specific activities of 17 aminoacyl-tRNA synthetases from liver, lung, heart, spleen, kidney, small intestine and skeletal muscle of young (2 months) and aged (39 months) female Han:NMRI mice were determined under standard conditions of sample preparation and assay. The average reduction of total activity during ageing is 70% for liver, 50% for lung and spleen, nearly 40% for heart and kidney and nearly 20% for intestine and skeletal muscle. Detailed comparison reveals no general, but an organ-specific pattern. Aminoacyl-tRNA synthetases were, furthermore, found to be ribosome-associated in higher proportions in liver tissue from aged mice.

Current concepts: I. The relationship between age-related changes in gene expression, protein turnover, and the responsiveness of an organism to stimuli

A general decline in gene expression, translation and transcription, has been observed to occur with increasing age in a wide variety of organisms and tissues. Because the level of most enzymes and proteins remains relatively constant with increasing age, one would predict that the decline in gene expression would result in an age-related decline in protein turnover. Recent studies show that protein turnover in mouse liver and nematodes declines with increasing age. The decline in protein turnover could lead to an age-related decrease in the response of inducible enzymes to stimuli. This could explain the molecular basis for the decline in aging organisms to respond to a variety of environmental factors.