The effect of age on the initiation of protein synthesis in Drosophila melanogaster

What do we mean by "aging"? Questions and perspectives revealed by studies in Drosophila

What is the nature of aging, and how best can we study it? Here, using a series of questions that highlight differing perspectives about the nature of aging, we ask how data from Drosophila melanogaster at the organismal, tissue, cellular, and molecular levels shed light on the complex interactions among the phenotypes associated with aging. Should aging be viewed as an individual's increasing probability of mortality over time or as a progression of physiological states? Are all age-correlated changes in physiology detrimental to vigor or are some compensatory changes that maintain vigor? Why do different age-correlated functions seem to change at different rates in a single individual as it ages? Should aging be considered as a single, integrated process across the scales of biological resolution, from organismal to molecular, or must we consider each level of biological scale as a separate, distinct entity? Viewing aging from these differing perspectives yields distinct but complementary interpretations about the properties and mechanisms of aging and may offer a path through the complexities related to understanding the nature of aging.

Protein translation paradox: Implications in translational regulation of aging

Protein translation is an essential cellular process playing key roles in growth and development. Protein translation declines over the course of age in multiple animal species, including nematodes, fruit flies, mice, rats, and even humans. In all these species, protein translation transiently peaks in early adulthood with a subsequent drop over the course of age. Conversely, lifelong reductions in protein translation have been found to extend lifespan and healthspan in multiple animal models. These findings raise the protein synthesis paradox: age-related declines in protein synthesis should be detrimental, but life-long reductions in protein translation paradoxically slow down aging and prolong lifespan. This article discusses the nature of this paradox and complies an extensive body of work demonstrating protein translation as a modulator of lifespan and healthspan.

Caloric restriction and lifespan: a role for protein turnover?

Oxidative damage to cellular macromolecules has been postulated to be a major contributor to the ageing of diverse organisms. Oxidative damage can be limited by maintaining high anti-oxidant defenses and by clearing/repairing damage efficiently. Protein turnover is one of the main routes by which functional proteins are maintained and damaged proteins are removed. Protein turnover rates decline with age, which might contribute to the accumulation of damaged proteins in ageing cells. Interestingly, protein turnover rates are maintained at high levels in caloric restricted animals. Whether changes in protein turnover are a cause or a consequence of ageing is not clear, and this question has not been a focal point of modern ageing research. Here we survey work on protein turnover and ageing and suggest that powerful genetic models such as the nematode Caenorhabditis elegans are well suited for a thorough investigation of this long-standing question.

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.

Age-dependent changes in proteins of Drosophila melanogaster

Several molecular theories of aging postulate that there are age-dependent changes in gene expression and that these changes contribute to the reduction in the viability of senescent cells. High-resolution, semiautomated, quantitative two-dimensional gel electrophoresis of many soluble proteins was used to test this hypothesis in Drosophila. Two-dimensional protein gel patterns were analyzed for each of three age groups of [(35)S]methionine-labeled adult male Drosophila melanogaster, which, except for their spermatocytes, consist entirely of fixed postmitotic cells. Seven relatively abundant polypeptides expressed in middle-aged (28-day-old) flies were absent in both young(10-day-old) and old (44-day-old) flies. Quantitative analyses of an additional 100 polypeptides were carried out by computer-assisted microdensitometry of fluorograms of the gel preparations. These analyses revealed a significant age-related heterogeneity in the quantitative distribution of radiolabel in these proteins. The data indicate that the qualitative pattern of gene expression is identical in young and old flies, but that profound quantitative changes occur in the expression of proteins during the Drosophila life-span.

Age dependent changes in the expression of Drosophila mitochondrial proteins

A subset of 43 [35S]methionine labeled mitochondrial proteins, identified on two dimensional electrophoresis gels of whole body extracts of adult Drosophila, were compared between two age groups. These flies, which except for their spermatocytes consist entirely of postmitotic cells, were 6 days old and 38 days old, with the mean life span of this inbred strain being 32.5 days. Qualitative changes in any of these mitochondrial polypeptides were not found, but significant quantitative differences were observed. Quantitation of the 43 mitochondrial proteins was carried out by computer assisted microdensitometry of autoradiograms of the gel patterns. The total amount of isotope incorporated into the mitochondrial proteins of the senescent flies decreased to 71% of that of the young insects. This decrease was heterogeneously distributed among the 43 proteins; however, six remained unchanged and one protein (mol. wt = 75; pI = 7.4) was increased 2.4 times in the senescent flies. These data clearly provide evidence that the age-dependent changes in the expression of mitochondrial proteins are quantitative and not qualitative.

Heterogeneity of protein-synthesis initiation factors in developing and aging rat brain

Protein synthesis initiation factors from rat brain were assayed in vitro through the formation of the initiator Met-tRNA X initiation factor 2 X GTP ternary complex. The initiation factor showed different intrinsic activity with increase in age. This difference was elicited by additions of high-salt ribosomal-wash protein to incubation mixtures that were already saturated with protein preparations from brains of older animals. This qualitative difference was further documented by examining the sensitivity of the activities of the initiation factor to spermidine and to temperature. The sensitivity to these effectors varied with age. By fitting an exponential decay model to data from the temperature experiments, it was possible to demonstrate that the preparations of the initiation factor from older brains behaved as a multicomponent system. The brain preparations from older animals contained at least two subpopulations of initiation factor. The fractions of these two gross subclasses varied, inversely one to the other, with age.

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.

Effect of age on peptide chain initiation and elongation in preparations from brain, liver, kidney and skeletal muscle of the C57B1/6J mouse

The effects of age on the initiation and elongation stages of protein synthesis were measured in cell-free preparations from brain, liver, kidney and skeletal muscle of young (3-5 months) and senescent (23-27 months) female C57B1/6J mice. The ability to form initiation complexes from isolated 40 S and 60 S ribosomal subunits decreased only slightly with age. In contrast, the rate of peptide chain elongation decreased by 67% in brain preparations, 80% in liver, 81% in kidney and 85% in skeletal muscle of the senescent mice when compared with the young mice.

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.

Decline in synthesis of elongation factor one (EF-1) precedes the decreased synthesis of total protein in aging Drosophila melanogaster

The decrease in the rate of protein synthesis in aging adult Drosophila melanogaster was found previously to be due, to a great extent, to a drop in the rate of peptide chain elongation, and principally to lowered activity of elongation factor one (EF-1). This decrease does not appear to be caused by appearance of an inhibitor of peptide chain elongation. Instead, the synthesis of EF-1 declines markedly early in adult life. This decrease is followed by lowered activity of EF-1 and by a drop in the synthesis of most of the cellular proteins.

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.

Effects of age on the post-initiation stages of protein synthesis

The peptide chain elongation stage of protein synthesis in Drosophila melanogaster was found to decrease markedly with age. The decrease paralleled the age-related decrease in overall protein synthesis. In contrast, the termination stage showed little decrease until the organisms were very old. Of the three reactions that comprise peptide chain elongation, the binding of aminoacyl-tRNA to ribosomes decreased greatly with age, and the decrease paralleled that of peptide chain elongation and of overall protein synthesis. The peptidyl transfer reaction decreased moderately, and the translocation reaction exhibited no measurable decrease with age. Thus, decreased binding of aminoacyl-tRNA to ribosomes appeared to be a major contributor to the age-related decreases in peptide chain elongation and overall protein synthesis.