Ribosomes from aging mice are not generally deficient in cell-free protein synthesis

Strategies and criteria for the development of molecular biomarkers of senescence

While it may be possible to employ panels of molecular parameters which correlate with senescence in vivo or in vitro, in a manner analogous to the use of mutagenesis assays for economic carcinogen screening, such an endeavor would at present be impeded by the absence of a clear mechanistic rationale for focusing on particular biomarkers, and by the complexity of the senescent phenotype and its multilevel interactions. Nevertheless, insight into the mechanism(s) of senescence may derive indirectly from correlative studies, or directly from strategies of molecular intervention, provided that such studies meet reasonable criteria for relevance and functionality. Even the control of expression of a single gene may be quite complex, with multigenic interactions and the potential to produce a cascade affecting many downstream genes. In order to understand such processes, functional assays and selective systems will need to be developed.

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.

Influence of fasting and refeeding on the elongation step of protein synthesis in rat liver of young (3M) and aging (18M) animals

The in vitro binding capacity for AA-tRNA of ribosomes, catalysed by EF1 enzyme, was studied in livers of rats, which had been starved for 5 days and re-fed for 3, 9, and 24 hours. The experiments were carried out with 3 and 18-month-old rats. The results were compared to those obtained with normally fed rats of the same ages. For both age groups, there was a similar change in the binding capacity, including a return to normal values after 24 hours of refeeding. When ribosomes of old control animals were treated with different enzymes, we obtained lower binding activities for all the experimental groups than in homologous cell-free systems (ribosomes and EF1 isolated from the same group). The ribosomes of young animals, tested in the presence of different enzymes, display an increased binding capacity, when the enzymes used were those of old (control and re-fed) animals. The recovery of the catalytic activity of EF1 on the binding process was observed after 9 hours in old animals, whereas it was not seen for young re-fed animals at that time of refeeding. EF1 seems to play an important role in the adaptation of older animals to fasting and refeeding.

The content and size distribution of membrane-bound and free polyribosomes in mouse liver during aging

The method of Ramsey and Steele [Anal. Biochem., 92 (1979) 305--313] was used to examine the size distribution of membrane-bound and free polyribosomes from the liver of male C57 BL/6 mice over the life span. Optimization of the concentration of Mg2+ and liver cell sap suppressed breakdown by ribonuclease and presumably gave preparations that approximate the integrity of native polyribosome populations. The content of polysomes per unit liver tissue from 7 groups of fed mice aged 10--35 months showed an age-related increase of subunits and monomers (+54%) and dimers-to-pentamers (+76%) in membrane-bound polyribosomes. The dimer-to-pentamer class of free polyribosomes also increased (+52%). Polysomes larger than nonamers showed 13% and 21% decreases that were not statistically significant. Total tissue ribosomes increased 15%, while the membrane-bound/free polyribosome ratio remained nearly constant at about 1.15. In a subsequent study, both 6 h-fasted and fed mice showed decreases (-20% to -33%) in membrane-bound and free polyribosomes larger than nonamers during aging from 15 to 35 months. The dimer-to-pentamer class of free polyribosomes in fed mice increased (+26%), while this class in the other groups underwent increases (+26 to +39%) that were not statistically significant. We conclude that the liver in old mice is not obviously deficient in either quantity or general quality of ribosomes. Old animals do tend to show an increase in small polysomes and a decrease in large polysomes, which is consistent with a reduction in the rate of translation.

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.

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.