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

Paradoxical microRNAs: individual gene repressors, global translation enhancers

In mammalian cells, microRNAs regulate the expression of target mRNAs generally by reducing their stability and/or translation, and thereby control diverse cellular processes such as senescence. We recently reported the differential abundance of microRNAs in young (early-passage, proliferating) relative to senescent (late-passage, non-proliferating) WI-38 human diploid fibroblasts. Here we report that the levels of the vast majority of mRNAs were unaltered in senescent compared to young WI-38 cells, while overall mRNA translation was potently reduced in senescent cells. Downregulation of Dicer or Drosha, two major enzymes in microRNA biogenesis, lowered microRNA levels, but, unexpectedly, it also reduced global translation. While a reduction in Dicer levels markedly enhanced cellular senescence, reduction of Drosha levels did not, suggesting that the Drosha/Dicer effects on translation may be independent of senescence, and further suggesting that microRNAs may directly or indirectly enhance mRNA translation in WI-38 cells. We discuss possible scenarios through which Dicer/Drosha/microRNAs could enhance translation.

Mechanisms of aging and energy metabolism in Caenorhabditis elegans

Aging studies on diverse species ranging from yeast to man have culminated in the delineation of several signaling pathways that influence the process of senescent decline and aging. While understanding these interlinked signal-transduction cascades is becoming even more detailed and comprehensive, the cellular and biochemical processes they impinge upon to modulate the rate of senescent decline and aging have lagged considerably behind. This fundamental question is one of the most important challenges of modern aging research and has been the focus of recent research efforts. Emerging findings provide insight into the facets of cellular metabolism which can be fine-tuned by upstream signaling events to ultimately promote longevity. Here, we survey the mechanisms regulating aging in the simple nematode worm Caenorhabditis elegans, aiming to highlight recent discoveries that shed light into the interface between aging signaling pathways and cellular energy metabolism. Our objective is to review the current understanding of the processes involved and discuss mechanisms that are likely conserved in higher organisms.

Signaling pathways regulating protein synthesis during ageing

Ageing in many organisms, including humans, is accompanied by marked alterations in both general and specific protein synthesis. Protein synthesis is normally under tight control by a broad array of regulatory factors, which facilitate appropriate rates of mRNA translation. Are the wide changes in protein synthesis simply a corollary of the ageing process or do they have a causative role in senescent decline? The jury is still out on this important question. Nevertheless, recent studies reveal an intimate interface between mechanisms that govern the translation of mRNA and molecular pathways implicated in ageing. In our manuscript we consider these links, which potentially underlie age-associated changes in protein synthesis.

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.

Effects of aging on the various steps of protein synthesis: fragmentation of elongation factor 2

The possible mechanism responsible for the in vivo protein synthesis decline during aging was studied. In order to determine the effect of aging on the various steps of protein synthesis, we determined the ribosomal state of aggregation and the time of assembly and release of polypeptide chains in the process of protein synthesis in rat liver. The results suggest that elongation is the most sensitive step to aging. A molecular study of the Elongation Factor 2 (EF-2), the main protein involved in the elongation step, shows that this protein has a higher content of carbonyl groups and is less active in old rats. In addition, the molecular mass analysis of EF-2 shows that this protein becomes fragmented in old rats. A similar pattern of fragmentation is found in 3-month-old rats suffering oxidative stress, in that the decline in protein synthesis is similar to that found in old rats. These data suggest that: i) oxidative stress seems to be involved in the modifications of EF-2 observed during aging, and ii) the observed modifications (oxidation and fragmentation) of EF-2 could account for the decline in protein synthesis in old animals.

The influence of tetracyclines on the aggregation of free and membrane-bound polysomes and on the RNA content of the liver of mice

Tetracycline (100 micrograms/g) and, to a lesser extent, doxycycline (50 and 100 micrograms/g) inhibited the aggregation of free but not of membrane-bound polysomes in the livers of female NMRI mice. In addition tetracycline decreased the RNA content of the fraction of membrane-bound polysomes probably due to detachment from their membrane sites.

Intracellular water and ionic shifts during growth and ageing of rats

As measured by X-ray microanalysis of frozen-dried cryosections, physiological concentrations of phosphorus, Na+, K+ and Mg2+ show a biphasic age dependence in some compartments of rat hepatocytes, especially in the regions of condensed chromatin associated with the actively transcribed genes. This could be consistent with a reactivation of the protein synthesizing apparatus in old livers. The decrease in intracellular water concentrations in hepatocytes and myocytes during ageing is shown to be solely due to a preferential decrease in the water concentration in mitochondria. As a consequence, ionic strength in mitochondria increases with age. This result could provide a rationale for a better understanding of functional declines found in mitochondria from old donors.

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.

Protein synthesis activity in vitro in mouse tissues in relation to methionine-cysteine deficiency and age

The objective of this study was to determine whether ageing in the mouse is associated with changes in protein synthesis activity and response to dietary methionine-cysteine deficiency. Growing subadult mice (30-day-old) and adult mice (70-day-old) were compared. The amino acid restricted diet contained methionine and cysteine at a level of 10% that of the control diet. After a feeding period of 6 days protein synthesis activity was measured in liver, kidney and lung. In control liver, the level of protein synthesis activity was the same for the two age groups; in kidney and lung, activity was diminished in the adult group. After methionine-cysteine deficiency protein synthesis activity was unchanged in the subadult and diminished in the adult liver. In kidney, both age groups responded with an increase in activity. In lung, the subadult group remained unchanged and activity in the adult group was elevated. The amount of DNA/g wet weight of tissue was higher in the subadult than the adult group throughout. Methionine-cysteine deficiency resulted in an increase in DNA/g tissue wet weight, while the total amount of DNA in liver decreased, but was unchanged in kidney and lung. Total liver activity was decreased less in subadult than adult mice. Total kidney and lung activities were diminished in subadult but not adult mice. The results suggest that the specific protein synthesis activity of the ribosomes in the kidney and lung tends to decrease with age more readily than in liver, and that subadult liver displays less sensitivity towards methionine-cysteine deficiency than does adult liver.