Fight Aging!

Cellular biochemistry is a messy process, a soup of colliding molecules moving at high speed and reacting with one another. Within this soup, complex processes of assembly and interaction take place. The blueprints of genes in DNA are converted into RNA via one complicated set of reading and assembly mechanisms in the cell nucleus. That RNA exits the nucleus and is then processed in ribosomes to produce proteins from amino acid fragments. Proteins are then folded in to the correct shape in the endoplasmic reticulum, and then must further be transported to a final destination within the cell.

All of this takes place within a dense storm of fast-moving molecules of all sorts, and any number of inappropriate interactions and reactions. Quality control is important, as all of the processes mentioned above can fail. Errant proteins and damaged structures are promptly identified and removed, broken down into amino acid fragments for recycling. Evolution has produced a system of assembly and quality control that has a high fidelity, suggesting that it is important for cell and tissue function for protein manufacture to produce few errors. But are variations in error rate an important contribution to differences in life span between species?

Thermophiles reveal the clues to longevity: precise protein synthesis

During the lifetime of an organism, proteins are constantly exposed to stressors that impair their function. Protein homeostasis networks have evolved to monitor and regulate the synthesis, folding, trafficking, and degradation of proteins. The earliest process in the protein life cycle that modulates cellular proteostasis is translation. Changes in levels or mutations in the components of translational machinery have a significant impact on longevity in several animals. For instance, mutation or depletion of ribosomal proteins and translation factors extend lifespan in yeast worms and flies. Similarly, blocking the mTOR (mammalian Target of Rapamycin) system through rapamycin reduces protein synthesis and increases lifespan. While the significance of protein synthesis in the aging process is widely accepted, it is unclear whether errors in protein synthesis or translational fidelity may play a part in aging per se.

Protein synthesis error rates in bacterial systems are estimated to be as high as 1 in 10^3 per amino acid. According to this estimation, up to 15%-20% of the cellular protein pool may contain mistranslation errors. Mistranslated proteins may not have a significant impact on cellular proteostasis at a young age, due in part to the quick turnover and efficient clearance of cellular proteins. However, in the older animal, protein turnover rates, proteasome activity, and autophagy decline, making them more sensitive to error-prone protein translation. Therefore, mistakes in protein translation have the potential to cause a variety of age-related diseases. So far, however, it was unclear whether accurate translation can affect cellular aging and if the rate and extent of mistranslation increase with aging.

Researchers recently proposed a mechanism of aging that is highly dependent on translational accuracy. The authors study ribosomal protein RPS23, a homolog of prokaryotic S12 protein that is conserved in all three domains of life, and implicate it in the maintenance of protein translational accuracy. The ribosomal protein S12 (eukaryotic homolog RPS23) contributes to translation accuracy. A single amino acid alteration in a ribosomal protein present in archaea can improve metazoan translation fidelity and survival. The authors conducted a comprehensive phylogenetic analysis of RPS23 and discovered that a lysine residue at position 60 is evolutionarily conserved, except in hyperthermophilic archaea, where arginine replaces it (RPS23-K60R).

CRISPR gene editing was used to insert a mutation into the Drosophila melanogaster Rps23 gene, which led to the substitution of the lysine 60 with an arginine residue. The translational accuracy of mutant and wild-type flies was determined with a reporter construct that contained a renilla luciferase gene followed by a firefly luciferase gene separated by an in-frame linker sequence containing a stop codon. Inclusion of a stop codon produces firefly only if a read-through error had occurred, and thus served as a measure of translational accuracy. It was discovered that the K60R mutation in the RPS23 protein decreased the frequency of mistakes made during protein synthesis, thereby leading to a reduction in firefly luciferase production in the mutants. Specifically, the stop-codon read-through increased considerably with aging in the control flies, but not in the RPS23-K60R mutant. Similar results were obtained in Caenorhabditis elegans and Schizosaccharomyces pombe (yeast), where the expression of the RPS23-K60R protein also reduced stop-codon read-through.

Given that the variant (RPS23 K60R) was initially discovered in hyperthermophilic archaea, the authors contended that the protein should provide an evolutionarily conserved mechanism of heat tolerance. Indeed, flies, worms, and yeast expressing RPS23-K60R protein were able to grow and survive at temperatures above the optimum. Finally, the physiological benefit of this mutation was demonstrated by an increase in healthy lifespan. The authors demonstrate that when the mutation was introduced, it improved lifespan by 9%-23% in all three model organisms. Additionally, the authors extend their findings by noting that treatments that prolong lifespan (such as rapamycin) do so by enhancing translational fidelity in controls but not in mutant K60R strains. Thus, they concluded that the primary determinant of the aging process was a reduction in translational fidelity.