The ribosome is an important type of cell structure, the location of protein synthesis. Like most cell structures, ribosomes are recycled and rebuilt on a regular basis, and their construction takes place in the nucleolus. The paper here considers the evidence for altered rates or disruptions in the manufacture of ribosomes to relate to aging. There are clear associations, particularly for calorie restriction, which both slows aging and the pace at which new ribosomes are produced.
The nucleolus has gained prominent attention in molecular research over the past two decades, due to its emerging role in various cellular processes. Among them, the production of ribosomes is seemingly the most important, as it controls translation of all proteins in the cell and thus governs cell growth and proliferation. A number of studies have demonstrated that the disruption of virtually any step in ribosome biogenesis can result in cell cycle arrest, primarily through activation of the tumor suppressor protein p53. This particular process was recently termed as the Impaired Ribosome Biogenesis Checkpoint (IRBC).
Numerous studies presented a direct connection between dysregulated ribosome biogenesis and aging. For instance, the downregulation of ribosome biogenesis components or nutrient sensing pathways, which stimulate ribosome production, have been shown to increase the lifespan of multiple organisms including C. elegans, D. melanogaster, yeast, mice, and human. Therefore, enhanced ribosome biogenesis, visualized by enlarged nucleoli, is believed to accelerate aging. Indeed, consistent with this idea, the size of the nucleoli and the amount of rRNA increases during aging in human primary fibroblasts and a single, large nucleolus is often observed in senescent cells. Furthermore, fibroblasts isolated from patients suffering from the premature aging disease Hutchinson-Gilford progeria, have enlarged nucleoli and upregulated ribosome biogenesis.
Since the rate of protein translation is proportional to the rate of ribosome biogenesis it was suggested that upregulation of protein synthesis and disruption of global proteostasis is the mechanism through which ribosome biogenesis promotes aging. This theory is supported by studies showing that reduction in the rate of translation can increase lifespan, and furthermore that altered proteostasis is a hallmark of aging. Additionally, caloric restriction that has been shown to promote longevity, leads to the downregulation of ribosome biogenesis by several mechanisms.