Cell size regulation during telomere-directed senescence in Saccharomyces cerevisiae

Genomic Instability and Cellular Senescence: Lessons From the Budding Yeast

Aging is a complex biological process that occurs in all living organisms. Aging is initiated by the gradual accumulation of biomolecular damage in cells leading to the loss of cellular function and ultimately death. Cellular senescence is one such pathway that leads to aging. The accumulation of nucleic acid damage and genetic alterations that activate permanent cell-cycle arrest triggers the process of senescence. Cellular senescence can result from telomere erosion and ribosomal DNA instability. In this review, we summarize the molecular mechanisms of telomere length homeostasis and ribosomal DNA stability, and describe how these mechanisms are linked to cellular senescence and longevity through lessons learned from budding yeast.

Phosphatase-dependent fluctuations in DNA-damage checkpoint activation at partially defective telomeres

Telomeres protect the ends of eukaryotic chromosomes, and telomere shortening causes irreversible cell-cycle arrest through activation of the DNA-damage checkpoint. In this study, we found that deletion of PPH3, encoding a 2A-like protein phosphatase, accelerated telomere-shortening-mediated senescence without affecting normal telomere length or the telomere erosion rate in Saccharomyces cerevisiae. Moreover, the loss of PPH3 increased sensitivity to telomere dysfunction. The detection of telomere abnormalities by DNA-damage sensors was not an all-or-none response, implying that Pph3 helps determine the border between normal and dysfunctional telomeres by suppressing premature activation of the DNA-damage checkpoint.

Quantitative assessment of changes in cell growth, size and morphology during telomere-initiated cellular senescence in Saccharomyces cerevisiae

Telomerase-deficient cells of the budding yeast S. cerevisiae experience progressive telomere shortening and undergo senescence in a manner similar to that seen in cultured human fibroblasts. The cells exhibit a DNA damage checkpoint-like stress response, undergo changes in size and morphology, and eventually stop dividing. In this study, a new assay is described that allowed quantitation of senescence in telomerase-deficient est2 cells with applied statistics. Use of the new technique revealed that senescence was strongly accelerated in est2 mutants that had homologous recombination genes RAD51, RAD52 or RAD54 co-inactivated, but was only modestly affected when RAD55, RAD57 or RAD59 were knocked out. Additionally, a new approach for calculating population doublings indicated that loss of growth capacity occurred after approximately 64 generations in est2 cells but only 42 generations in est2 rad52 cells. Phase contrast microscopy experiments demonstrated that senescing est2 cells became enlarged in a time-dependent manner, ultimately exhibiting a 60% increase in cell size. Progressive alterations in physical properties were also observed, including striking changes in light scattering characteristics and cellular sedimentation rates. The results described herein will facilitate future studies of genetic and environmental factors that affect telomere shortening-associated cell senescence rates using the yeast model system.

Reversible DNA damage checkpoint activation at the presenescent stage in telomerase-deficient cells of Saccharomyces cerevisiae

The telomere protects the ends of eukaryotic linear chromosomes, and its shortening or erosion is recognized as DNA damage, leading to loss of proliferation activity and, thus, cellular senescence at the population level. Here, using a GFP-based DNA damage checkpoint marker suited for single-cell observation of Saccharomyces cerevisiae cells, we correlated the checkpoint status of telomere-shortened cells with their behavior. We show that some cells possessing short telomeres retain proliferation capacity even after the DNA damage checkpoint is activated. At the presenescent stage, the activation of the checkpoint causes cell cycle delay, but does not induce permanent cell cycle arrest, eventually leading to the expansion of cell size that is characteristic of cellular senescence. Moreover, the proliferation capacity of checkpoint-activated cells is not dependent on homologous recombination or the checkpoint adaptation pathway. The retention of proliferation capacity is specific to the telomere-derived DNA damage response, suggesting that damaged telomeres differ functionally from other types of DNA damage. Our data establish the role of the presenescent stage in telomere shortening-induced senescence, which proceeds gradually and is associated with a variety of changes, including altered cell morphology and metabolism.

A role for the universal Kae1/Qri7/YgjD (COG0533) family in tRNA modification

The YgjD/Kae1 family (COG0533) has been on the top-10 list of universally conserved proteins of unknown function for over 5 years. It has been linked to DNA maintenance in bacteria and mitochondria and transcription regulation and telomere homeostasis in eukaryotes, but its actual function has never been found. Based on a comparative genomic and structural analysis, we predicted this family was involved in the biosynthesis of N(6)-threonylcarbamoyl adenosine, a universal modification found at position 37 of tRNAs decoding ANN codons. This was confirmed as a yeast mutant lacking Kae1 is devoid of t(6)A. t(6)A(-) strains were also used to reveal that t(6)A has a critical role in initiation codon restriction to AUG and in restricting frameshifting at tandem ANN codons. We also showed that YaeZ, a YgjD paralog, is required for YgjD function in vivo in bacteria. This work lays the foundation for understanding the pleiotropic role of this universal protein family.