Long Telomeres Do Not Affect Cellular Fitness in Yeast

Abstract

Telomeres, the ends of the eukaryotic chromosomes, help to maintain the genome's integrity and thus play important roles in aging and cancer. Telomere length is strictly controlled in all organisms. In humans, telomeres shorten with age, and it has been proposed that telomere shortening may play a causal role in aging. We took advantage of the availability of yeast strains with genetically or physiologically generated differences in telomere length to measure the effect that telomere length may have on cellular growth. By comparing the growth rates affecting telomere length of various yeast mutants we show that there is no correlation between their telomere length and cellular fitness. We also show that wild-type yeast cells carrying extremely long telomeres (~5 times longer than the average) showed no signs of mitotic or meiotic defects, and competition experiments found no differences in growth between strains with normal telomeres and strains with long telomeres. No advantage or disadvantage of cells with long telomeres was detected under stress conditions either. Finally, telomere length had no effect in a chronological life span assay, which measures survival of post-mitotic-stage cells. We conclude that extreme telomere length has no effects (positive or negative) on the fitness of yeast cells.IMPORTANCE Telomeres protect the chromosomal ends from fusion, degradation, and unwanted repair. Therefore, telomeres preserve genome stability and cell viability. In humans, telomeres shorten with each cell duplication event and with age. It has thus been proposed that telomere shortening may be responsible for human aging and that elongation of telomeres may be a way to rejuvenate cells and to combat aging. However, it is difficult to prove this hypothesis in human cells. Yeasts are easy to manipulate and have telomeres whose length is strictly maintained. Here we show that yeast cells manipulated to have extremely long telomeres (~5-fold those of normal cells) did not show any improvement or reduction in fitness compared to otherwise identical cells with telomeres of normal length under all the conditions tested. Moreover, an assay that measures cell aging showed no effect of the presence of extremely long telomeres. We thus conclude that extreme telomere length, at least in yeast cells, does not affect cellular fitness, aging, or senescence.

Keywords: aging; cancer; ethanol; fitness; telomere; yeasts.

FIG 1

The effect of tlm mutations on cell fitness. (A) Representative Telo-blot analysis of the different tlm mutants. (B) Relative growth rate measurements for different tlm mutants in YPD medium. For each strain, the growth rate was normalized to that of the wild-type (BY4741) strain (whose rate is shown with a value of 1). The different tlm strains are shown on the x axis according to their telomere length phenotypes. (C) A similar graph for strains grown in YPD–5% ethanol medium.

FIG 2

Cell fitness in wild-type strains with long telomeres. (A) Schematic cartoon representing the process of mating of haploid cells, followed by sporulation and tetrad dissection. (B) Telo-blot analysis of haploid meiotic products with long telomeres. (C) Relative doubling time measurements for untreated wild-type cells (BY4741) and haploid meiotic products of long telomeres in media consisting of either YPD (green bars) and YPD plus 5% ethanol (blue bars). For each candidate, doubling times in YPD media and in YPD–5% ethanol media are normalized according to the doubling time of BY4741 in YPD medium. (D) Relative doubling-time measurements for wild-type cells (BY4741) of normal telomere length and haploid meiotic products from a single tetrad in media consisting of either YPD (green bars) or YPD plus 5% ethanol (blue bars) (results were normalized as described for panel C).

FIG 3

Competition experiments. (A) Assay of competition between two differently marked wild-type cells of normal telomere length grown in YPD medium for 100 generations. Cell type abundance is presented in intervals of 10 generations. (B) Assay similar to that described for panel A, performed using YPD plus 5% ethanol. (C) Competition assay between two isogenic wt strains, one of normal telomere length, and one with long telomeres. (D) Assay similar to that described for panel C, performed using YPD plus 5% ethanol.

FIG 4

The effect of telomere length on chronological life span (CLS). (A) Chronological life span (CLS) assay of isogenic wild-type strains of yeast cells of normal telomere length (blue), short telomere length (orange), and long telomere length (gray). For each strain, cell viability (measured as the number of CFU per milliliter) is presented as a function of time. (B) Telo-blot analysis of the surviving cells within the senescent CLS cultures after day 43.

FIG 5

Spot assay (10-fold dilutions) of isogenic wild-type strains with normal and long telomeres on plates containing methyl methanesulfonate (MMS) or hydroxyurea (HU). The diploid BY4743 strain and the Δrad52 strain are shown as controls.