Role of Mitochondrial DNA in Yeast Replicative Aging.

Despite the diverse manifestations of aging across different species, some common aging features and underlying mechanisms are shared. In particular, mitochondria appear to be among the most vulnerable systems in both metazoa and fungi. In this review, we discuss how mitochondrial dysfunction is related to replicative aging in the simplest eukaryotic model, the baker's yeast Saccharomyces cerevisiae. We discuss a chain of events that starts from asymmetric distribution of mitochondria between mother and daughter cells. With age, yeast mother cells start to experience a decrease in mitochondrial transmembrane potential and, consequently, a decrease in mitochondrial protein import efficiency. This induces mitochondrial protein precursors in the cytoplasm, the loss of mitochondrial DNA (mtDNA), and at the later stages - cell death. Interestingly, yeast strains without mtDNA can have either increased or decreased lifespan compared to the parental strains with mtDNA. The direction of the effect depends on their ability to activate compensatory mechanisms preventing or mitigating negative consequences of mitochondrial dysfunction. The central role of mitochondria in yeast aging and death indicates that it is one of the most complex and, therefore, deregulation-prone systems in eukaryotic cells.

Protonophore FCCP provides fitness advantage to PDR-deficient yeast cells.

Pleiotropic drug resistance (PDR) plasma membrane transporters mediate xenobiotic efflux from the cells and thereby help pathogenic microorganisms to withstand antimicrobial therapies. Given that xenobiotic efflux is an energy-consuming process, cells with upregulated PDR can be sensitive to perturbations in cellular energetics. Protonophores dissipate proton gradient across the cellular membranes and thus increase ATP spendings to their maintenance. We hypothesised that chronic exposure of yeast cells to the protonophores can favour the selection of cells with inactive PDR. To test this, we measured growth rates of the wild type Saccharomyces cerevisiae and PDR-deficient Δpdr1Δpdr3 strains in the presence of protonophores carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP), pentachlorophenol (PCP) and niclosamide (NCA). Although the protonophore-induced respiration rates of these two strains were similar, the PDR-deficient strain outperformed the control one in the growth rate on non-fermentable carbon source supplemented with low concentrations of FCCP. Thus, active PDR can be deleterious under conditions of partially uncoupled oxidative-phosphorylation. Furthermore, our results suggest that tested anionic protonophores are poor substrates of PDR-transporters. At the same time, protonophores imparted azole tolerance to yeasts, pointing that they are potent PDR inducers. Interestingly, protonophore PCP led to a persistent increase in the levels of a major ABC-transporter Pdr5p, while azole clotrimazole induced only a temporary increase. Together, our data provides an insight into the effects of the protonophores in the eukaryotes at the cellular level and support the idea that cells with activated PDR can be selected out upon conditions of energy limitations.

Replicative aging as a source of cell heterogeneity in budding yeast.

While deviations from the optimal phenotype are deleterious, increased variation can prevent population extinction under severe stresses. Cell division asymmetry is an important source of microbial phenotypic heterogeneity. A consecutive set of asymmetric divisions can cause the gradual accumulation of deleterious factors and, at late stages, the death of old pole (mother) cells. This phenomenon is known as replicative aging. As the old cells are constantly being diluted by the progeny, the majority of a microbial population is represented by replicatively young cells. Therefore, early-age changes in yeast mother cells have a much greater impact on the integral performance of the microbial population than does functional deterioration at later ages. Here, we review the early manifestations of replicative aging in Saccharomyces cerevisiae mother cells that occur during the first ten cell cycles. Early age-dependent changes occur in stress resistance, genomic instability, protein aggregate levels, redox balance and metabolism. We speculate that some of these manifestations can be beneficial during stress exposure; therefore, early aging may be a bet-hedging mechanism. Together, the data suggest that the age component of variation in populations of asymmetrically dividing microorganisms is substantial and may play an important role in adaptations to changing environments.

The contribution of Saccharomyces cerevisiae replicative age to the variations in the levels of Trx2p, Pdr5p, Can1p and Idh isoforms.

Asymmetrical division can be a reason for microbial populations heterogeneity. In particular, budding yeast daughter cells are more vulnerable to stresses than the mothers. It was suggested that yeast mother cells could also differ from each other depending on their replicative age. To test this, we measured the levels of Idh1-GFP, Idh2-GFP, Trx2-GFP, Pdr5-GFP and Can1-GFP proteins in cells of the few first, most represented, age cohorts. Pdr5p and Can1p were selected because of the pronounced mother-bud asymmetry for these proteins distributions, Trx2p as indicator of oxidative stress. Isocitrate dehydrogenase subunits Idh1p and Idh2p were assessed because their levels are regulated by mitochondria. We found a small negative correlation between yeast replicative age and Idh1-GFP or Idh2-GFP but not Trx2-GFP levels. Mitochondrial network fragmentation was also confirmed as an early event of replicative aging. No significant difference in the membrane proteins levels Pdr5p and Can1p was found. Moreover, the elder mother cells showed lower coefficient of variation for Pdr5p levels compared to the younger ones and the daughters. Our data suggest that the levels of stress-response proteins Pdr5p and Trx2p in the mother cells are stable during the first few cell cycles regardless of their mother-bud asymmetry.