Biodemographic trajectories of age-specific reproliferation from stationary phase in the yeast Saccharomyces cerevisiae seem multiphasic

Unbalanced Growth, Senescence and Aging

Usually, cells balance their growth with their division. Coordinating growth inputs with cell division ensures the proper timing of division when sufficient cell material is available and affects the overall rate of cell proliferation. At a very fundamental level, cellular replicative lifespan-defined as the number of times a cell can divide, is a manifestation of cell cycle control. Hence, control of mitotic cell divisions, especially when the commitment is made to a new round of cell division, is intimately linked to replicative aging of cells. In this chapter, we review our current understanding, and its shortcomings, of how unbalanced growth and division, can dramatically influence the proliferative potential of cells, often leading to cellular and organismal aging phenotypes. The interplay between growth and division also underpins cellular senescence (i.e., inability to divide) and quiescence, when cells exit the cell cycle but still retain their ability to divide.

Drugs that modulate aging: the promising yet difficult path ahead

Once a backwater in medical sciences, aging research has emerged and now threatens to take the forefront. This dramatic change of stature is driven from 3 major events. First and foremost, the world is rapidly getting old. Never before have we lived in a demographic environment like today, and the trends will continue such that 20% percent of the global population of 9 billion will be over the age of 60 by 2050. Given current trends of sharply increasing chronic disease incidence, economic disaster from the impending silver tsunami may be ahead. A second major driver on the rise is the dramatic progress that aging research has made using invertebrate models such as worms, flies, and yeast. Genetic approaches using these organisms have led to hundreds of aging genes and, perhaps surprisingly, strong evidence of evolutionary conservation among longevity pathways between disparate species, including mammals. Current studies suggest that this conservation may extend to humans. Finally, small molecules such as rapamycin and resveratrol have been identified that slow aging in model organisms, although only rapamycin to date impacts longevity in mice. The potential now exists to delay human aging, whether it is through known classes of small molecules or a plethora of emerging ones. But how can a drug that slows aging become approved and make it to market when aging is not defined as a disease. Here, we discuss the strategies to translate discoveries from aging research into drugs. Will aging research lead to novel therapies toward chronic disease, prevention of disease or be targeted directly at extending lifespan?

Common genetic variants of the β2-adrenergic receptor affect its translational efficiency and are associated with human longevity

β-adrenoceptors are the common pharmacological targets for the treatment of cardiovascular diseases and asthma. Genetic modifications of β-adrenergic system in engineered mice affect their lifespan. Here, we tested whether genes encoding for key components of the β-adrenergic signaling pathway are associated with human longevity. We performed a 10-year follow-up study of the Chinese longitudinal healthy longevity survey. The Han Chinese population in this study consisted of 963 long-lived and 1028 geography-matched young individuals. Sixteen SNPs from ADRB1, ADRB2, ADCY5, ADCY6, and MAPK1 were selected and genotyped. Two SNPs, rs1042718 (C/A) and rs1042719 (G/C), of ADRB2 in linkage disequilibrium (D' = 1.0; r2 = 0.67) were found to be associated with enhanced longevity in men in two geographically isolated populations. Bonferroni-corrected P-values in a combined analysis were 0.00053-0.010. Men with haplotype A-C showed an increased probability to become centenarians (the frequency of A-C in long-lived and young individuals are 0.332 and 0.250, respectively, OR = 1.49, CI 95% = 1.17-1.88, P = 0.0007), in contrast to those with haplotype C-G (the frequency of C-G in long-lived and young individuals are 0.523 and 0.635, respectively, OR = 0.63, CI 95% = 0.51-0.78, P = 0.000018). The permuted P-values were 0.00005 and 0.0009, respectively. ADRB2 encodes the β2-adrenergic receptor; the haplotype A-C markedly reduced its translational efficiency compared with C-G (P = 0.002) in transfected HEK293 cells. Thus, our data indicate that enhanced production of β2-adrenergic receptors caused by genetic variants is inversely associated with human lifespan.

Differences in stationary-phase cells of a commercial Saccharomyces cerevisiae wine yeast grown in aerobic and microaerophilic batch cultures assessed by electric particle analysis, light diffraction and flow cytometry

We applied electric particle analysis, light diffraction and flow cytometry to obtain information on the morphological changes during the stationary phase of Saccharomyces cerevisiae. The reported analyses of S. cerevisiae populations were obtained under two different conditions, aerobic and microaerophilic, at 27°C. The samples analysed were taken at between 20 and 50 h from the beginning of culture. To assist in the interpretation of the observed distributions a complexity index was used. The aerobically grown culture reached significantly greater cell density. Under these conditions, the cell density experienced a much lower reduction (3%) compared with the microaerophilic conditions (30%). Under aerobic conditions, the mean cell size determined by both electric particle analysis and light diffraction was lower and remained similar throughout the experiment. Under microaerophilic conditions, the mean cell size determined by electric particle analysis decreased slightly as the culture progressed through the stationary phase. Forward and side scatter distributions revealed two cell subpopulations under both growth conditions. However, in the aerobic growing culture the two subpopulations were more separated and hence easier to distinguish. The distributions obtained with the three experimental techniques were analysed using the complexity index. This analysis suggested that a complexity index is a good descriptor of the changes that take place in a yeast population in the stationary phase, and that it aids in the discussion and understanding of the implications of these distributions obtained by these experimental techniques.

Isolation of two cell populations from yeast during high-level alcoholic fermentation that resemble quiescent and nonquiescent cells from the stationary phase on glucose

High-level production of bioethanol (140 g L(-1) in 45 h) in aerated fed-batch cultures of Saccharomyces cerevisiae was shown to be linked to the length of a production phase uncoupled to the growth. The induction of this phase was characterized by metabolic and morphologic changes reminiscent of those occurring in the stationary phase of growth on glucose. Global transcriptomic analysis of ethanol-stressed yeast cells in the uncoupling phase harboured features similar to those from stationary-phase cells on glucose. Two distinct cellular populations were isolated by Percoll density-gradient centrifugation in this uncoupling phase. The lower fraction was enriched by yeast cells that were mostly uniform in size and opalescent, containing a large amount of glycogen and trehalose, and exhibiting high respiratory activity. In contrast, the upper fraction was characterized by cells heterogeneous in size, with one to several small buds, which did not contain storage carbohydrates and which exhibited a poor respiratory competence while retaining a high relative glycolytic activity. These results are discussed in terms of a possible induction of a state similar to the quiescence state previously observed from yeast stationary-phase cultures, in response to ethanol toxicity, whose acquisition may be critical for performing high-level alcoholic fermentation.

Plasticity of death rates in stationary phase in Saccharomyces cerevisiae

For the species that have been most carefully studied, mortality rises with age and then plateaus or declines at advanced ages, except for yeast. Remarkably, mortality for yeast can rise, fall and rise again. In the present study we investigated (i) if this complicated shape could be modulated by environmental conditions by measuring mortality with different food media and temperature; (ii) if it is triggered by biological heterogeneity by measuring mortality in stationary phase in populations fractionated into subpopulations of young, virgin cells, and replicatively older, non-virgin cells. We also discussed the results of a staining method to measure viability instead of measuring the number of cells able to exit stationary phase and form a colony. We showed that different shapes of age-specific death rates were observed and that their appearance depended on the environmental conditions. Furthermore, biological heterogeneity explained the shapes of mortality with homogeneous populations of young, virgin cells exhibiting a simple shape of mortality in conditions under which more heterogeneous populations of older cells or unfractionated populations displayed complicated death rates. Finally, the staining method suggested that cells lost the capacity to exit stationary phase and to divide long before they died in stationary phase. These results explain a phenomenon that was puzzling because it appeared to reflect a radical departure from mortality patterns observed for other species.

Isolation of quiescent and nonquiescent cells from yeast stationary-phase cultures

Quiescence is the most common and, arguably, most poorly understood cell cycle state. This is in part because pure populations of quiescent cells are typically difficult to isolate. We report the isolation and characterization of quiescent and nonquiescent cells from stationary-phase (SP) yeast cultures by density-gradient centrifugation. Quiescent cells are dense, unbudded daughter cells formed after glucose exhaustion. They synchronously reenter the mitotic cell cycle, suggesting that they are in a G(0) state. Nonquiescent cells are less dense, heterogeneous, and composed of replicatively older, asynchronous cells that rapidly lose the ability to reproduce. Microscopic and flow cytometric analysis revealed that nonquiescent cells accumulate more reactive oxygen species than quiescent cells, and over 21 d, about half exhibit signs of apoptosis and necrosis. The ability to isolate both quiescent and nonquiescent yeast cells from SP cultures provides a novel, tractable experimental system for studies of quiescence, chronological and replicative aging, apoptosis, and the cell cycle.

Natural variation in replicative and chronological life spans of Saccharomyces cerevisiae

Natural variation in the lifespan of natural yeast populations has not been systematically investigated. Here, we have quantified the variation in the replicative and chronological life spans (RLS and CLS) in natural isolates of Saccharomyces cerevisiae and found that genotypic variation accounts for about 22% of the total variation of RLS. Strikingly, the average RLS of 14 natural isolates is about 30% longer than that of 13 laboratory strains (32 versus 21 cell divisions). As is the case for aging in mammals, there is a negative correlation between the logarithmic transformation of the initial mortality rate and the Gompertz coefficient for RLS. Thus this characteristic feature of aging is conserved from yeast to mammals. The average CLS of the natural isolates is about 7 days, significantly shorter than that of the laboratory strains. There is no correlation between RLS and CLS in natural isolates. Possible reasons for the differences between natural and laboratory strains are discussed.