PerspectiveAging

Old moms say, no Sir

See allHide authors and affiliations

Science  17 Mar 2017:
Vol. 355, Issue 6330, pp. 1126-1127
DOI: 10.1126/science.aam9740

Is aging ineluctable, or are there genetic programs of aging that could be manipulated to extend life span? Research over the past two decades has provided powerful evidence that aging is indeed regulated by genes that control highly conserved pathways (1). For example, mutations in single genes in model organisms like flies and worms not only allow these animals to live longer, but rejuvenate them as well (2). Even the single-celled budding yeast Saccharomyces cerevisiae ages, and specific genes likewise control this process (1). Indeed, one of the most powerful genetic manipulations to extend life span was discovered in yeast. The histone deacetylase silent information regulator 2 (Sir2) is required for repressing the transcription of certain mating-type loci, telomeres, and ribosomal DNA (35). The latter had been linked to aging in yeast, inspiring studies that revealed Sir2's importance for this process (6). Subsequent work in yeast and animal models established that changes in Sir2 activity are responsible for much of the life span–extending effects of caloric restriction (7). On page 1184 of this issue, Schlissel et al. (8) report that a particular facet of aging, which had long been attributed to age-dependent changes in Sir2 function, is caused by a new mechanism.

To study aging in budding yeast, researchers count the number of times a daughter cell buds off from a mother cell. A mother has only a fixed number of times to divide to produce a daughter. As she gets older (produces more daughters), she undergoes characteristic physiological changes, including sterility. That is, haploid mother cells lose the ability to mate. Haploid cells exist as either one of two mating types—a or α. Mating-type information is encoded by the MAT locus. Extra mating-type information is expressed from the HML and HMR loci (so-called hidden mating-type loci). The expression of genes from these loci is controlled by silencing, whereby chromosomal regions are packaged tightly into chromatin so that the genes located there are not transcribed (4). Sir2, the key aging regulator, is critical for this silencing in young cells.

It was long thought that decreasing Sir2 function in an old mother causes sterility because of a loss of silencing at the HML and HMR loci (9). This would cause both mating-type genes to be expressed simultaneously, making mating impossible. However, Schlissel et al. discovered that silencing at these loci is intact in old cells. They used a clever strategy to build a highly sensitive detector of loss of silencing at the HML locus. The HMLα2 gene was replaced with a gene encoding the enzyme Cre recombinase. The authors also inserted a fluorescent reporter gene elsewhere in the yeast genome. This reporter is designed to be Cre-responsive, such that cells in which Cre is absent (silenced) express a red fluorescent protein and cells in which Cre is expressed (loss of silencing) express a green fluorescent protein. The authors did not observe any silencing events (switch from red to green) as the mothers aged. This was so surprising that they expanded the analysis to over 1500 yeast pedigrees. In total, only 13 loss-of-silencing events were identified, none of which correlated with the yeast cell's age. Moreover, nearly all the old mothers eventually stopped dividing without showing any loss of silencing. A chemical inhibitor of Sir2 did cause a loss of silencing, suggesting that a decline in Sir2 activity would be sufficient to promote HML expression, but that this doesn't happen during aging.

To further test the hypothesis that decline in Sir2-mediated transcriptional silencing underlies aging yeast cell sterility, Schlissel et al. compared the genes expressed (or repressed) in old versus young cells and in cells in which Sir2 activity is inhibited. Sir2 inhibition caused high expression (loss of silencing) of HMRα1 messenger RNA compared to old cells. However, when the authors analyzed publicly available RNA sequencing data sets from yeast cells lacking Sir2 and compared them to RNA sequencing results from old and young yeast cells, they found no overlap.

What, then, is the true cause of age-induced sterility? When haploid cells of the a mating type (MATa) are exposed to α-factor pheromone from haploid cells of the α mating type (MATα), they arrest their cell cycle and start to grow a mating projection (see the figure). Old yeast cells are less responsive to pheromone, but this has nothing to do with loss of silencing at the HML locus. Then what does cause decreased sensitivity to pheromone and impaired mating response in old cells? One way of thinking about the problem is to consider the asymmetry of the phenotype. Whereas old mothers respond poorly to pheromone, their daughters do so proficiently. Could there be a factor that is asymmetrically inherited between an old mother and her daughter that governs the mating response? This scenario was reminiscent of a prion-like aggregation-prone RNA-binding protein called whiskey 3 (Whi3). Whi3 inhibits the G1-to-S phase transition of the cell division cycle and is inactivated when it aggregates in old mother cells (10). These aggregates also permit yeast cells to “memorize” deceptive mating attempts. Unlike true prions, the Whi3 “mnemon” is asymmetrically retained in mother cells. Schlissel et al. asked whether the asymmetric aggregation of Whi3 in old mother cells caused age-dependent sterility. The authors deleted a glutamine-rich prion-like domain in Whi3, which is required for its capacity to aggregate. Remarkably, this simple manipulation prevented old cells from losing sensitivity to the mating pheromone. Indeed, when the authors visualized Whi3 (by expressing it in yeast as a fusion with a fluorescent protein), they observed its aggregation in old cells. Thus, Whi3 aggregation provides a compelling explanation for the sterility of old age, and suggests that the selective retention of aggregated Whi3 by mother cells allows daughter cells to become responsive to pheromone.

The sterility of old age

Aging yeast mother cells accumulate aggregated Whi3, which blocks mating. Daughter cells are spared.

GRAPHIC: A. KITTERMAN/SCIENCE

The study by Schlissel et al. shows that contrary to previous assumptions, loss of transcriptional silencing is not responsible for sterility in old haploid yeast, and invokes the intriguing hypothesis that aggregation of the RNA-binding protein Whi3 instead accounts for age-related sterility. They also propose the provocative idea that by accumulating protein aggregates, old cells are “differentiated” relative to young cells, which may be beneficial to the ecology of wild yeast. The study of Schlissel et al. is exciting in terms of resolving a longstanding question in the aging field and providing a new link between the asymmetric retention of aggregates and phenotypes classically associated with aging.

References

Navigate This Article