SummaryBudding yeast shows a progressive decline in viability after entering stationary phase, a phenomenon known as chronological aging. We show here that the fission yeast Schizosaccharomyces pombe also undergoes chronological aging and that the process is regulated by genes controlling two related nutrient signalling pathways. The first pathway includes the serine/threonine cAMP-activated protein kinase Pka1 and the second pathway comprises the serine/threonine kinase Sck2, a homologue of Saccharomyces cerevisiae SCH9. A double mutant for pka1 and sck2 displayed an additive effect on prolonging the fission yeast lifespan, suggesting that these genes regulate related but independent pathways. These long-lived mutants also accumulated less reactive oxygen species and had a delayed initiation of apoptosis compared with wild-type cells. We also found that strains carrying pka1 deletion but not those with sck2 deletion gained resistance to oxidative stress due to exposure to H 2 O 2 or menadione. On the other hand, the additional increase in lifespan shown by the ∆ ∆ ∆ ∆ pka1∆ ∆ ∆ ∆ sck2 double-mutant strain correlated with an increased resistance to both oxidative stress and heat shock. These results underscore the importance of nutrient signalling pathways and reactive oxygen species on organismal lifespan and establish S. pombe as a new model organism to study the molecular mechanisms underlying aging.

Glucose is the preferred carbon and energy source in prokaryotes, unicellular eukaryotes, and metazoans. However, excess of glucose has been associated with several diseases, including diabetes and the less understood process of aging. On the contrary, limiting glucose (i.e., calorie restriction) slows aging and age-related diseases in most species. Understanding the mechanism by which glucose limits life span is therefore important for any attempt to control aging and age-related diseases. Here, we use the yeast Schizosaccharomyces pombe as a model to study the regulation of chronological life span by glucose. Growth of S. pombe at a reduced concentration of glucose increased life span and oxidative stress resistance as reported before for many other organisms. Surprisingly, loss of the Git3 glucose receptor, a G protein-coupled receptor, also increased life span in conditions where glucose consumption was not affected. These results suggest a role for glucose-signaling pathways in life span regulation. In agreement, constitutive activation of the Gα subunit acting downstream of Git3 accelerated aging in S. pombe and inhibited the effects of calorie restriction. A similar pro-aging effect of glucose was documented in mutants of hexokinase, which cannot metabolize glucose and, therefore, are exposed to constitutive glucose signaling. The pro-aging effect of glucose signaling on life span correlated with an increase in reactive oxygen species and a decrease in oxidative stress resistance and respiration rate. Likewise, the anti-aging effect of both calorie restriction and the Δgit3 mutation was accompanied by increased respiration and lower reactive oxygen species production. Altogether, our data suggest an important role for glucose signaling through the Git3/PKA pathway to regulate S. pombe life span.

Stress conditions affecting the functions of the endoplasmic reticulum (ER) cause the accumulation of unfolded proteins. ER stress is counteracted by the unfolded-protein response (UPR). However, under prolonged stress the UPR initiates a proapoptotic response. Mounting evidence indicate that the ER chaperone calnexin is involved in apoptosis caused by ER stress. Here, we report that overexpression of calnexin in Schizosaccharomyces pombe induces cell death with apoptosis markers. Cell death was partially dependent on the Ire1p ER-stress transducer. Apoptotic death caused by calnexin overexpression required its transmembrane domain (TM), and involved sequences on either side of the ER membrane. Apoptotic death caused by tunicamycin was dramatically reduced in a strain expressing endogenous levels of calnexin lacking its TM and cytosolic tail. This demonstrates the involvement of calnexin in apoptosis triggered by ER stress. A genetic screen identified the S. pombe homologue of the human antiapoptotic protein HMGB1 as a suppressor of apoptotic death due to calnexin overexpression. Remarkably, overexpression of human calnexin in S. pombe also provoked apoptotic death. Our results argue for the conservation of the role of calnexin in apoptosis triggered by ER stress, and validate S. pombe as a model to elucidate the mechanisms of calnexin-mediated cell death. INTRODUCTIONThe endoplasmic reticulum (ER) is a specialized organelle playing essential and central roles in the biology of the cell. The ER is the site of synthesis and folding of secreted, membrane-bound and some organelle-targeted proteins (Bukau et al., 2000;Fewell et al., 2001). Protein folding in the ER is assisted by a battery of molecular chaperones and foldases (Bukau et al., 2000;Fewell et al., 2001;Trombetta and Parodi, 2003;Helenius and Aebi, 2004). In addition, the ER contains several factors required for optimum protein folding, including, ATP, Ca 2ϩ , and an oxidizing environment to allow disulphide-bond formation. Proper ER function is critical for numerous aspects of cell physiology, including vesicle trafficking and lipid and membrane biogenesis, as well as protein targeting and secretion (Lai et al., 2007).The ER is highly sensitive to stresses perturbing the cellular energy levels and ER lipid or glycolipid imbalances or changes in the redox state or Ca 2ϩ concentration (Breckenridge et al., 2003;Boyce and Yuan, 2006;Szegezdi et al., 2006). Such stresses reduce the protein folding capacity of the ER, which results in the accumulation and aggregation of unfolded proteins, a condition referred to as ER stress. When the capacity of the ER to fold proteins properly is compromised or overwhelmed, a highly conserved unfolded-protein response (UPR) signal-transduction pathway is activated. The ER response to stress is basically conserved from yeast to mammalian cells (Patil and Walter, 2001;Ron and Walter, 2007). To counter ER stress, the UPR halts general protein synthesis and up-regulates the transcription of genes encoding ER resident chaperone...

We have cloned a Schizosaccharomyces pombe gene, here designated cnx1, encoding the homologue of the endoplasmic reticulum molecular chaperone calnexin. Disruption of the cnx1 gene was lethal, demonstrating that it has an essential cellular function. Transcription of cnx1 mRNA is initiated at multiple sites, and it can be induced by various stress treatments that lead to the accumulation of unfolded and/or misfolded proteins in the endoplasmic reticulum. The encoded Cnx1p protein more closely resembles its plant and animal calnexin homologues than that of Saccharomyces cerevisiae. Cnx1p is acidic and migrates aberrantly on SDS-polyacrylamide gel electrophoresis, similar to its mammalian counterparts. Cnx1p contains the hallmark KPEDWD motifs that are found in all members of the calnexin/calreticulin family of proteins. Using an in vitro translation-processing system, we have shown that Cnx1p has the characteristic type I topology of calnexin proteins. Unlike its higher eukaryotic homologues, Cnx1p has a site for N-glycosylation that was modified in an in vitro translation-processing assay.

In the past years, simple organisms such as yeasts and worms have contributed a great deal to aging research. Studies pioneered in Saccharomyces cerevisiae were useful to elucidate a significant number of molecular mechanisms underlying cellular aging and to discover novel longevity genes. Importantly, these genes proved many times to be conserved in multicellular eukaryotes. Consequently, such discovery approaches are being extended to other yeast models, such as Schizosaccharomyces pombe, Candida albicans, Kluyveromyces lactis, and Cryptococcus neoformans. In fission yeast, researchers have found links between asymmetrical cell division and nutrient signaling pathways with aging. In this review, we discuss the state of knowledge on the mechanisms controlling both replicative and chronological aging in S pombe and the other emergent yeast models.