Table of contents

Volume 1, Issue 6, pp. 163 - 209, June 2014

Issue cover
Cover: Fungicidal action of the combination of posaconazole (PCZ) with the heat shock protein 90 inhibitor (17-demethoxy-17-(2-propenylamino) geldanamycin, 17AAG) against S. prolificans germlings as depicted by the morbidity dye DiBAC (original colors were modified for cover design). Picture by Fazal Shirazi and Dimitrios P. Kontoyiannis (The University of Texas MD Anderson Cancer Center, USA). The cover is published under the Creative Commons Attribution (CC BY) license. Enlarge issue cover

Reviews

Cell-autonomous mechanisms of chronological aging in the yeast Saccharomyces cerevisiae

Anthony Arlia-Ciommo, Anna Leonov, Amanda Piano, Veronika Svistkova, Vladimir I. Titorenko

page 163-178 | 10.15698/mic2014.06.152 | Full text | PDF | Abstract

A body of evidence supports the view that the signaling pathways governing cellular aging – as well as mechanisms of their modulation by longevity-extending genetic, dietary and pharmacological interventions – are conserved across species. The scope of this review is to critically analyze recent advances in our understanding of cell-autonomous mechanisms of chronological aging in the budding yeast Saccharomyces cerevisiae. Based on our analysis, we propose a concept of a biomolecular network underlying the chronology of cellular aging in yeast. The concept posits that such network progresses through a series of lifespan checkpoints. At each of these checkpoints, the intracellular concentrations of some key intermediates and products of certain metabolic pathways – as well as the rates of coordinated flow of such metabolites within an intricate network of intercompartmental communications – are monitored by some checkpoint-specific ʺmaster regulatorʺ proteins. The concept envisions that a synergistic action of these master regulator proteins at certain early-life and late-life checkpoints modulates the rates and efficiencies of progression of such processes as cell metabolism, growth, proliferation, stress resistance, macromolecular homeostasis, survival and death. The concept predicts that, by modulating these vital cellular processes throughout lifespan (i.e., prior to an arrest of cell growth and division, and following such arrest), the checkpoint-specific master regulator proteins orchestrate the development and maintenance of a pro- or anti-aging cellular pattern and, thus, define longevity of chronologically aging yeast.

Research Articles

Heat shock protein 90 and calcineurin pathway inhibitors enhance the efficacy of triazoles against Scedosporium prolificans via induction of apoptosis

Fazal Shirazi, Dimitrios P. Kontoyiannis

page 179-188 | 10.15698/mic2014.06.150 | Full text | PDF | Abstract

Scedosporium prolificans is a pathogenic mold resistant to current antifungals, and infection results in high mortality. Simultaneous targeting of both ergosterol biosynthesis and heat shock protein 90 (Hsp90) or the calcineurin pathway in S. prolificans may be an important strategy for enhancing the potency of antifungal agents. We hypothesized that the inactive triazoles posaconazole (PCZ) and itraconazole (ICZ) acquire fungicidal activity when combined with the calcineurin inhibitor tacrolimus (TCR) or Hsp90 inhibitor 17-demethoxy-17-(2-propenylamino) geldanamycin (17AAG). PCZ, ICZ, TCR and 17AAG alone were inactive in vitro against S. prolificans spores (MICs > 128 μg/ml). In contrast, MICs for PCZ or ICZ in combination with TCR or 17AAG (0.125-0.50 μg/ml) were much lower compared with drug alone. In addition PCZ and ICZ in combination with TCR or 17AAG became fungicidal. Because apoptosis is regulated by the calcineurin pathway in fungi and is under the control of Hsp90, we hypothesized that this synergistic fungicidal effect is mediated via apoptosis. This observed fungicidal activity was mediated by increased apoptosis of S. prolificans germlings, as evidenced by reactive oxygen species accumulation, decreased mitochondrial membrane potential, phosphatidylserine externalization, and DNA fragmentation. Furthermore, induction of caspase-like activity was correlated with TCR or 17AAG + PCZ/ICZ-induced cell death. In conclusion, we report for the first time that PCZ or ICZ in combination with TCR or 17AAG renders S. prolificans exquisitely sensitive to PCZ or ICZ via apoptosis. This finding may stimulate the development of new therapeutic strategies for patients infected with this recalcitrant fungus.

At neutral pH the chronological lifespan of Hansenula polymorpha increases upon enhancing the carbon source concentrations

Adam Kawałek, Ida J. van der Klei

page 189-202 | 10.15698/mic2014.06.149 | Full text | PDF | Abstract

Dietary restriction is generally assumed to increase the lifespan in most eukaryotes, including the simple model organism Saccharomyces cerevisiae. However, recent data questioned whether this phenomenon is indeed true for yeast. We studied the effect of reduction of the carbon source concentration on the chronological lifespan of the yeast Hansenula polymorpha using four different carbon sources. Our data indicate that reduction of the carbon source concentration has a negative (glucose, ethanol, methanol) or positive (glycerol) effect on the chronological lifespan. We show that the actual effect of carbon source concentrations largely depends on extracellular factor(s). We provide evidence that H. polymorpha acidifies the medium and that a low pH of the medium alone is sufficient to significantly decrease the chronological lifespan. However, glucose-grown cells are less sensitive to low pH compared to glycerol-grown cells, explaining why only the reduction of the glycerol-concentration (which leads to less medium acidification) has a positive effect on the chronological lifespan. Instead, the positive effect of enhancing the glucose concentrations is much larger than the negative effect of the medium acidification at these conditions, explaining the increased lifespan with increasing glucose concentrations. Importantly, at neutral pH, the chronological lifespan also decreases with a reduction in glycerol concentrations. We show that for glycerol cultures this effect is related to acidification independent changes in the composition of the spent medium. Altogether, our data indicate that in H. polymorpha at neutral pH the chronological lifespan invariably extends upon increasing the carbon source concentration.

Microreviews

Decoding the biosynthesis and function of diphthamide, an enigmatic modification of translation elongation factor 2 (EF2)

Raffael Schaffrath, Michael J. R. Stark

page 203-205 | 10.15698/mic2014.06.151 | Full text | PDF | Abstract

Diphthamide is a highly conserved modification of archaeal and eukaryal translation elongation factor 2 (EF2) and yet why cells need EF2 to contain diphthamide is unclear. In yeast, the first steps of diphthamide synthesis and the genes (DPH1-DPH5) required to form the intermediate diphthine are well-documented. However, the last step, amidation of diphthine to diphthamide, had largely been ill-defined. Remarkably, through mining genome-wide synthetic gene array (SGA) and chemical genomics databases, recent studies by Uthman et al. [PLoS Genetics (2013) 9, e1003334] and Su et al. [Proc. Natl. Acad. Sci. USA (2012) 109, 19983-19987] have identified two more diphthamide players, DPH6 and DPH7. Consistent with roles in the amidation step, dph6 and dph7 deletion strains fail to complete diphthamide synthesis and accumulate diphthine-modified EF2. In contrast to Dph6, the catalytically relevant amidase, Dph7 appears to be regulatory. As shown by Uthman et al., it promotes dissociation of diphthine synthase (Dph5) from EF2, allowing diphthine amidation by Dph6 to occur and thereby coupling diphthine synthesis to the terminal step in the pathway. Remarkably, the study by Uthman et al. suggests that Dph5 has a novel role as an EF2 inhibitor that affects cell growth when diphthamide synthesis is blocked or incomplete and, importantly, shows that diphthamide promotes the accuracy of EF2 performance during translation.

Genome evolution in yeast reveals connections between rare mutations in human cancer

Xinchen Teng, J. Marie Hardwick

page 206-209 | 10.15698/mic2014.06.153 | Full text | PDF | Abstract

Cancer cells are riddled with mutations. Less than one percent of these are thought to be mutations that drive cancer phenotypes. However, a recent study conducted on the yeast knockout collections by Teng et al. [Mol. Cell (2013) 52: 485–494] provides hard evidence that single gene deletions/mutations in most non-essential genes can drive the selection for cancer-like mutations.