Table of contents

Volume 3, Issue 8, pp. 302 - 360, August 2016

Issue cover
Cover: False-colored scanning electron microscopy imagery of bloodstream form Trypanosoma brucei. Image by Torsten Ochsenreiter (University of Bern, Switzerland); modified by MIC. The cover is published under the Creative Commons Attribution (CC BY) license. Enlarge issue cover

Viewpoint

Similar environments but diverse fates: Responses of budding yeast to nutrient deprivation.

Saul M. Honigberg

page 302-328 | 10.15698/mic2016.08.516 | Full text | PDF | Abstract

Diploid budding yeast (Saccharomyces cerevisiae) can adopt one of several alternative differentiation fates in response to nutrient limitation, and each of these fates provides distinct biological functions. When different strain backgrounds are taken into account, these various fates occur in response to similar environmental cues, are regulated by the same signal transduction pathways, and share many of the same master regulators. I propose that the relationships between fate choice, environmental cues and signaling pathways are not Boolean, but involve graded levels of signals, pathway activation and master-regulator activity. In the absence of large differences between environmental cues, small differences in the concentration of cues may be reinforced by cell-to-cell signals. These signals are particularly essential for fate determination within communities, such as colonies and biofilms, where fate choice varies dramatically from one region of the community to another. The lack of Boolean relationships between cues, signaling pathways, master regulators and cell fates may allow yeast communities to respond appropriately to the wide range of environments they encounter in nature.

Reviews

Functions and regulation of the MRX complex at DNA double-strand breaks

Elisa Gobbini, Corinne Cassani, Matteo Villa, Diego Bonetti and Maria Pia Longhese

page 329-337 | 10.15698/mic2016.08.517 | Full text | PDF | Abstract

DNA double-strand breaks (DSBs) pose a serious threat to genome stability and cell survival. Cells possess mechanisms that recognize DSBs and promote their repair through either homologous recombination (HR) or non-homologous end joining (NHEJ). The evolutionarily conserved Mre11-Rad50-Xrs2 (MRX) complex plays a central role in the cellular response to DSBs, as it is implicated in controlling end resection and in maintaining the DSB ends tethered to each other. Furthermore, it is responsible for DSB signaling by activating the checkpoint kinase Tel1 that, in turn, supports MRX function in a positive feedback loop. The present review focuses mainly on recent works in the budding yeast Saccharomyces cerevisiae to highlight structure and regulation of MRX as well as its interplays with Tel1.

Research Articles

Attenuation of polyglutamine-induced toxicity by enhancement of mitochondrial OXPHOS in yeast and fly models of aging

Andrea L. Ruetenik, Alejandro Ocampo, Kai Ruan, Yi Zhu, Chong Li, R. Grace Zhai and Antoni Barrientos

page 338-351 | 10.15698/mic2016.08.518 | Full text | PDF | Abstract

Defects in mitochondrial biogenesis and function are common in many neurodegenerative disorders, including Huntington’s disease (HD). We have previously shown that in yeast models of HD, enhancement of mitochondrial biogenesis through overexpression of Hap4, the catalytic subunit of the transcriptional complex that regulates mitochondrial gene expression, alleviates the growth arrest induced by expanded polyglutamine (polyQ) tract peptides in rapidly dividing cells. However, the mechanism through which HAP4 overexpression exerts this protection remains unclear. Furthermore, it remains unexplored whether HAP4 overexpression and increased respiratory function during growth can also protect against polyQ-induced toxicity during yeast chronological lifespan. Here, we show that in yeast, mitochondrial respiration and oxidative phosphorylation (OXPHOS) are essential for protection against the polyQ-induced growth defect by HAP4 overexpression. In addition, we show that not only increased HAP4 levels, but also alternative interventions, including calorie restriction, that result in enhanced mitochondrial biogenesis confer protection against polyQ toxicity during stationary phase. The data obtained in yeast models guided experiments in a fly model of HD, where we show that enhancement of mitochondrial biogenesis can also protect against neurodegeneration and behavioral deficits. Our results suggest that therapeutic interventions aiming at the enhancement of mitochondrial respiration and OXPHOS could reduce polyQ toxicity and delay disease onset.

Microreviews

A novel component of the mitochondrial genome segregation machinery in trypanosomes

Anneliese Hoffmann, Martin Jakob, and Torsten Ochsenreiter

page 352-354 | 10.15698/mic2016.08.519 | Full text | PDF | Abstract

We recently described a new component (TAC102) of the mitochondrial genome segregation machinery (mtGSM) in the protozoan parasite Trypanosoma brucei. T. brucei belongs to a group of organisms that contain a single mitochondrial organelle with a single mitochondrial genome (mt-genome) per cell. The mt-genome consists of 5000 minicircles (1 kb) and 25 maxicircles (23 kb) that are catenated into a large network. After replication of the network its segregation is driven by the separating basal bodies, which are homologous structures to the centrioles organizing the spindle apparatus in many eukaryotes. The structure connecting the basal body to the mt-genome was named the Tripartite Attachment Complex (TAC) owing its name to the distribution across three areas in the cell including the two mitochondrial membranes.

Bacterial genotoxin functions as immune-modulator and promotes host survival

Guidi, L. Del Bell Belluz, T. Frisan

page 355-357 | 10.15698/mic2016.08.520 | Full text | PDF | Abstract

Bacterial genotoxins are effectors that cause DNA damage in target cells. Many aspects of the biology of these toxins have been characterised in vitro, such as structure, cellular internalisation pathways and effects on the target cells. However, little is known about their function in vivo. Salmonella enterica serovar Typhi (S. Typhi) is a Gram-negative, intracellular bacterium that causes typhoid fever, a debilitating disease infecting more than 20 million people every year. S. Typhi produce a genotoxin named typhoid toxin (TT), but its role in the contest of host infection is poorly characterized. The major obstacle in addressing this issue is that S. Typhi is exclusively a human pathogen. To overcome this limitation, we have used as model bacterium S. Typhimurium, and engineered it to produce endogenous levels of an active and inactive typhoid toxin, hereby named as TT (or genotoxic) and cdtB (or control), respectively. To our surprise, infection with the genotoxin strain strongly suppressed intestinal inflammation, leading to a better survival of the host during the acute phase of infection, suggesting typhoid toxin may exert a protective role. The presence of a functional genotoxin was also associated with an increased frequency of asymptomatic carriers.

Cryptococcus flips its lid – membrane phospholipid asymmetry modulates antifungal drug resistance and virulence

Erika Shor, Yina Wang, David S. Perlin, and Chaoyang Xue

page 358-360 | 10.15698/mic2016.08.521 | Full text | PDF | Abstract

Human fungal infections are increasing in prevalence and acquisition of antifungal drug resistance, while our antifungal drug armamentarium remains very limited, constituting a significant public health problem. Despite the fact that prominent antifungal drugs target the fungal cell membrane, very little is known about how fungal membrane biology regulates drug-target interactions. Asymmetrical phospholipid distribution is an essential property of biological membranes, which is maintained by a group of transporters that dynamically translocate specific phospholipid groups across the membrane bilayer. Lipid flippase is the enzyme responsible for translocation of certain phospholipids, including phosphatidylserine (PS), across the plasma membrane from the exocytoplasmic to the cytoplasmic leaflet. Loss of lipid flippase leads to abnormal phospholipid distribution and impaired intracellular vesicular trafficking. The recent research article by Huang et al. reported that in pathogenic fungus Cryptococcus neoformans loss of lipid flippase activity sensitized cryptococcal cells to multiple classes of antifungal drugs, including the cell wall active echinocandins, and abolished fungal virulence in murine models. This finding demonstrates that lipid flippase may promote fungal drug resistance and virulence and indicates that this enzyme may represent a novel antifungal drug target.