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Volume 4, Issue 12, pp. 390 - 427, December 2017

Issue cover
Cover: The image is a molecular surface representation of the the aminoglycoside acetyltransferase AAC(6')-Im, with a bound kanamycin shown in ball-and-stick (image by Clyde Smith, Stanford University, USA); image modified by MIC. The cover is published under the Creative Commons Attribution (CC BY) license. Enlarge issue cover

Research Articles

A genome-wide screen for FTY720-sensitive mutants reveals genes required for ROS homeostasis

Kanako Hagihara, Kanako Kinoshita, Kouki Ishida, Shihomi Hojo, Yoshinori Kameoka, Ryosuke Satoh, Teruaki Takasaki and Reiko Sugiura

page 390-401 | 10.15698/mic2017.12.601 | Full text | PDF | Abstract

Fingolimod hydrochloride (FTY720), a sphingosine-1-phosphate (S1P) analogue, is an approved immune modulator for the treatment of multiple sclerosis (MS). Notably, in addition to its well-known mode of action as an S1P modulator, accumulating evidence suggests that FTY720 induces apoptosis in various cancer cells via reactive oxygen species (ROS) generation. Although the involvement of multiple signaling molecules, such as JNK (Jun N-terminal kinase), Akt (alpha serine/threonine-protein kinase) and Sphk has been reported, the exact mechanisms how FTY720 induces cell growth inhibition and the functional relationship between FTY720 and these signaling pathways remain elusive. Our previous reports using the fission yeast Schizosaccharomyces pombe as a model system to elucidate FTY720-mediated signaling pathways revealed that FTY720 induces an increase in intracellular Ca2+ concentrations and ROS generation, which resulted in the activation of the transcriptional responses downstream of Ca2+/calcineurin signaling and stress-activated MAPK signaling, respectively. Here, we performed a genome-wide screening for genes whose deletion induces FTY720-sensitive growth in S. pombe and identified 49 genes. These gene products are related to the biological processes involved in metabolic processes, transport, transcription, translation, chromatin organization, cytoskeleton organization and intracellular signal transduction. Notably, most of the FTY720-sensitive deletion cells exhibited NAC-remedial FTY720 sensitivities and dysregulated ROS homeostasis. Our results revealed a novel gene network involving ROS homeostasis and the possible mechanisms of the FTY720 toxicity.

Aminoglycoside resistance profile and structural architecture of the aminoglycoside acetyltransferase AAC(6’)-Im

Clyde A. Smith, Monolekha Bhattacharya, Marta Toth, Nichole K. Stewart and Sergei B. Vakulenko

page 402-410 | 10.15698/mic2017.12.602 | Full text | PDF | Abstract

Aminoglycoside 6’-acetyltransferase-Im (AAC(6’)-Im) is the closest monofunctional homolog of the AAC(6’)-Ie acetyltransferase of the bifunctional enzyme AAC(6’)-Ie/APH(2”)-Ia. The AAC(6’)-Im acetyltransferase confers 4- to 64-fold higher MICs to 4,6-disubstituted aminoglycosides and the 4,5-disubstituted aminoglycoside neomycin than AAC(6’)-Ie, yet unlike AAC(6’)-Ie, the AAC(6’)-Im enzyme does not confer resistance to the atypical aminoglycoside fortimicin. The structure of the kanamycin A complex of AAC(6’)-Im shows that the substrate binds in a shallow positively-charged pocket, with the N6’ amino group positioned appropriately for an efficient nucleophilic attack on an acetyl-CoA cofactor. The AAC(6’)-Ie enzyme binds kanamycin A in a sufficiently different manner to position the N6’ group less efficiently, thereby reducing the activity of this enzyme towards the 4,6-disubstituted aminoglycosides. Conversely, docking studies with fortimicin in both acetyltransferases suggest that the atypical aminoglycoside might bind less productively in AAC(6’)-Im, thus explaining the lack of resistance to this molecule.

Research Reports

Mitochondrial energy metabolism is required for lifespan extension by the spastic paraplegia-associated protein spartin

Julia Ring, Patrick Rockenfeller, Claudia Abraham, Jelena Tadic, Michael Poglitsch, Katherina Schimmel, Julia Westermayer, Simon Schauer, Bettina Achleitner, Christa Schimpel, Barbara Moitzi, Gerald N. Rechberger, Stephan J. Sigrist, Didac Carmona-Gutierrez, Guido Kroemer, Sabrina Büttner, Tobias Eisenberg, Frank Madeo

page 411-422 | 10.15698/mic2017.12.603 | Full text | PDF | Abstract

Hereditary spastic paraplegias, a group of neurodegenerative disorders, can be caused by loss-of-function mutations in the protein spartin. However, the physiological role of spartin remains largely elusive. Here we show that heterologous expression of human or Drosophila spartin extends chronological lifespan of yeast, reducing age-associated ROS production, apoptosis, and necrosis. We demonstrate that spartin localizes to the proximity of mitochondria and physically interacts with proteins related to mitochondrial and respiratory metabolism. Interestingly, Nde1, the mitochondrial external NADH dehydrogenase, and Pda1, the core enzyme of the pyruvate dehydrogenase complex, are required for spartin-mediated cytoprotection. Furthermore, spartin interacts with the glycolysis enhancer phospo-fructo-kinase-2,6 (Pfk26) and is sufficient to complement for PFK26-deficiency at least in early aging. We conclude that mitochondria-related energy metabolism is crucial for spartin’s vital function during aging and uncover a network of specific interactors required for this function.


A new role for the nuclear basket network

Paola Gallardo, Silvia Salas-Pino and Rafael R. Daga

page 423-425 | 10.15698/mic2017.12.604 | Full text | PDF | Abstract

Our view of the nuclear pore complexes (NPCs) as gateways between the nuclear and cytoplasmic compartments has been largely expanded in recent years. NPCs have now demonstrated roles in genome regulation and maintenance from single cells to multicellular organisms. Both NPC proteins as well as components of the NPC basket act as dynamic scaffolds for silencing factors, and chromatin and cell cycle regulators. Components of the NPC basket also couple mRNA production and export, and prevent the exit of unprocessed mRNAs from the nucleus. Our recent work describes a novel function of the fission yeast nuclear basket component – the translocated promoter region (TPR) nucleoporin Alm1 – in proper localization of the proteasome to the nuclear envelope. Here we discuss how regulation of proteasome localization to the nuclear envelope by Alm1 is key to maintain kinetochores homeostasis and proper chromosome segregation.

VAMP8 mucin exocytosis attenuates intestinal pathogenesis by Entamoeba histolytica

Steve Cornick, France Moreau, Herbert Y. Gaisano, Kris Chadee

page 426-427 | 10.15698/mic2017.12.605 | Full text | PDF | Abstract

The intestinal mucosa encounters a barrage of ingested insults within the host yet under homeostasis elegantly facilitates nutrient absorption and sustenance of the commensal microbiota. An essential defence mechanism employed by the host is limiting the spatial niche various microbes may occupy as executed by the fluid mucus layer. Pathogens that violate their restricted niche within the intestinal mucosa are first expelled by robust mucus secretion from goblet cells thus by-passing the need for an immune response. Surprisingly, while many pathogens are known to exert hyper-secretion of mucus from goblet cells, the mechanisms governing this event remain elusive. In a recent report by Cornick et al (MBio 8: e01323-17), we nominate SNARE-mediated exocytosis as the putative mechanism responsible for pathogen-induced mucus secretion from goblet cells. The vesicle SNARE VAMP8 on mucin granules within goblet cells is specifically activated following infection with the protozoan parasite Entamoeba histolytica that is known to induce potent hyper-secretion and coordinates mucin exocytosis. This secretion event is critical in fending off a pathogen, as cells lacking VAMP8 are prone to increased E. histolytica colonization and cytolysis through apoptosis. Failing coordinated mucus exocytosis and subsequent epithelial barrier destruction, the host mounts an immune response as a last line of defence.