Activation of S phase checkpoint by cigarette smoke extract in Schizosaccharomyces pombe

Cigarette smoke affects ESCRT-mediated vacuolar activity in Saccharomyces cerevisiae

Cigarette smoking is a risk factor for developing chronic obstructive pulmonary disease and protein aggresome formation is considered to be a hallmark event for the disease. Since dysfunction of lysosome-mediated protein degradation leads to enhanced accumulation of misfolded proteins and subsequent aggresome formation, we examined the effect of cigarette smoke extract (CSE) on ESCRT-mediated sorting in S. cerevisiae as this process is necessary for the functioning of the vacuole, the lysosomal equivalent in yeast. An operational ESCRT pathway is essential for ion homeostasis and our observation that exposure to CSE caused increased sensitivity to LiCl indicated CSE-induced impairment of ESCRT function. To confirm the inhibition of ESCRT function, the targeting of carboxypeptidase S (CPS), which reaches the vacuole lumen via the ESCRT pathway, was examined. Treatment with CSE resulted in the mislocalization of GFP-tagged CPS to the vacuolar membrane, instead of the vacuolar lumen, confirming defective functioning of the ESCRT machinery in CSE-treated cells. Further analysis revealed that CSE-treatment inhibited the recruitment of the ESCRT-0 component, Vps27, to the endosome surface, which is a key event is for the functioning of the ESCRT pathway. This lack of endosomal recruitment of Vps27 most likely results from a depletion of the endosomally-enriched lipid, phosphatidylinositol 3-phosphate (PI3-P), which is the target of Vps27. This is supported by our observation that the presence of excess leucine, a known activator of the lipid kinase responsible for the generation of PI3-P, Vps34, in the medium can rescue the CSE-induced ESCRT misfunctioning. Thus, the current study provides an insight into CSE-induced aggresome formation as it documents that CSE treatment compromises vacuolar degradation due to an impairment of the ESCRT pathway, which likely stems from the inhibition of Vps34. It also indicates that leucine has the potential to attenuate the CSE-induced accumulation of misfolded proteins.

Communication between Cyclin-dependent kinase Cdc2 and the Wis1-Spc1 MAPK pathway determines mitotic timing in Schizosaccharomyces pombe

Checkpoint activation and gene expression modulation represent key determinants of cellular survival in adverse conditions. The former is regulated by cyclin-dependent kinases (CDKs) while the latter can be controlled by mitogen-activated protein kinases (MAPKs). Association between cell-cycle progression and MAPK-dependent gene expression exists in cells growing in optimal environments. While MAPK-mediated regulation of the cell cycle is well characterised, the reciprocal influence of mitotic CDK on stress response is not well studied. We present evidence that CDK activity can regulate the extent of MAPK activation in Schizosaccharomyces pombe cells. We show that increasing or decreasing mitotic CDK (Cdc2) activity in S. pombe cells can affect the activation of stress responsive MAPK (Spc1) even in the absence of stress stimuli. Our results indicate that the strong correlation between Cdc2 activity and Spc1 MAPK-activity in S. pombe is important in regulating mitotic timing.This article has an associated First Person interview with the first author of the paper.

Expression of the atf1+ gene is upregulated in fission yeast under nitrosative and nutritional stresses

This work was designed to assess regulation of the atf1+ gene in the fission yeast Schizosaccharomyces pombe under nitrosative and nutritional stresses, using the atf1+-lacZ fusion gene and RT-PCR. Nitric oxide (NO)-generating sodium nitroprusside (SNP; 10 micromol/L) and nitrogen depletion significantly enhanced synthesis of beta-galactosidase from the atf1+-lacZ fusion gene in S. pombe Pap1-positive KP1 cells, but not in S. pombe Pap1-negative TP108-3C cells. SNP (10 micromol/L) and nitrogen depletion also caused a significant increase in atf1+ mRNA levels in Pap1-positive cells, but not in Pap1-negative cells. Depletion of glucose marginally increased synthesis of beta-galactosidase from the fusion gene in S. pombe Pap1-positive cells. Taken together, the S. pombe atf1+ gene is upregulated by nitrosative and nutritional stresses on a transcriptional level, possibly via the mediation of Pap1.

Characterization of Sro1, a novel stress responsive protein in Schizosaccharomyces pombe

The large amount of available genome sequencing data presents a huge challenge in the form of orphan sequences. This study reports the detailed functional characterization of one such orphan sequence in Schizosaccharomyces pombe. We identified this gene as a prominently upregulated 1.4 kb transcript in a screen for Cigarette smoke extract responsive genes in S. pombe and named it Stress Responsive Orphan 1 (Sro1). We report various functions of Sro1 in regulation of cellular behaviour under stress conditions. We show that this gene (Sro1) responds to a variety of stress conditions and that the expression of the gene is regulated mainly through the stress activated protein kinase (SAPK) Sty1 and its downstream transcription factor Atf1. Deletion of Sro1 also significantly alters the reactive oxygen species (ROS) generation profiles and the cell-cycle progression of S. pombe during stress conditions. The stress-specific alteration of the ROS generation profiles and checkpoint activation resulting from deletion of the gene suggest that Sro1 might be a key player in determining cellular responses/fate under stress conditions.