Analysis of mitogen-activated protein kinase signaling specificity in response to hyperosmotic stress: use of an analog-sensitive HOG1 allele

Role of the cell wall integrity and filamentous growth mitogen-activated protein kinase pathways in cell wall remodeling during filamentous growth

Many fungal species including pathogens exhibit filamentous growth (FG) as a means of foraging for nutrients. Genetic screens were performed to identify genes required for FG in the budding yeast Saccharomyces cerevisiae. Genes encoding proteins with established functions in transcriptional activation (MCM1, MATalpha2, PHD1, MSN2, SIR4, and HMS2), cell wall integrity (MPT5, WSC2, and MID2), and cell polarity (BUD5) were identified as potential regulators of FG. The transcription factors MCM1 and MATalpha2 induced invasive growth by promoting diploid-specific bipolar budding in haploid cells. Components of the cell wall integrity pathway including the cell surface proteins Slg1p/Wsc1p, Wsc2p, Mid2p, and the mitogen-activated protein kinase (MAPK) Slt2p/Mpk1p contributed to multiple aspects of the FG response including cell elongation, cell-cell adherence, and agar invasion. Mid2p and Wsc2p stimulated the FG MAPK pathway through the signaling mucin Msb2p and components of the MAPK cascade. The FG pathway contributed to cell wall integrity in parallel with the cell wall integrity pathway and in opposition with the high osmolarity glycerol response pathway. Mass spectrometry approaches identified components of the filamentous cell wall including the mucin-like proteins Msb2p, Flo11p, and subtelomeric (silenced) mucin Flo10p. Secretion of Msb2p, which occurs as part of the maturation of the protein, was inhibited by the ss-1,3-glucan layer of the cell wall, which highlights a new regulatory aspect to cell wall remodeling in this organism. Disruption of ss-1,3-glucan linkages induced mucin shedding and resulted in defects in cell-cell adhesion and invasion of cells into the agar matrix.

The signaling mucins Msb2 and Hkr1 differentially regulate the filamentation mitogen-activated protein kinase pathway and contribute to a multimodal response

A central question in the area of signal transduction is why pathways utilize common components. In the budding yeast Saccharomyces cerevisiae, the HOG and filamentous growth (FG) MAPK pathways require overlapping components but are thought to be induced by different stimuli and specify distinct outputs. To better understand the regulation of the FG pathway, we examined FG in one of yeast's native environments, the grape-producing plant Vitis vinifera. In this setting, different aspects of FG were induced in a temporal manner coupled to the nutrient cycle, which uncovered a multimodal feature of FG pathway signaling. FG pathway activity was modulated by the HOG pathway, which led to the finding that the signaling mucins Msb2p and Hkr1p, which operate at the head of the HOG pathway, differentially regulate the FG pathway. The two mucins exhibited different expression and secretion patterns, and their overproduction induced nonoverlapping sets of target genes. Moreover, Msb2p had a function in cell polarization through the adaptor protein Sho1p that Hkr1p did not. Differential MAPK activation by signaling mucins brings to light a new point of discrimination between MAPK pathways.

Dynamic signaling in the Hog1 MAPK pathway relies on high basal signal transduction

Appropriate regulation of the Hog1 mitogen-activated protein kinase (MAPK) pathway is essential for cells to survive osmotic stress. Here, we show that the two sensing mechanisms upstream of Hog1 display different signaling properties. The Sho1 branch is an inducible nonbasal system, whereas the Sln1 branch shows high basal signaling that is restricted by a MAPK-mediated feedback mechanism. A two-dimensional mathematical model of the Snl1 branch, including high basal signaling and a Hog1-regulated negative feedback, shows that a system with basal signaling exhibits higher efficiency, with faster response times and higher sensitivity to variations in external signals, than would systems without basal signaling. Analysis of two other yeast MAPK pathways, the Fus3 and Kss1 signaling pathways, indicates that high intrinsic basal signaling may be a general property of MAPK pathways allowing rapid and sensitive responses to environmental changes.

Hog1 mitogen-activated protein kinase (MAPK) interrupts signal transduction between the Kss1 MAPK and the Tec1 transcription factor to maintain pathway specificity

In Saccharomyces cerevisiae, the mating, filamentous growth (FG), and high-osmolarity glycerol (HOG) mitogen-activated protein kinase (MAPK) signaling pathways share components and yet mediate distinct responses to different extracellular signals. Cross talk is suppressed between the mating and FG pathways because mating signaling induces the destruction of the FG transcription factor Tec1. We show here that HOG pathway activation results in phosphorylation of the FG MAPK, Kss1, and the MAPKK, Ste7. However, FG transcription is not activated because HOG signaling prevents the activation of Tec1. In contrast to the mating pathway, we find that the mechanism involves the inhibition of DNA binding by Tec1 rather than its destruction. We also find that nuclear accumulation of Tec1 is not affected by HOG signaling. Inhibition by Hog1 is apparently indirect since it does not require any of the consensus S/TP MAPK phosphorylation sites on Tec1, its DNA-binding partner Ste12, or the associated regulators Dig1 or Dig2. It also does not require the consensus MAPK sites of the Ste11 activator Ste50, in contrast to a recent proposal for a role for negative feedback in specificity. Our results demonstrate that HOG signaling interrupts the FG pathway signal transduction between the phosphorylation of Kss1 and the activation of DNA binding by Tec1.

Stress resistance and signal fidelity independent of nuclear MAPK function

Elevated external solute stimulates a conserved MAPK cascade that elicits responses that maintain osmotic balance. The yeast high-osmolarity glycerol (HOG) pathway activates Hog1 MAPK (mammalian ortholog p38alpha/SAPKalpha), which enters the nucleus and induces expression of >50 genes, implying that transcriptional up-regulation is necessary to cope with hyperosmotic stress. Contrary to this expectation, we show here that cells lacking the karyopherin required for Hog1 nuclear import or in which Hog1 is anchored at the plasma membrane (or both) can withstand long-term hyperosmotic challenge by ionic and nonionic solutes without exhibiting the normal change in transcriptional program (comparable with hog1Delta cells), as judged by mRNA hybridization and microarray analysis. For such cells to survive hyperosmotic stress, systematic genetic analysis ruled out the need for any Hog1-dependent transcription factor, the Hog1-activated MAPKAP kinases, or ion, glycerol, and water channels. By contrast, enzymes needed for glycerol production were essential for viability. Thus, control of intracellular glycerol formation by Hog1 is critical for maintenance of osmotic balance but not transcriptional induction of any gene.

Mitogen-activated protein kinase Hog1 mediates adaptation to G1 checkpoint arrest during arsenite and hyperosmotic stress

Cells slow down cell cycle progression in order to adapt to unfavorable stress conditions. Yeast (Saccharomyces cerevisiae) responds to osmotic stress by triggering G(1) and G(2) checkpoint delays that are dependent on the mitogen-activated protein kinase (MAPK) Hog1. The high-osmolarity glycerol (HOG) pathway is also activated by arsenite, and the hog1Delta mutant is highly sensitive to arsenite, partly due to increased arsenite influx into hog1Delta cells. Yeast cell cycle regulation in response to arsenite and the role of Hog1 in this process have not yet been analyzed. Here, we found that long-term exposure to arsenite led to transient G(1) and G(2) delays in wild-type cells, whereas cells that lack the HOG1 gene or are defective in Hog1 kinase activity displayed persistent G(1) cell cycle arrest. Elevated levels of intracellular arsenite and "cross talk" between the HOG and pheromone response pathways, observed in arsenite-treated hog1Delta cells, prolonged the G(1) delay but did not cause a persistent G(1) arrest. In contrast, deletion of the SIC1 gene encoding a cyclin-dependent kinase inhibitor fully suppressed the observed block of G(1) exit in hog1Delta cells. Moreover, the Sic1 protein was stabilized in arsenite-treated hog1Delta cells. Interestingly, Sic1-dependent persistent G(1) arrest was also observed in hog1Delta cells during hyperosmotic stress. Taken together, our data point to an important role of the Hog1 kinase in adaptation to stress-induced G(1) cell cycle arrest.

Comparative genomics of the environmental stress response in ascomycete fungi

Unicellular fungi thrive in diverse niches around the world, and many of these niches present unique and stressful challenges that must be contended with by their inhabitants. Numerous studies have investigated the genomic expression responses to environmental stress in 'model' ascomycete fungi, including Saccharomyces cerevisiae, Candida albicans and Schizosaccharomyces pombe. This review presents a comparative-genomics perspective on the environmental stress response, a common response to diverse stresses. Implications for the role of this response, based on its presence or absence in fungi from disparate ecological niches, are discussed.

MOS1 osmosensor of Metarhizium anisopliae is required for adaptation to insect host hemolymph

Entomopathogenic fungi such as Metarhizium anisopliae infect insects by direct penetration of the cuticle, after which the fungus adapts to the high osmotic pressure of the hemolymph and multiplies. Here we characterize the M. anisopliae Mos1 gene and demonstrate that it encodes the osmosensor required for this process. MOS1 contains transmembrane regions and a C-terminal Src homology 3 domain similar to those of yeast osmotic adaptor proteins, and homologs of MOS1 are widely distributed in the fungal kingdom. Reverse transcription-PCR demonstrated that Mos1 is up-regulated in insect hemolymph as well as artificial media with high osmotic pressure. Transformants containing an antisense vector directed to the Mos1 mRNA depleted transcript levels by 80%. This produced selective alterations in regulation of genes involved in hyphal body formation, cell membrane stiffness, and generation of intracellular turgor pressure, suggesting that these processes are mediated by MOS1. Consistent with a role in stress responses, transcript depletion of Mos1 increased sensitivity to osmotic and oxidative stresses and to compounds that interfere with cell wall biosynthesis. It also disrupted developmental processes, including formation of appressoria and hyphal bodies. Insect bioassays confirmed that Mos1 knockdown significantly reduces virulence. Overall, our data show that M. anisopliae MOS1 mediates cellular responses to high osmotic pressure and subsequent adaptations to colonize host hemolymph.

Function and regulation in MAPK signaling pathways: lessons learned from the yeast Saccharomyces cerevisiae

Signaling pathways that activate different mitogen-activated protein kinases (MAPKs) elicit many of the responses that are evoked in cells by changes in certain environmental conditions and upon exposure to a variety of hormonal and other stimuli. These pathways were first elucidated in the unicellular eukaryote Saccharomyces cerevisiae (budding yeast). Studies of MAPK pathways in this organism continue to be especially informative in revealing the molecular mechanisms by which MAPK cascades operate, propagate signals, modulate cellular processes, and are controlled by regulatory factors both internal to and external to the pathways. Here we highlight recent advances and new insights about MAPK-based signaling that have been made through studies in yeast, which provide lessons directly applicable to, and that enhance our understanding of, MAPK-mediated signaling in mammalian cells.

Dissecting yeast Hog1 MAP kinase pathway using a chemical genetic approach

Using a chemical genetic approach, we identified four novel physiological substrates of Hog1 kinase (Krs1, Tdh3, Hsp26, and Shm2). These substrates suggest plausible mechanisms for actin reorganization, cell cycle arrest and regulation of protein synthesis observed upon osmotic stress. We further show that the human homolog of Shm2 (SHMT1) is a novel physiological substrate of p38 MAP kinase in vitro and in vivo. Down-regulation of its enzymatic activity was observed following p38-mediated phosphorylation revealing a potential cancer-modulating property of p38 MAP kinase. This screen has uncovered several novel Hog1 substrates that provide new avenues for investigation into the mechanism of osmoadaptation by this kinase.