Yeast Cip1 is activated by environmental stress to inhibit Cdk1-G1 cyclins via Mcm1 and Msn2/4

Neuroretinal Cell Culture Model as a Tool for the Development of New Therapeutic Approaches for Oxidative Stress-Induced Ocular Diseases, with a Focus on Glaucoma

Glaucoma is a heterogeneous group of optic neuropathies characterized by a progressive degeneration of the retinal ganglion cells (RGCs), leading to irreversible vision loss. Nowadays, the traditional therapeutic approach to glaucoma consists of lowering the intraocular pressure (IOP), which does not address the neurodegenerative features of the disease. Besides animal models of glaucoma, there is a considerable need for in vitro experimental models to propose new therapeutic strategies for this ocular disease. In this study, we elucidated the pathological mechanisms leading to neuroretinal R28 cell death after exposure to glutamate and hydrogen peroxide (H2O2) in order to develop new therapeutic approaches for oxidative stress-induced retinal diseases, including glaucoma. We were able to show that glutamate and H2O2 can induce a decrease in R28 cell viability in a concentration-dependent manner. A cell viability of about 42% was found after exposure to 3 mM of glutamate and about 56% after exposure to 100 µM of H2O2 (n = 4). Label-free quantitative mass spectrometry analysis revealed differential alterations of 193 and 311 proteins in R28 cells exposed to 3 mM of glutamate and 100 µM of H2O2, respectively (FDR < 1%; p < 0.05). Bioinformatics analysis indicated that the protein changes were associated with the dysregulation of signaling pathways, which was similar to those observed in glaucoma. Thus, the proteomic alteration induced by glutamate was associated with the inhibition of the PI3K/AKT signaling pathway. On the other hand, H2O2-induced toxicity in R28 cells was linked to the activation of apoptosis signaling and the inhibition of the mTOR and ERK/MAPK signaling pathways. Furthermore, the data show a similarity in the inhibition of the EIF2 and AMPK signaling pathways and the activation of the sumoylation and WNT/β-catenin signaling pathways in both groups. Our findings suggest that the exposure of R28 cells to glutamate and H2O2 could induce glaucoma-like neurodegenerative features and potentially provide a suitable tool for the development of new therapeutic strategies for retinal diseases.

Oxidative stress, inflammation, and steatosis elucidate the complex dynamics of HgCl2 induced liver damage in Channa punctata

Water bodies are highly pollution-prone areas in which mercury (Hg) is considered as a major menace to aquatic organisms. However, the information about the toxicity of mercuric chloride (HgCl2) in a vital organ such as the liver of fish is still inadequate. This study aimed to assess the impact of mercuric chloride (HgCl2) exposure on the liver of Channa punctata fish over 15, 30, and 45 days, at two different concentrations (0.039 mg/L and 0.078 mg/L). Mercury is known to be a significant threat to aquatic life, and yet, information regarding its effects on fish liver remains limited. The results of this study demonstrate that exposure to HgCl2 significantly increases oxidative stress markers, such as lipid peroxidation (LPO) and protein carbonyls (PC), as well as the levels of serum glutamic-oxaloacetic transaminase (SGOT) and serum glutamic pyruvic transaminase (SGPT) in the fish. Additionally, the transcriptional and protein analysis of specific genes and molecules associated with necroptosis and inflammation, such as ABCG2, TNF α, Caspase 3, RIPK 3, IL-1β, Caspase-1, IL-18, and RIPK1, confirm the occurrence of necroptosis and inflammation in the liver. Histopathological and ultrastructural examinations of the liver tissue further reveal a significant presence of liver steatosis. Interestingly, the upregulation of PPARα suggests that the fish's body is actively responding to counteract the effects of liver steatosis. This study provides a comprehensive analysis of oxidative stress, biochemical changes, gene expression, protein profiles, and histological findings in the liver tissue of fish exposed to mercury pollution in freshwater environments.

Identification of a transcription factor AoMsn2 of the Hog1 signaling pathway contributes to fungal growth, development and pathogenicity in Arthrobotrys oligospora

INTRODUCTION

Arthrobotrys oligospora has been utilized as a model strain to study the interaction between fungi and nematodes owing to its ability to capture nematodes by developing specialized traps. A previous study showed that high-osmolarity glycerol (Hog1) signaling regulates the osmoregulation and nematocidal activity of A. oligospora. However, the function of downstream transcription factors of the Hog1 signaling in the nematode-trapping (NT) fungi remains unclear.

OBJECTIVE

This study aimed to investigate the functions and potential regulatory network of AoMsn2, a downstream transcription factor of the Hog1 signaling pathway in A. oligospora.

METHODS

The function of AoMsn2 was characterized using targeted gene deletion, phenotypic experiments, real-time quantitative PCR, RNA sequencing, untargeted metabolomics, and yeast two-hybrid analysis.

RESULTS

Loss of Aomsn2 significantly enlarged and swollen the hyphae, with an increase in septa and a significant decrease in nuclei. In particular, spore yield, spore germination rate, traps, and nematode predation efficiency were remarkably decreased in the mutants. Phenotypic and transcriptomic analyses revealed that AoMsn2 is essential for fatty acid metabolism and autophagic pathways. Additionally, untargeted metabolomic analysis identified an important function of AoMsn2 in the modulation of secondary metabolites. Furtherly, we analyzed the protein interaction network of AoMsn2 based on the Kyoto Encyclopedia of Genes and Genomes pathway map and the online website STRING. Finally, Hog1 and six putative targeted proteins of AoMsn2 were identified by Y2H analysis.

CONCLUSION

Our study reveals that AoMsn2 plays crucial roles in the growth, conidiation, trap development, fatty acid metabolism, and secondary metabolism, as well as establishes a broad basis for understanding the regulatory mechanisms of trap morphogenesis and environmental adaptation in NT fungi.

TOR Complex 1: Orchestrating Nutrient Signaling and Cell Cycle Progression

The highly conserved TOR signaling pathway is crucial for coordinating cellular growth with the cell cycle machinery in eukaryotes. One of the two TOR complexes in budding yeast, TORC1, integrates environmental cues and promotes cell growth. While cells grow, they need to copy their chromosomes, segregate them in mitosis, divide all their components during cytokinesis, and finally physically separate mother and daughter cells to start a new cell cycle apart from each other. To maintain cell size homeostasis and chromosome stability, it is crucial that mechanisms that control growth are connected and coordinated with the cell cycle. Successive periods of high and low TORC1 activity would participate in the adequate cell cycle progression. Here, we review the known molecular mechanisms through which TORC1 regulates the cell cycle in the budding yeast Saccharomyces cerevisiae that have been extensively used as a model organism to understand the role of its mammalian ortholog, mTORC1.

Two-way communication between cell cycle and metabolism in budding yeast: what do we know?

Coordination of cell cycle and metabolism exists in all cells. The building of a new cell is a process that requires metabolic commitment to the provision of both Gibbs energy and building blocks for proteins, nucleic acids, and membranes. On the other hand, the cell cycle machinery will assess and regulate its metabolic environment before it makes decisions on when to enter the next cell cycle phase. Furthermore, more and more evidence demonstrate that the metabolism can be regulated by cell cycle progression, as different biosynthesis pathways are preferentially active in different cell cycle phases. Here, we review the available literature providing a critical overview on how cell cycle and metabolism may be coupled with one other, bidirectionally, in the budding yeast Saccharomyces cerevisiae.

Mitotic chromosome condensation resets chromatin to safeguard transcriptional homeostasis during interphase

Mitotic entry correlates with the condensation of the chromosomes, changes in histone modifications, exclusion of transcription factors from DNA, and the broad downregulation of transcription. However, whether mitotic condensation influences transcription in the subsequent interphase is unknown. Here, we show that preventing one chromosome to condense during mitosis causes it to fail resetting of transcription. Rather, in the following interphase, the affected chromosome contains unusually high levels of the transcription machinery, resulting in abnormally high expression levels of genes in cis, including various transcription factors. This subsequently causes the activation of inducible transcriptional programs in trans, such as the GAL genes, even in the absence of the relevant stimuli. Thus, mitotic chromosome condensation exerts stringent control on interphase gene expression to ensure the maintenance of basic cellular functions and cell identity across cell divisions. Together, our study identifies the maintenance of transcriptional homeostasis during interphase as an unexpected function of mitosis and mitotic chromosome condensation.

When yeast cells change their mind: cell cycle "Start" is reversible under starvation

Eukaryotic cells decide in late G1 phase of the cell cycle whether to commit to another round of division. This point of cell cycle commitment is termed "Restriction Point" in mammals and "Start" in the budding yeast Saccharomyces cerevisiae. At Start, yeast cells integrate multiple signals such as pheromones and nutrients, and will not pass Start if nutrients are lacking. However, how cells respond to nutrient depletion after the Start decision remains poorly understood. Here, we analyze how post-Start cells respond to nutrient depletion, by monitoring Whi5, the cell cycle inhibitor whose export from the nucleus determines Start. Surprisingly, we find that cells that have passed Start can re-import Whi5 into the nucleus. In these cells, the positive feedback loop activating G1/S transcription is interrupted, and the Whi5 repressor re-binds DNA. Cells which re-import Whi5 become again sensitive to mating pheromone, like pre-Start cells, and CDK activation can occur a second time upon replenishment of nutrients. These results demonstrate that upon starvation, the commitment decision at Start can be reversed. We therefore propose that cell cycle commitment in yeast is a multi-step process, similar to what has been suggested for mammalian cells.

Cip1 tunes cell cycle arrest duration upon calcineurin activation

SignificanceTo ensure their survival, cells arrest the cell division cycle when they are exposed to environmental stress. The duration of this arrest is dependent upon the time it takes a cell to adapt to a particular environment. How cells adjust the amount of time they remain arrested is not known. This study investigates the role of the phosphatase calcineurin in controlling cell cycle arrest duration in yeast. We show that calcineurin lengthens arrest by prolonging Hog1-dependent activation of the poorly characterized cyclin-dependent kinase inhibitor Cip1. Cip1 only impacts cell cycle arrest in response to stressors that robustly activate calcineurin, suggesting that Cip1 is a context-specific regulator that differentially adjusts the length of arrest depending on the particular stressor.

LncRNAs of Saccharomyces cerevisiae bypass the cell cycle arrest imposed by ethanol stress

Ethanol alters many subsystems of Saccharomyces cerevisiae, including the cell cycle. Two ethanol-responsive lncRNAs in yeast interact with cell cycle proteins, and here, we investigated the role of these RNAs in cell cycle. Our network dynamic modeling showed that higher and lower ethanol-tolerant strains undergo cell cycle arrest in mitosis and G1 phases, respectively, during ethanol stress. The higher population rebound of the lower ethanol-tolerant phenotype after stress relief responds to the late phase arrest. We found that the lncRNA lnc9136 of SEY6210 (a lower ethanol-tolerant strain) induces cells to skip mitosis arrest. Simulating an overexpression of lnc9136 and analyzing CRISPR–Cas9 mutants lacking this lncRNA suggest that lnc9136 induces a regular cell cycle even under ethanol stress, indirectly regulating Swe1p and Clb1/2 by binding to Gin4p and Hsl1p. Notably, lnc10883 of BY4742 (a higher ethanol-tolerant strain) does not prevent G1 arrest in this strain under ethanol stress. However, lnc19883 circumvents DNA and spindle damage checkpoints, maintaining a functional cell cycle by interacting with Mec1p or Bub1p even in the presence of DNA/spindle damage. Overall, we present the first evidence of direct roles for lncRNAs in regulating yeast cell cycle proteins, the dynamics of this system in different ethanol-tolerant phenotypes, and a new yeast cell cycle model.

Ethanol is a cell stressor in yeast that dampen ethanol production. LncRNAs are RNAs that control many cellular processes. Computational simulations allow us to study the dynamism of cell systems. Therefore, we built a computational model of the yeast cell cycle to investigate how cells respond to ethanol stress. Simulations showed that ethanol stress or spindle damage arrests the cell cycle. Furthermore, the performance of higher and lower ethanol-tolerant strains in poststress recovery growth seems to be related to the cell cycle phase in which cells are stalled. However, two lncRNAs maintain the activity of the cell cycle even in yeast cells under these stresses by repressing specific cell cycle proteins. Finally, this model facilitates analyses of the yeast cell cycle for applied or basic science purposes.

Purine Auxotrophic Starvation Evokes Phenotype Similar to Stationary Phase Cells in Budding Yeast

Purine auxotrophy is an abundant trait among eukaryotic parasites and a typical marker for many budding yeast strains. Supplementation with an additional purine source (such as adenine) is necessary to cultivate these strains. If not supplied in adequate amounts, purine starvation sets in. We explored purine starvation effects in a model organism, a budding yeast Saccharomyces cerevisiae ade8 knockout, at the level of cellular morphology, central carbon metabolism, and global transcriptome. We observed that purine-starved cells stopped their cycle in G1/G0 state and accumulated trehalose, and the intracellular concentration of AXP decreased, but adenylate charge remained stable. Cells became tolerant to severe environmental stresses. Intracellular RNA concentration decreased, and massive downregulation of ribosomal biosynthesis genes occurred. We proved that the expression of new proteins during purine starvation is critical for cells to attain stress tolerance phenotype Msn2/4p targets are upregulated in purine-starved cells when compared to cells cultivated in purine-rich media. The overall transcriptomic response to purine starvation resembles that of stationary phase cells. Our results demonstrate that the induction of a strong stress resistance phenotype in budding yeast can be caused not only by natural starvation, but also starvation for metabolic intermediates, such as purines.

The high osmolarity glycerol (HOG) pathway in fungi†

While fungi are widely occupying nature, many species are responsible for devastating mycosis in humans. Such niche diversity explains how quick fungal adaptation is necessary to endow the capacity of withstanding fluctuating environments and to cope with host-imposed conditions. Among all the molecular mechanisms evolved by fungi, the most studied one is the activation of the phosphorelay signalling pathways, of which the high osmolarity glycerol (HOG) pathway constitutes one of the key molecular apparatus underpinning fungal adaptation and virulence. In this review, we summarize the seminal knowledge of the HOG pathway with its more recent developments. We specifically described the HOG-mediated stress adaptation, with a particular focus on osmotic and oxidative stress, and point out some lags in our understanding of its involvement in the virulence of pathogenic species including, the medically important fungi Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus, compared to the model yeast Saccharomyces cerevisiae. Finally, we also highlighted some possible applications of the HOG pathway modifications to improve the fungal-based production of natural products in the industry.

Cdc42-Specific GTPase-Activating Protein Rga1 Squelches Crosstalk between the High-Osmolarity Glycerol (HOG) and Mating Pheromone Response MAPK Pathways

Eukaryotes utilize distinct mitogen/messenger-activated protein kinase (MAPK) pathways to evoke appropriate responses when confronted with different stimuli. In yeast, hyperosmotic stress activates MAPK Hog1, whereas mating pheromones activate MAPK Fus3 (and MAPK Kss1). Because these pathways share several upstream components, including the small guanosine-5'-triphosphate phosphohydrolase (GTPase) cell-division-cycle-42 (Cdc42), mechanisms must exist to prevent inadvertent cross-pathway activation. Hog1 activity is required to prevent crosstalk to Fus3 and Kss1. To identify other factors required to maintain signaling fidelity during hypertonic stress, we devised an unbiased genetic selection for mutants unable to prevent such crosstalk even when active Hog1 is present. We repeatedly isolated truncated alleles of RGA1, a Cdc42-specific GTPase-activating protein (GAP), each lacking its C-terminal catalytic domain, that permit activation of the mating MAPKs under hyperosmotic conditions despite Hog1 being present. We show that Rga1 down-regulates Cdc42 within the high-osmolarity glycerol (HOG) pathway, but not the mating pathway. Because induction of mating pathway output via crosstalk from the HOG pathway takes significantly longer than induction of HOG pathway output, our findings suggest that, under normal conditions, Rga1 contributes to signal insulation by limiting availability of the GTP-bound Cdc42 pool generated by hypertonic stress. Thus, Rga1 action contributes to squelching crosstalk by imposing a type of “kinetic proofreading”. Although Rga1 is a Hog1 substrate in vitro, we eliminated the possibility that its direct Hog1-mediated phosphorylation is necessary for its function in vivo. Instead, we found first that, like its paralog Rga2, Rga1 is subject to inhibitory phosphorylation by the S. cerevisiae cyclin-dependent protein kinase 1 (Cdk1) ortholog Cdc28 and that hyperosmotic shock stimulates its dephosphorylation and thus Rga1 activation. Second, we found that Hog1 promotes Rga1 activation by blocking its Cdk1-mediated phosphorylation, thereby allowing its phosphoprotein phosphatase 2A (PP2A)-mediated dephosphorylation. These findings shed light on why Hog1 activity is required to prevent crosstalk from the HOG pathway to the mating pheromone response pathway.

The Negative Effect of Protein Phosphatase Z1 Deletion on the Oxidative Stress Tolerance of Candida albicans Is Synergistic with Betamethasone Exposure

The glucocorticoid betamethasone (BM) has potent anti-inflammatory and immunosuppressive effects; however, it increases the susceptibility of patients to superficial Candida infections. Previously we found that this disadvantageous side effect can be counteracted by menadione sodium bisulfite (MSB) induced oxidative stress treatment. The fungus specific protein phosphatase Z1 (CaPpz1) has a pivotal role in oxidative stress response of Candida albicans and was proposed as a potential antifungal drug target. The aim of this study was to investigate the combined effects of CaPPZ1 gene deletion and MSB treatment in BM pre-treated C. albicans cultures. We found that the combined treatment increased redox imbalance, enhanced the specific activities of antioxidant enzymes, and reduced the growth in cappz1 mutant (KO) strain. RNASeq data demonstrated that the presence of BM markedly elevated the number of differentially expressed genes in the MSB treated KO cultures. Accumulation of reactive oxygen species, increased iron content and fatty acid oxidation, as well as the inhibiting ergosterol biosynthesis and RNA metabolic processes explain, at least in part, the fungistatic effect caused by the combined stress exposure. We suggest that the synergism between MSB treatment and CaPpz1 inhibition could be considered in developing of a novel combinatorial antifungal strategy accompanying steroid therapy.

Inositol polyphosphate-protein interactions: Implications for microbial pathogenicity

Inositol polyphosphates (IPs) and inositol pyrophosphates (PP-IPs) regulate diverse cellular processes in eukaryotic cells. IPs and PP-IPs are highly negatively charged and exert their biological effects by interacting with specific protein targets. Studies performed predominantly in mammalian cells and model yeasts have shown that IPs and PP-IPs modulate target function through allosteric regulation, by promoting intra- and intermolecular stabilization and, in the case of PP-IPs, by donating a phosphate from their pyrophosphate (PP) group to the target protein. Technological advances in genetics have extended studies of IP function to microbial pathogens and demonstrated that disrupting PP-IP biosynthesis and PP-IP-protein interaction has a profound impact on pathogenicity. This review summarises the complexity of IP-mediated regulation in eukaryotes, including microbial pathogens. It also highlights examples of poor conservation of IP-protein interaction outcome despite the presence of conserved IP-binding domains in eukaryotic proteomes.

Rapid and reversible cell volume changes in response to osmotic stress in yeast

Saccharomyces cerevisiae has evolved diverse mechanisms to osmotic changes: the cell wall, ion and water transport systems, and signaling cascades. At the present time, little is known about the mechanisms involved in short-term responses of osmotic stress in yeast or their physiological state during this process. We conducted studies of flow cytometry, wet weight measurements, and electron microscopy to evaluate the modifications in cell volume and the cell wall induced by osmotic stress. In response to osmotic challenges, we show very fast and drastic changes in cell volume (up to 60%), which were completed in less than eight seconds. This dramatic change was completely reversible approximately 16 s after returning to an isosmotic solution. Cell volume changes were also accompanied by adaptations in yeast metabolism observed as a reduction by 50% in the respiratory rate, measured as oxygen consumption. This effect was also fully reversible upon returning to an isosmotic solution. It is noteworthy that we observed a significant recovery in oxygen consumption during the first 10 min of the osmotic shock. The rapid adjustment of the cellular volume may represent an evolutionary advantage, allowing greater flexibility for survival.

The budding yeast Start repressor Whi7 differs in regulation from Whi5, emerging as a major cell cycle brake in response to stress

Start is the main decision point in the eukaryotic cell cycle at which cells commit to a new round of cell division. It involves the irreversible activation of a transcriptional programme through the inactivation of Start transcriptional repressors: the retinoblastoma family in mammals, or Whi5 and its recently identified paralogue Whi7 (also known as Srl3) in budding yeast. Here, we provide a comprehensive comparison of Whi5 and Whi7 that reveals significant qualitative differences. Indeed, the expression, subcellular localization and functionality of Whi7 and Whi5 are differentially regulated. Importantly, Whi7 shows specific properties in its association with promoters not shared by Whi5, and for the first time, we demonstrate that Whi7, and not Whi5, can be the main contributor to Start inhibition such as it occurs in the response to cell wall stress. Our results help to improve understanding of the interplay between multiple differentially regulated Start repressors in order to face specific cellular conditions.

Highlighted Article: Cells can use the interplay between functionally redundant but differentially regulated cell-cycle repressors in order to confer new repression capabilities and to respond to specific cellular conditions.

Tumor suppressor stars in yeast G1/S transition

Yeast is one of the best-understood biological systems for genetic research. Over the last 40 years, geneticists have striven to search for homologues of tumor suppressors in yeast to simplify cancer research. The star tumor suppressor p21, downstream target of p53, is one of the primary factors on the START point through negatively regulating CycD/E-CDK, the yeast counterpart Cln3-Cdk1. Not like yeast Whi5 that was identified as the analog of the retinoblastoma tumor suppressor protein (Rb) and hence promoted to uncover the mechanism of its cancer suppression, homologue of p21 had not been found in yeast. Our lab identified Cip1 in budding yeast as a novel negative regulator of G1-Cdk1 and proposed that Cip1 is an analog of human p21. Recently, we demonstrated a dual repressive function of Cip1 on START timing via the redundant Cln3 and Ccr4 pathways. This work in yeast may help clarify the complex regulation in human p53-p21 signaling cascade. In this review, we will discuss the yeast paralogs of star tumor suppressors in the control of G1/S transition and present the new findings in this field.

Dual Repressive Function by Cip1, a Budding Yeast Analog of p21, in Cell-Cycle START Regulation

Cip1, a newly identified yeast analog of p21, is a Cln3-CDK inhibitor that negatively regulates cell-cycle START. However, its function remains poorly understood. In this study, we found that deletion of CLN3 did not result in bypass of G1-phase arrest caused by Cip1 overexpression. Cip1 depletion in cln3-null mutants significantly advanced the timing of Cln2 expression, supporting the idea that Cip1 represses START in a Cln3-independent manner. We set to search for novel Cip1 interacting proteins and found that Ccr4, a known START regulator, and its associated factor Caf120, interact with Cip1. Ccr4-Caf120 acts redundantly with Cdk1-Cln3 to inhibit Whi5-mediated regulation of START. This interaction was conserved between human Ccr4 and p21. In addition, deletion of WHI5 robustly suppressed G1-phase arrest caused by Cip1 overexpression. We conclude that Cip1 negatively regulates START by acting as a dual repressor of Ccr4 in parallel with Cln3.

The coordinate actions of calcineurin and Hog1 mediate the stress response through multiple nodes of the cell cycle network

Upon exposure to environmental stressors, cells transiently arrest the cell cycle while they adapt and restore homeostasis. A challenge for all cells is to distinguish between stress signals and coordinate the appropriate adaptive response with cell cycle arrest. Here we investigate the role of the phosphatase calcineurin (CN) in the stress response and demonstrate that CN activates the Hog1/p38 pathway in both yeast and human cells. In yeast, the MAPK Hog1 is transiently activated in response to several well-studied osmostressors. We show that when a stressor simultaneously activates CN and Hog1, CN disrupts Hog1-stimulated negative feedback to prolong Hog1 activation and the period of cell cycle arrest. Regulation of Hog1 by CN also contributes to inactivation of multiple cell cycle-regulatory transcription factors (TFs) and the decreased expression of cell cycle-regulated genes. CN-dependent downregulation of G1/S genes is dependent upon Hog1 activation, whereas CN inactivates G2/M TFs through a combination of Hog1-dependent and -independent mechanisms. These findings demonstrate that CN and Hog1 act in a coordinated manner to inhibit multiple nodes of the cell cycle-regulatory network. Our results suggest that crosstalk between CN and stress-activated MAPKs helps cells tailor their adaptive responses to specific stressors.

In order to survive exposure to environmental stress, cells transiently arrest the cell division cycle while they adapt to the stress. Several kinases and phosphatases are known to control stress adaptation programs, but the extent to which these signaling pathways work together to tune the stress response is not well understood. This study investigates the role of the phosphatase calcineurin in the stress response and shows that calcineurin inhibits the cell cycle in part by stimulating the activity of the Hog1/p-38 stress-activated MAPK in both yeast and human cells. Crosstalk between stress response pathways may help cells mount specific responses to diverse stressors and to survive changes in their environment.

Hog1 activation delays mitotic exit via phosphorylation of Net1

Proper chromosome segregation is critical for the maintenance of genomic information in every cell division, which is required for cell survival. Cells have orchestrated a myriad of control mechanisms to guarantee proper chromosome segregation. Upon stress, cells induce a number of adaptive responses to maximize survival that range from regulation of gene expression to control of cell-cycle progression. We have found here that in response to osmostress, cells also regulate mitosis to ensure proper telomeric and rDNA segregation during adaptation. Osmostress induces a Hog1-dependent delay of cell-cycle progression in early mitosis by phosphorylating Net1, thereby impairing timely nucleolar release and activation of Cdc14, core elements of mitosis regulation. Thus, Hog1 activation prevents segregation defects to maximize survival.

Adaptation to environmental changes is crucial for cell fitness. In Saccharomyces cerevisiae, variations in external osmolarity trigger the activation of the stress-activated protein kinase Hog1 (high-osmolarity glycerol 1), which regulates gene expression, metabolism, and cell-cycle progression. The activation of this kinase leads to the regulation of G1, S, and G2 phases of the cell cycle to prevent genome instability and promote cell survival. Here we show that Hog1 delays mitotic exit when cells are stressed during metaphase. Hog1 phosphorylates the nucleolar protein Net1, altering its affinity for the phosphatase Cdc14, whose activity is essential for mitotic exit and completion of the cell cycle. The untimely release of Cdc14 from the nucleolus upon activation of Hog1 is linked to a defect in ribosomal DNA (rDNA) and telomere segregation, and it ultimately delays cell division. A mutant of Net1 that cannot be phosphorylated by Hog1 displays reduced viability upon osmostress. Thus, Hog1 contributes to maximizing cell survival upon stress by regulating mitotic exit.

Effect of Lactobacillus acidophilus CICC 6074 S-Layer Protein on Colon Cancer HT-29 Cell Proliferation and Apoptosis

The objective of this research was to investigate the effects of Lactobacillus acidophilus CICC 6074 S-layer protein on the viability, adhesion, cell cycle, and apoptosis of human colon cancer HT-29 cells and to explore their molecular mechanism of tumor suppression. The S-layer protein at doses of 0, 25, 50, and 100 mg/L significantly suppressed the proliferation of HT-29 cells. The S-layer protein exerts its cytotoxic activities against colon cancer HT-29 cells by arresting the cell cycle in the G1 phase through upregulating the expression of p53, p21, and p16 and downregulating the expression of CDK1 (cyclin-dependent kinases) and cyclin B. Morphological changes were further observed by transmission electron microscopy, and the cells treated with the S-layer protein showed obvious characteristic changes of apoptosis including chromatin condensation, nuclear fragmentation, vacuoles, and so on. Furthermore, our mechanism studies indicated that the S-layer protein may induce HT-29 cell apoptosis through the death receptor apoptotic pathway and mitochondrial pathway and impede cell invasion by inhibiting the synthesis of the PI3K/AKT pathway and FasL. These results demonstrated that the L. acidophilus CICC 6074 S-layer protein may be a potential anticarcinogenic agent.

Novel interconnections of HOG signaling revealed by combined use of two proteomic software packages

Modern quantitative mass spectrometry (MS)-based proteomics enables researchers to unravel signaling networks by monitoring proteome-wide cellular responses to different stimuli. MS-based analysis of signaling systems usually requires an integration of multiple quantitative MS experiments, which remains challenging, given that the overlap between these datasets is not necessarily comprehensive. In a previous study we analyzed the impact of the yeast mitogen-activated protein kinase (MAPK) Hog1 on the hyperosmotic stress-affected phosphorylome. Using a combination of a series of hyperosmotic stress and kinase inhibition experiments, we identified a broad range of direct and indirect substrates of the MAPK. Here we re-evaluate this extensive MS dataset and demonstrate that a combined analysis based on two software packages, MaxQuant and Proteome Discoverer, increases the coverage of Hog1-target proteins by 30%. Using protein-protein proximity assays we show that the majority of new targets gained by this analysis are indeed Hog1-interactors. Additionally, kinetic profiles indicate differential trends of Hog1-dependent versus Hog1-independent phosphorylation sites. Our findings highlight a previously unrecognized interconnection between Hog1 signaling and the RAM signaling network, as well as sphingolipid homeostasis.

Competition in the chaperone-client network subordinates cell-cycle entry to growth and stress

The precise coordination of growth and proliferation has a universal prevalence in cell homeostasis. As a prominent property, cell size is modulated by the coordination between these processes in bacterial, yeast, and mammalian cells, but the underlying molecular mechanisms are largely unknown. Here, we show that multifunctional chaperone systems play a concerted and limiting role in cell-cycle entry, specifically driving nuclear accumulation of the G1 Cdk-cyclin complex. Based on these findings, we establish and test a molecular competition model that recapitulates cell-cycle-entry dependence on growth rate. As key predictions at a single-cell level, we show that availability of the Ydj1 chaperone and nuclear accumulation of the G1 cyclin Cln3 are inversely dependent on growth rate and readily respond to changes in protein synthesis and stress conditions that alter protein folding requirements. Thus, chaperone workload would subordinate Start to the biosynthetic machinery and dynamically adjust proliferation to the growth potential of the cell.

CgHog1-Mediated CgRds2 Phosphorylation Alters Glycerophospholipid Composition To Coordinate Osmotic Stress in Candida glabrata

Under stress conditions, Hog1 is required for cell survival through transiently phosphorylating downstream targets and reprogramming gene expression. Here, we report that Candida glabrata Hog1 (CgHog1) interacts with and phosphorylates CgRds2, a zinc cluster transcription factor, in response to osmotic stress. Additionally, we found that deletion of CgRDS2 led to decreases in cell growth and cell survival by 23.4% and 39.6%, respectively, at 1.5 M NaCl, compared with levels of the wild-type strain. This is attributed to significant downregulation of the expression levels of glycerophospholipid metabolism genes. As a result, the content of total glycerophospholipid decreased by 30.3%. Membrane integrity also decreased 47.6% in the Cgrds2Δ strain at 1.5 M NaCl. In contrast, overexpression of CgRDS2 increased the cell growth and cell survival by 10.2% and 6.3%, respectively, owing to a significant increase in the total glycerophospholipid content and increased membrane integrity by 27.2% and 12.1%, respectively, at 1.5 M NaCl, compared with levels for the wild-type strain. However, a strain in which the CgRDS2 gene encodes the replacement of Ser64 and Thr97 residues with alanines (Cgrds2^2A ), harboring a CgRds2 protein that was not phosphorylated by CgHog1, failed to promote glycerophospholipid metabolism and membrane integrity at 1.5 M NaCl. Thus, the above results demonstrate that CgHog1-mediated CgRds2 phosphorylation enhanced glycerophospholipid composition and membrane integrity to resist osmotic stress in C. glabrata IMPORTANCE This study explored the role of CgHog1-mediated CgRds2 phosphorylation in response to osmotic stress in Candida glabrata CgHog1 interacts with and phosphorylates CgRds2, a zinc cluster transcription factor, under osmotic stress. Phosphorylated CgRds2 plays an important role in increasing glycerophospholipid composition and membrane integrity, thereby enhancing cell growth and survival.

Candida glabrata Med3 Plays a Role in Altering Cell Size and Budding Index To Coordinate Cell Growth

Candida glabrata is a promising microorganism for the production of organic acids. Here, we report deletion and quantitative-expression approaches to elucidate the role of C. glabrata Med3AB (CgMed3AB), a subunit of the mediator transcriptional coactivator, in regulating cell growth. Deletion of CgMed3AB caused an 8.6% decrease in final biomass based on growth curve plots and 10.5% lower cell viability. Based on transcriptomics data, the reason for this growth defect was attributable to changes in expression of genes involved in pyruvate and acetyl-coenzyme A (CoA)-related metabolism in a Cgmed3abΔ strain. Furthermore, the mRNA level of acetyl-CoA synthetase was downregulated after deleting Cgmed3ab, resulting in 22.8% and 21% lower activity of acetyl-CoA synthetase and cellular acetyl-CoA, respectively. Additionally, the mRNA level of CgCln3, whose expression depends on acetyl-CoA, was 34% lower in this strain. As a consequence, the cell size and budding index in the Cgmed3abΔ strain were both reduced. Conversely, overexpression of Cgmed3ab led to 16.8% more acetyl-CoA and 120% higher CgCln3 mRNA levels, as well as 19.1% larger cell size and a 13.3% higher budding index than in wild-type cells. Taken together, these results suggest that CgMed3AB regulates cell growth in C. glabrata by coordinating homeostasis between cellular acetyl-CoA and CgCln3.IMPORTANCE This study demonstrates that CgMed3AB can regulate cell growth in C. glabrata by coordinating the homeostasis of cellular acetyl-CoA metabolism and the cell cycle cyclin CgCln3. Specifically, we report that CgMed3AB regulates the cellular acetyl-CoA level, which induces the transcription of Cgcln3, finally resulting in alterations to the cell size and budding index. In conclusion, we report that CgMed3AB functions as a wheel responsible for driving cellular acetyl-CoA metabolism, indirectly inducing the transcription of Cgcln3 and coordinating cell growth. We propose that Mediator subunits may represent a vital regulatory target modulating cell growth in C. glabrata.

How yeast coordinates metabolism, growth and division

All cells, especially single cell organisms, need to adapt their metabolism, growth and division coordinately to the available nutrients. This coordination is mediated by extensive cross-talk between nutrient signaling, metabolism, growth, and the cell division cycle, which is only gradually being uncovered: Nutrient signaling not only controls entry into the cell cycle at the G1/S transition, but all phases of the cell cycle. Metabolites are even sensed directly by cell cycle regulators to prevent cell cycle progression in absence of sufficient metabolic fluxes. In turn, cell cycle regulators such as the cyclin-dependent kinase directly control metabolic fluxes during cell cycle progression. In this review, I highlight some recent advances in our understanding of how metabolism and the cell division cycle are coordinated in the model organism Saccharomyces cerevisiae.