A novel ER membrane protein Ehg1/May24 plays a critical role in maintaining multiple nutrient permeases in yeast under high-pressure perturbation

Mtc6/Ehg2 is a novel endoplasmic reticulum-resident glycoprotein essential for high-pressure tolerance

Hydrostatic pressure is a common mechanical stressor that modulates metabolism and reduces cell viability. Eukaryotic cells have genetic programs to cope with hydrostatic pressure stress and maintain intracellular homeostasis. However, the mechanism underlying hydrostatic pressure tolerance remains largely unknown. We have recently demonstrated that maintenance of telomere capping protein 6 (Mtc6) plays a protective role in the survival of the budding yeast Saccharomyces cerevisiae under hydrostatic pressure stress by supporting the integrity of nutrient permeases. The current study demonstrates that Mtc6 acts as an endoplasmic reticulum (ER) membrane protein. Mtc6 comprises two transmembrane domains, a C-terminal cytoplasmic domain and a luminal region with 12 Asn (N)-linked glycans attached to it. Serial mutational analyses showed that the cytoplasmic C-terminal amino acid residues GVPS Mtc6 activity. Multiple N-linked glycans in the luminal region are involved in the structural conformation of Mtc6. Moreover, deletion of MTC6 led to increased degradation of the leucine permease Bap2 under hydrostatic pressure, suggesting that Mtc6 facilitates the proper folding of nutrient permeases in the ER under stress conditions. We propose a novel model of molecular function in which the glycosylated luminal domain and cytoplasmic GVPS sequences of Mtc6 cooperatively support the nutrient permease activity.

Amino acid metabolism and MAP kinase signaling pathway play opposite roles in the regulation of ethanol production during fermentation of sugarcane molasses in budding yeast

Sugarcane molasses is one of the main raw materials for bioethanol production, and Saccharomyces cerevisiae is the major biofuel-producing organism. In this study, a batch fermentation model has been used to examine ethanol titers of deletion mutants for all yeast nonessential genes in this yeast genome. A total of 42 genes are identified to be involved in ethanol production during fermentation of sugarcane molasses. Deletion mutants of seventeen genes show increased ethanol titers, while deletion mutants for twenty-five genes exhibit reduced ethanol titers. Two MAP kinases Hog1 and Kss1 controlling the high osmolarity and glycerol (HOG) signaling and the filamentous growth, respectively, are negatively involved in the regulation of ethanol production. In addition, twelve genes involved in amino acid metabolism are crucial for ethanol production during fermentation. Our findings provide novel targets and strategies for genetically engineering industrial yeast strains to improve ethanol titer during fermentation of sugarcane molasses.

Activation of CWI pathway through high hydrostatic pressure, enhancing glycerol efflux via the aquaglyceroporin Fps1 in Saccharomyces cerevisiae

The fungal cell wall is the initial barrier for the fungi against diverse external stresses, such as osmolarity changes, harmful drugs, and mechanical injuries. This study explores the roles of osmoregulation and the cell-wall integrity (CWI) pathway in response to high hydrostatic pressure in the yeast Saccharomyces cerevisiae. We demonstrate the roles of the transmembrane mechanosensor Wsc1 and aquaglyceroporin Fps1 in a general mechanism to maintain cell growth under high-pressure regimes. The promotion of water influx into cells at 25 MPa, as evident by an increase in cell volume and a loss of the plasma membrane eisosome structure, activates the CWI pathway through the function of Wsc1. Phosphorylation of Slt2, the downstream mitogen-activated protein kinase, was increased at 25 MPa. Glycerol efflux increases via Fps1 phosphorylation, which is initiated by downstream components of the CWI pathway, and contributes to the reduction in intracellular osmolarity under high pressure. The elucidation of the mechanisms underlying adaptation to high pressure through the well-established CWI pathway could potentially translate to mammalian cells and provide novel insights into cellular mechanosensation.

Pleiotropic roles of N-glycans for enzyme activities and stabilities of MIPC synthases, Csh1 and Sur1/Csg1, in Saccharomyces cerevisiae

Mannosyl phosphorylceramide (MIPC) is a membrane lipid classified as a complex sphingolipid in Saccharomyces cerevisiae. MIPC is synthesized by two redundant enzymes, Sur1/Csg1 and Csh1, in the Golgi lumen. MIPC consists of five subtypes (A, B', B, C, and D-type) according to the position and number of hydroxyl groups on the ceramide moiety. Sur1 exerts higher impact on synthesis of MIPC-B and MIPC-C than Csh1. In this study, we elucidated the roles played by N-glycans attached to Sur1 and Csh1, and dissected the mechanisms underlying substrate recognition by these two enzymes. Sur1 carries an N-glycan on Asn-224, while Csh1 has N-glycans on Asn-51 and Asn-247. Although intracellular proteins usually harbor core-type N-glycans, the N-glycan on Asn-51 of Csh1 exhibited a unique mannan-like structure containing a long backbone of mannose. Sur1 N224Q and Csh1 N51Q mutants exhibited a decrease in the activity to synthesize specific MIPC subtypes for each enzyme, suggesting that these N-glycans play a role in substrate recognition through their catalytic domains. Moreover, ectopic insertion of an N-glycosylation consensus sequence (NST) at codon 51 of Sur1 (Sur1-NST51) resulted in an artificial modification with mannan, which markedly decreased protein stability. Our results suggest that the diminished stability of the Sur1-NST51 mutant protein could be attributable to potential structural alterations by the mannan. Collectively, the present study reveals essential luminal domains of Sur1 and Csh1 that dictate substrate specificity and/or the protein stabilities via mannan modification.

SOD1 mutations cause hypersensitivity to high-pressure-induced oxidative stress in Saccharomyces cerevisiae

Living organisms are subject to various mechanical stressors, such as high hydrostatic pressure. Empirical evidence shows that under high pressure, the oxidative stress response is activated in Saccharomyces cerevisiae. However, the mechanisms involved in its antioxidant systems are unclear. Here, we demonstrate that superoxide dismutase 1 (Sod1) plays a role in resisting high pressure for cell growth. Mutants lacking Sod1 or Ccs1, the copper chaperone for Sod1, displayed growth defects under 25 MPa. Of the various SOD1 mutations associated with familial amyotrophic lateral sclerosis, H46Q and S134N substitutions diminished SOD activity to levels comparable to those of catalytically deficient H63A and null mutants. When these mutant cells were cultured under 25 MPa, their intracellular O2•- levels increased while sod1∆ mutant genome stability was unaffected. The high-pressure sensitive sod1 mutants were also susceptible to sublethal levels of the O2•- generator paraquat. The sod1∆ mutant is known to exhibit methionine and lysine auxotrophy. However, excess methionine addition or overexpression of the lysine permease gene LYP1 did not counteract high-pressure sensitivity in the sod1 mutants, suggesting that their amino acid availability might be intact under 25 MPa. Interestingly, an exclusive localization of Sco2-Sod1 to the intermembrane space (IMS) of mitochondria appeared to partially restore the high-pressure growth ability in the sod1 mutants. Taken these results together, we suggest that high pressure enhances O2•- production and Sod1 within the IMS plays a role in scavenging O2•- allowing the cells to grow under high pressure.

BACKGROUND

Empirical evidence shows that under high hydrostatic pressure, the oxidative stress response is activated in Saccharomyces cerevisiae. However, the mechanisms involved in its antioxidant systems are unclear. In the current study, we aimed to explore the role of superoxide dismutase 1 (Sod1) in yeast able to grow under high pressure.

METHODS

Wild type and sod1 mutant cells were cultured in high-pressure chambers under 25 MPa (~250 kg/cm2). The SOD activity in whole cell extracts and 6His-tagged Sod1 recombinant proteins was analyzed using an SOD assay kit. The O2•- generation in cells was estimated by fluorescence staining.

RESULTS

Mutants lacking Sod1 or Ccs1, the copper chaperone for Sod1, displayed growth defects under 25 MPa. Of the various SOD1 mutations associated with familial amyotrophic lateral sclerosis, H46Q and S134N substitutions diminished SOD activity to levels comparable to those of catalytically deficient H63A and null mutants. The high-pressure sensitive sod1 mutants were also susceptible to sublethal levels of the O2•- generator paraquat. Exclusive localization of Sco2-Sod1 to the intermembrane space (IMS) of mitochondria partially restored the high-pressure growth ability in the sod1 mutants.

CONCLUSIONS

High pressure enhances O2•- production and Sod1 within the IMS plays a role in scavenging O2•- allowing the cells to grow under high pressure.

GENERAL SIGNIFICANCE

Unlike external free radical-generating compounds, high-pressure treatment appeared to increase endogenous O2•- levels in yeast cells. Our experimental system offers a unique approach to investigating the physiological responses to mechanical and oxidative stresses in human body.

Molecular Responses to High Hydrostatic Pressure in Eukaryotes: Genetic Insights from Studies on Saccharomyces cerevisiae

Simple Summary

High hydrostatic pressure generally has an adverse effect on the biological systems of organisms inhabiting lands or shallow sea regions. Deep-sea piezophiles that prefer high hydrostatic pressure for growth have garnered considerable scientific attention. However, the underlying molecular mechanisms of their adaptation to high pressure remains unclear owing to the challenges of culturing and manipulating the genome of piezophiles. Humans also experience high hydrostatic pressure during exercise. A long-term stay in space can cause muscle weakness in astronauts. Thus, the human body indubitably senses mechanical stresses such as hydrostatic pressure and gravity. Nonetheless, the mechanisms underlying biological responses to high pressures are not clearly understood. This review summarizes the occurrence and significance of high-pressure effects in eukaryotic cells and how the cell responds to increasing pressure by particularly focusing on the physiology of S. cerevisiae at the molecular level.

Abstract

High hydrostatic pressure is common mechanical stress in nature and is also experienced by the human body. Organisms in the Challenger Deep of the Mariana Trench are habitually exposed to pressures up to 110 MPa. Human joints are intermittently exposed to hydrostatic pressures of 3–10 MPa. Pressures less than 50 MPa do not deform or kill the cells. However, high pressure can have various effects on the cell’s biological processes. Although Saccharomyces cerevisiae is not a deep-sea piezophile, it can be used to elucidate the molecular mechanism underlying the cell’s responses to high pressures by applying basic knowledge of the effects of pressure on industrial processes involving microorganisms. We have explored the genes associated with the growth of S. cerevisiae under high pressure by employing functional genomic strategies and transcriptomics analysis and indicated a strong association between high-pressure signaling and the cell’s response to nutrient availability. This review summarizes the occurrence and significance of high-pressure effects on complex metabolic and genetic networks in eukaryotic cells and how the cell responds to increasing pressure by particularly focusing on the physiology of S. cerevisiae at the molecular level. Mechanosensation in humans has also been discussed.

Research and application of hydrostatic high pressure in tumor vaccines (Review)

It is well known that hydrostatic pressure (HP) is a physical parameter that is now regarded as an important variable for life. High hydrostatic pressure (HHP) technology has influenced biological systems for more than 100 years. Food and bioscience researchers have shown great interest in HHP technology over the past few decades. The development of knowledge related to this area can better facilitate the application of HHP in the life sciences. Furthermore, new applications for HHP may come from these current studies, particularly in tumor vaccines. Currently, cancer recurrence and metastasis continue to pose a serious threat to human health. The limited efficacy of conventional treatments has led to the need for breakthroughs in immunotherapy and other related areas. Research into tumor vaccines is providing new insights for cancer treatment. The purpose of this review is to present the main findings reported thus far in the relevant scientific literature, focusing on knowledge related to HHP technology and tumor vaccines, and to demonstrate the potential of applying HHP technology to tumor vaccine development.

Amino acid homeostatic control by TORC1 in Saccharomyces cerevisiae under high hydrostatic pressure

Mechanical stresses, including high hydrostatic pressure, elicit diverse physiological effects on organisms. Gtr1, Gtr2, Ego1 (also known as Meh1) and Ego3 (also known as Slm4), central regulators of the TOR complex 1 (TORC1) nutrient signaling pathway, are required for the growth of Saccharomyces cerevisiae cells under high pressure. Here, we showed that a pressure of 25 MPa (∼250 kg/cm2) stimulates TORC1 to promote phosphorylation of Sch9, which depends on the EGO complex (EGOC) and Pib2. Incubation of cells at this pressure aberrantly increased glutamine and alanine levels in the ego1Δ, gtr1Δ, tor1Δ and pib2Δ mutants, whereas the polysome profiles were unaffected. Moreover, we found that glutamine levels were reduced by combined deletions of EGO1, GTR1, TOR1 and PIB2 with GLN3 These results suggest that high pressure leads to the intracellular accumulation of amino acids. Subsequently, Pib2 loaded with glutamine stimulates the EGOC-TORC1 complex to inactivate Gln3, downregulating glutamine synthesis. Our findings illustrate the regulatory circuit that maintains intracellular amino acid homeostasis and suggest critical roles for the EGOC-TORC1 and Pib2-TORC1 complexes in the growth of yeast under high hydrostatic pressure.