High hydrostatic pressure and the cell membrane: stress response of Saccharomyces cerevisiae

Insight into the adaptation mechanisms of high hydrostatic pressure in physiology and metabolism of hadal fungi from the deepest ocean sediment

High hydrostatic pressure (HHP) influences the life processes of organisms living at depth in the oceans. While filamentous fungi are one of the essential members of deep-sea microorganisms, few works have explored their piezotolerance to HHP. Here, we obtained three homogeneous Aspergillus sydowii from terrestrial, shallow, and hadal areas, respectively, to compare their pressure resistance. A set of all-around evaluation methods including determination of growth rate, metabolic activity, and microscopic staining observation was established and indicated that A. sydowii DM1 from the hadal area displayed significant piezotolerance. Global analysis of transcriptome data under elevated HHP revealed that A. sydowii DM1 proactively modulated cell membrane permeability, hyphae morphology, and septal quantities for seeking a better livelihood under mild pressure. Besides, differentially expressed genes were mainly enriched in the biosynthesis of amino acids, carbohydrate metabolism, cell process, etc., implying how the filamentous fungi respond to elevated pressure at the molecular level. We speculated that A. sydowii DM1 could acclimatize itself to HHP by adopting several strategies, including environmental response pathway HOG-MAPK, stress proteins, and cellular metabolisms.IMPORTANCEFungi play an ecological and biological function in marine environments, while the physiology of filamentous fungi under high hydrostatic pressure (HHP) is an unknown territory due to current technologies. As filamentous fungi are found in various niches, Aspergillus sp. from deep-sea inspire us to the physiological trait of eukaryotes under HHP, which can be considered as a prospective research model. Here, the evaluation methods we constructed would be universal for most filamentous fungi to assess their pressure resistance, and we found that Aspergillus sydowii DM1 from the hadal area owned better piezotolerance and the active metabolisms under HHP indicated the existence of undiscovered metabolic strategies for hadal fungi. Since pressure-related research of marine fungi has been unexpectedly neglected, our study provided an enlightening strategy for them under HHP; we believed that understanding their adaptation and ecological function in original niches will be accelerated in the perceivable future.

Mechanical Static Force Negatively Regulates Vitality and Early Skeletal Development in Zebrafish Embryos

Skeletal system development and remodelling is regulated by several different factors, including hormones, cytokines, and mechanical forces. It is known that gravity and pressure stimulate mechanosensors on bone cells which transduce mechanical signals to chemical ones. Nevertheless, few data have been provided about the role of mechanical forces on embryo osteogenesis in vivo. Since the zebrafish is an elective model for developmental studies, in particular on bone formation and tissue mineralization, we analyzed in vivo the effects of a static mechanical force generated by a water column on fertilized zebrafish eggs. The results have shown that an increase in the hydrostatic pressure (HP) of up to 5.9% was lethal for 100% of treated embryos at 48 h post fertilization (hpf). A small decrease in length (−2%) and 49% mortality were found in the +4.4% HP embryos compared with the controls. To analyze skeletal development, we evaluated the number of mineralized vertebral bodies in the trunk at five days post fertilization. The embryos grown under +2.4% HP showed a physiological intramembranous mineralization of vertebral bodies whereas the embryos which grew with +3.4% HP showed a significant decrease in mineralization rate (−54%). Morphological analysis of cartilage and bones in embryos at +3.4% HP revealed a delay of both intramembranous and chondrogenic mineralization, respectively, in axial and head bones, whereas the chondrogenesis appeared normal. These data suggested that developing osteoblasts and different mineralization programs are sensitive to mechanical pressure when applied to early embryogenesis.

Characterization and Role of Sterols in Saccharomyces cerevisiae during White Wine Alcoholic Fermentation

Responsible for plasma membrane structure maintenance in eukaryotic organisms, sterols are essential for yeast development. The role of two sterol sources in Saccharomyces cerevisiae during wine fermentation is highlighted in this review: ergosterol (yeast sterol produced by yeast cells under aerobic conditions) and phytosterols (plant sterols imported by yeast cells from grape musts in the absence of oxygen). These compounds are responsible for the maintenance of yeast cell viability during white wine fermentation under stress conditions, such as ethanol stress and sterol starvation, to avoid sluggish and stuck fermentations.

Synergetic Inactivation Mechanism of Protocatechuic Acid and High Hydrostatic Pressure against Escherichia coli O157:H7

With the wide application of high hydrostatic pressure (HHP) technology in the food industry, safety issues regarding food products, resulting in potential food safety hazards, have arisen. To address such problems, this study explored the synergetic bactericidal effects and mechanisms of protocatechuic acid (PCA) and HHP against Escherichia coli O157:H7. At greater than 200 MPa, PCA (1.25 mg/mL for 60 min) plus HHP treatments had significant synergetic bactericidal effects that positively correlated with pressure. After a combined treatment at 500 MPa for 5 min, an approximate 9.0 log CFU/mL colony decline occurred, whereas the individual HHP and PCA treatments caused 4.48 and 1.06 log CFU/mL colony decreases, respectively. Mechanistically, membrane integrity and morphology were damaged, and the permeability increased when E. coli O157: H7 was exposed to the synergetic stress of PCA plus HHP. Inside cells, the synergetic treatment additionally targeted the activities of enzymes such as superoxide dismutase, catalase and ATPase, which were inhibited significantly (p ≤ 0.05) when exposed to high pressure. Moreover, an analysis of circular dichroism spectra indicated that the synergetic treatment caused a change in DNA structure, which was expressed as the redshift of the characteristic absorption peak. Thus, the synergetic treatment of PCA plus HHP may be used as a decontamination method owing to the good bactericidal effects on multiple targets.

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.

Difference between the cell wall roughnesses of mothers and daughters of Saccharomyces cerevisiae subjected to high pressure stress

High hydrostatic pressure (HHP) stress generates cellular responses similar to those to other stresses that yeasts endure in fermentation tanks. Structural and spatial compaction of molecules, as well as weakening and stretching of plasma membranes and cell walls, are often observed and have a significant influence on the fermentative process. Atomic force microscopy (AFM) yields accurate data on the morphological characteristics of yeast cell walls, providing important insights for the development of more productive yeast strains. Saccharomyces cerevisiae cell wall assessment using AFM in the intermittent contact reading mode using a silicon cantilever, before and after application of a pressure of 100 MPa for 30 min, demonstrated that mother and daughter cells have different responses. Daughter cells were more sensitive to the effects of HHP, presenting lower average Ra (arithmetic roughness), Rz (ten-point average roughness), and Rq (root-mean-square roughness) after exposure to high pressure. Better adaptation to stress in mother cells leads to higher cell wall resistance and, therefore, to better protection.

Ethanol and H2O2 stresses enhance lipid production in an oleaginous Rhodotorula toruloides thermotolerant mutant L1-1

Stress tolerance is a desired characteristic of yeast strains for industrial applications. Stress tolerance has been well described in Saccharomyces yeasts but has not yet been characterized in oleaginous Rhodotorula yeasts even though they are considered promising platforms for lipid production owing to their outstanding lipogenicity. In a previous study, the thermotolerant strain L1–1 was isolated from R. toruloides DMKU3-TK16 (formerly Rhodosporidium toruloides). In this study, we aimed to further examine the ability of this strain to tolerate other stresses and its lipid productivity under various stress conditions. We found that the L1–1 strain could tolerate not only thermal stress but also oxidative stress (ethanol and H2O2), osmotic stress (glucose) and a cell membrane disturbing reagent (DMSO). Our results also showed that the L1–1 strain exhibited enhanced ability to maintain ROS homeostasis, stronger cell wall strength and increased levels of unsaturated membrane lipids under various stresses. Moreover, we also demonstrated that ethanol-induced stress significantly increased the lipid productivity of the thermotolerant L1–1. The thermotolerant L1–1 was also found to produce a higher lipid titer under the dual ethanol-H2O2 stress than under non-stress conditions. This is the first report to indicate that ethanol stress can induce lipid production in an R. toruloides thermotolerant strain.

LC-MS/MS analysis of free fatty acid composition and other lipids in skins and seeds of Vitis vinifera grape cultivars

Lipids are important constituents of plant and animal cells, being essential due to their biological functions. Despite the significant role of these compounds in grape berries, knowledge of grape lipid composition is still limited. This study addresses the free fatty acid composition and other lipids in skins and seeds of several Vitis vinifera grape cultivars using LC-MS/MS. A different profile of free fatty acids was determined in the grape tissues, showing a higher proportion of unsaturated fatty acids in seeds (ca. 60%). Lignoceric acid was one of the main free saturated fatty acids found in grape skins, together with palmitic and stearic acids. Berry skins showed similar ranges of C18-unsaturated fatty acids, whereas linoleic acid was predominant in the seed fatty acid composition. Higher content of glycerophospholipids was determined in grape seeds. Uvaol and oleanolic acid were only quantified in skins (1.5-3.9 and 38.6-57.6 mg/kg fresh weight, respectively). These preliminary results suggest a certain diversity in grape lipids according to their location in the berry tissues and cultivar, providing useful information for the fermentation process due to their role in wine sensory profile and yeast growth.

Mechanical stimuli modulate intracellular calcium oscillations: a pathological model without chemical cues

OBJECTIVE

To control the oscillatory behavior of the intracellular calcium ([Ca²⁺]_i) concentration in endothelial cells via mechanical factors (i.e., various hydrostatic pressures) because [Ca²⁺]_i in these cells is affected by blood pressure.

RESULTS

Quantitative analyses based on real-time imaging showed that [Ca²⁺]_i oscillation frequency and relative concentration increased significantly when 200 mm Hg pressure, mimicking hypertension, was applied for >10 min. Peak height and peak width decreased significantly at 200 mm Hg. These trends were more marked as the duration of the 200 mm Hg pressure was increased. However, no change was observed under normal blood pressure conditions 100 mm Hg.

CONCLUSION

We generated a simple in vitro model to study [Ca²⁺]_i behavior in relation to various pathologies and diseases by eliminating possible complicating effects induced by chemical cues.

Molecular and Translational Classifications of DAMPs in Immunogenic Cell Death

The immunogenicity of malignant cells has recently been acknowledged as a critical determinant of efficacy in cancer therapy. Thus, besides developing direct immunostimulatory regimens, including dendritic cell-based vaccines, checkpoint-blocking therapies, and adoptive T-cell transfer, researchers have started to focus on the overall immunobiology of neoplastic cells. It is now clear that cancer cells can succumb to some anticancer therapies by undergoing a peculiar form of cell death that is characterized by an increased immunogenic potential, owing to the emission of the so-called “damage-associated molecular patterns” (DAMPs). The emission of DAMPs and other immunostimulatory factors by cells succumbing to immunogenic cell death (ICD) favors the establishment of a productive interface with the immune system. This results in the elicitation of tumor-targeting immune responses associated with the elimination of residual, treatment-resistant cancer cells, as well as with the establishment of immunological memory. Although ICD has been characterized with increased precision since its discovery, several questions remain to be addressed. Here, we summarize and tabulate the main molecular, immunological, preclinical, and clinical aspects of ICD, in an attempt to capture the essence of this phenomenon, and identify future challenges for this rapidly expanding field of investigation.

Cell membrane fatty acid changes and desaturase expression of Saccharomyces bayanus exposed to high pressure homogenization in relation to the supplementation of exogenous unsaturated fatty acids

Aims: The aim of this work was to study the responses of Saccharomyces bayanus cells exposed to sub-lethal high-pressure homogenization (HPH) and determine whether the plasmatic membrane can sense HPH in the presence, or absence, of exogenous unsaturated fatty acids (UFAs) in the growth medium.

Methods and Results: High-pressure homogenization damaged and caused the collapse of cell walls and membranes of a portion of cells; however, HPH did not significantly affect S. bayanus cell viability (less than 0.3 Log CFU ml^-1). HPH strongly affected the membrane fatty acid (FA) composition by increasing the percentage of total UFA when compared with saturated fatty acids. The gene expression showed that the transcription of OLE1, ERG3, and ERG11 increased after HPH. The presence of exogenous UFA abolished HPH-induced effects on the OLE1 and ERG3 genes, increased the percentage of membrane lipids and decreased the expression of OLE1 and ERG3 within 30 min of treatment.

Conclusion: The results suggest a key role for UFA in the microbial cell response to sub-lethal stress. In addition, these data provide insight into the molecular basis of the response of S. bayanus to this innovative technology.

Significance and Impact of the Study: Elucidation of the mechanism of action for sub-lethal HPH will enable the utilization of this technology to modulate the starter performance at the industrial scale.

Changes in SAM2 expression affect lactic acid tolerance and lactic acid production in Saccharomyces cerevisiae

Background

The great interest in the production of highly pure lactic acid enantiomers comes from the application of polylactic acid (PLA) for the production of biodegradable plastics. Yeasts can be considered as alternative cell factories to lactic acid bacteria for lactic acid production, despite not being natural producers, since they can better tolerate acidic environments. We have previously described metabolically engineered Saccharomyces cerevisiae strains producing high amounts of L-lactic acid (>60 g/L) at low pH. The high product concentration represents the major limiting step of the process, mainly because of its toxic effects. Therefore, our goal was the identification of novel targets for strain improvement possibly involved in the yeast response to lactic acid stress.

Results

The enzyme S-adenosylmethionine (SAM) synthetase catalyses the only known reaction leading to the biosynthesis of SAM, an important cellular cofactor. SAM is involved in phospholipid biosynthesis and hence in membrane remodelling during acid stress. Since only the enzyme isoform 2 seems to be responsive to membrane related signals (e.g. myo-inositol), Sam2p was tagged with GFP to analyse its abundance and cellular localization under different stress conditions. Western blot analyses showed that lactic acid exposure correlates with an increase in protein levels. The SAM2 gene was then overexpressed and deleted in laboratory strains. Remarkably, in the BY4741 strain its deletion conferred higher resistance to lactic acid, while its overexpression was detrimental. Therefore, SAM2 was deleted in a strain previously engineered and evolved for industrial lactic acid production and tolerance, resulting in higher production.

Conclusions

Here we demonstrated that the modulation of SAM2 can have different outcomes, from clear effects to no significant phenotypic responses, upon lactic acid stress in different genetic backgrounds, and that at least in one genetic background SAM2 deletion led to an industrially relevant increase in lactic acid production. Further work is needed to elucidate the molecular basis of these observations, which underline once more that strain robustness relies on complex cellular mechanisms, involving regulatory genes and proteins. Our data confirm cofactor engineering as an important tool for cell factory improvement.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-014-0147-7) contains supplementary material, which is available to authorized users.

The use of trehalose in the preparation of specimens for molecular electron microscopy

Biological specimens have to be prepared for imaging in the electron microscope in a way that preserves their native structure. Two-dimensional (2D) protein crystals to be analyzed by electron crystallography are best preserved by sugar embedding. One of the sugars often used to embed 2D crystals is trehalose, a disaccharide used by many organisms for protection against stress conditions. Sugars such as trehalose can also be added to negative staining solutions used to prepare proteins and macromolecular complexes for structural studies by single-particle electron microscopy (EM). In this review, we describe trehalose and its characteristics that make it so well suited for preparation of EM specimens and we review specimen preparation methods with a focus on the use of trehalose.