Aquaporin expression correlates with freeze tolerance in baker's yeast, and overexpression improves freeze tolerance in industrial strains

Enhancing freeze-thaw tolerance in baker's yeast: strategies and perspectives

Frozen dough technology is important in modern bakery operations, facilitating the transportation of dough at low temperatures to downstream sales points. However, the freeze-thaw process imposes significant stress on baker's yeast, resulting in diminished viability and fermentation capacity. Understanding the mechanisms underlying freeze-thaw stress is essential for mitigating its adverse effects on yeast performance. This review delves into the intricate mechanisms underlying freeze-thaw stress, focusing specifically on Saccharomyces cerevisiae, the primary yeast used in baking, and presents a wide range of biotechnological approaches to enhance freeze-thaw resistance in S. cerevisiae. Strategies include manipulating intracellular metabolites, altering membrane composition, managing antioxidant defenses, mediating aquaporin expression, and employing adaptive evolutionary and breeding techniques. Addressing challenges and strategies associated with freeze-thaw stress, this review provides valuable insights for future research endeavors, aiming to enhance the freeze-thaw tolerance of baker's yeast and contribute to the advancement of bakery science.

Just passing through: Deploying aquaporins in microbial cell factories

As microbial membranes are naturally impermeable to even the smallest biomolecules, transporter proteins are physiologically essential for normal cell functioning. This makes transporters a key target area for engineering enhanced cell factories. As part of the wider cellular transportome, aquaporins (AQPs) are responsible for transporting small polar solutes, encompassing many compounds which are of great interest for industrial biotechnology, including cell feedstocks, numerous commercially relevant polyols and even weak organic acids. In this review, examples of cell factory engineering by targeting AQPs are presented. These AQP modifications aid in redirecting carbon fluxes and boosting bioconversions either by enhanced feedstock uptake, improved intermediate retention, increasing product export into the media or superior cell viability against stressors with applications in both bacterial and yeast production platforms. Additionally, the future potential for AQP deployment and targeting is discussed, showcasing hurdles and considerations of this strategy as well as recent advances and future directions in the field. By leveraging the natural diversity of AQPs and breakthroughs in channel protein engineering, these transporters are poised to be promising tools capable of enhancing a wide variety of biotechnological processes.

Polyextremophile engineering: a review of organisms that push the limits of life

Nature exhibits an enormous diversity of organisms that thrive in extreme environments. From snow algae that reproduce at sub-zero temperatures to radiotrophic fungi that thrive in nuclear radiation at Chernobyl, extreme organisms raise many questions about the limits of life. Is there any environment where life could not "find a way"? Although many individual extremophilic organisms have been identified and studied, there remain outstanding questions about the limits of life and the extent to which extreme properties can be enhanced, combined or transferred to new organisms. In this review, we compile the current knowledge on the bioengineering of extremophile microbes. We summarize what is known about the basic mechanisms of extreme adaptations, compile synthetic biology's efforts to engineer extremophile organisms beyond what is found in nature, and highlight which adaptations can be combined. The basic science of extremophiles can be applied to engineered organisms tailored to specific biomanufacturing needs, such as growth in high temperatures or in the presence of unusual solvents.

High-throughput RNA sequencing reveals differences between the transcriptomes of the five spore forms of Puccinia striiformis f. sp. tritici, the wheat stripe rust pathogen

The devastating wheat stripe (yellow) rust pathogen, Puccinia striiformis f. sp. tritici (Pst), is a macrocyclic and heteroecious fungus. Pst produces urediniospores and teliospores on its primary host, wheat, and pycniospores and aeciospores are produced on its alternate hosts, barberry (Berberis spp.) or mahonia (Mahonia spp.). Basidiospores are developed from teliospores and infect alternate hosts. These five spore forms play distinct roles in Pst infection, disease development, and fungal survival, etc. However, the specific genes and mechanisms underlying these functional differences are largely unknown. In this study, we performed, for the first time in rust fungi, the deep RNA sequencing to examine the transcriptomic shift among all five Pst spore forms. Among a total of 29,591 identified transcripts, 951 were specifically expressed in basidiospores, whereas 920, 761, 266, and 110 were specific for teliospores, pycniospores, aeciospores, and urediniospores, respectively. Additionally, transcriptomes of sexual spores, namely pycniospores and basidiospores, showed significant differences from those of asexual spores (urediniospores, teliospores, and aeciospores), and transcriptomes of urediniospores and aeciospores were more similar to each other than to the three other spore forms. Especially, the basidiospores and pycniospores which infected the berberis shows wide differences in the cell wall degrading-enzymes and mating and pheromone response genes. Besides, we also found that there are 6234 differential expressed genes between the urediniospores and pycniospores, while only have 3 genes have alternative splicing enents, suggesting that differential genes expression may make more contribution than AS. This comprehensive transcriptome profiling can substantially improve our understanding of the developmental biology of the wheat stripe rust fungus.

A high-throughput yeast approach to characterize aquaporin permeabilities: Profiling the Arabidopsis PIP aquaporin sub-family

INTRODUCTION

Engineering membrane transporters to achieve desired functionality is reliant on availability of experimental data informing structure-function relationships and intelligent design. Plant aquaporin (AQP) isoforms are capable of transporting diverse substrates such as signaling molecules, nutrients, metalloids, and gases, as well as water. AQPs can act as multifunctional channels and their transport function is reliant on many factors, with few studies having assessed transport function of specific isoforms for multiple substrates.

METHODS

High-throughput yeast assays were developed to screen for transport function of plant AQPs, providing a platform for fast data generation and cataloguing of substrate transport profiles. We applied our high-throughput growth-based yeast assays to screen all 13 Arabidopsis PIPs (AtPIPs) for transport of water and several neutral solutes: hydrogen peroxide (H2O2), boric acid (BA), and urea. Sodium (Na+) transport was assessed using elemental analysis techniques.

RESULTS

All AtPIPs facilitated water and H2O2 transport, although their growth phenotypes varied, and none were candidates for urea transport. For BA and Na+ transport, AtPIP2;2 and AtPIP2;7 were the top candidates, with yeast expressing these isoforms having the most pronounced toxicity response to BA exposure and accumulating the highest amounts of Na+. Linking putative AtPIP isoform substrate transport profiles with phylogenetics and gene expression data, enabled us to align possible substrate preferences with known and hypothesized biological roles of AtPIPs.

DISCUSSION

This testing framework enables efficient cataloguing of putative transport functionality of diverse AQPs at a scale that can help accelerate our understanding of AQP biology through big data approaches (e.g. association studies). The principles of the individual assays could be further adapted to test additional substrates. Data generated from this framework could inform future testing of AQP physiological roles, and address knowledge gaps in structure-function relationships to improve engineering efforts.

Protein Structure and Modification of Aquaporins

Aquaporins (AQPs) allow water molecules and other small, neutral solutes to quickly pass through membrane. The protein structures of AQPs solved by crystallographic methods or cryo-electron microscopy technology show that AQP monomer consists of six membrane-spanning alpha-helices that form the central water-transporting pore. AQP monomers assemble to form tetramers, forming the functional units in the membrane, to transport water or other small molecules. The biological functions of AQPs are regulated by posttranslational modifications, e.g., phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation and protein interactions. Modifications of AQP combined with structural properties contribute to a better functional mechanism of AQPs. Insight into the molecular mechanisms responsible for AQP modifications as well as gating and transport properties proved to be fundamental to the development of new therapeutic targets or reliable diagnostic and prognostic biomarkers.

Improving carotenoid production in recombinant yeast, Saccharomyces cerevisiae, using ultrasound-irradiated two-phase extractive fermentation

Carotenoids are hydrophobic compounds that exhibit excellent bioactivity and can be produced by recombinant S. cerevisiae. Irradiating microorganisms with ultrasonic waves increase the productivity of various useful chemicals. Ultrasonic waves are also used to extract useful chemicals that accumulate in microbial cells. In this study, we aimed to improve the carotenoid production efficiency of a recombinant S. cerevisiae using an ultrasonic-irradiation based two-phase extractive fermentation process. When isopropyl myristate was used as the extraction solvent, a total of 264 mg/L of carotenoid was produced when batches were subjected to ultrasonic-irradiation at 10 W, which was a 1.3-fold increase when compared to the control. Transcriptome analysis suggested that one of the reasons for this improvement was an increase in the number of living cells. In fact, after 96 h of fermentation, the number of living cells increased by 1.4-fold upon irradiation with ultrasonic waves. Consequently, we succeeded in improving the carotenoid production in a recombinant S. cerevisiae strain using a ultrasonic-irradiated two-phase extractive fermentation and isopropyl myristate as the solvent. This fermentation strategy has the potential to be widely applied during the production of hydrophobic chemicals in recombinant yeast, and future research is expected to further develop this process.

Expression of a CO2-permeable aquaporin enhances mesophyll conductance in the C4 species Setaria viridis

A fundamental limitation of photosynthetic carbon fixation is the availability of CO₂. In C₄ plants, primary carboxylation occurs in mesophyll cytosol, and little is known about the role of CO₂ diffusion in facilitating C₄ photosynthesis. We have examined the expression, localization, and functional role of selected plasma membrane intrinsic aquaporins (PIPs) from Setaria italica (foxtail millet) and discovered that SiPIP2;7 is CO₂-permeable. When ectopically expressed in mesophyll cells of Setaria viridis (green foxtail), SiPIP2;7 was localized to the plasma membrane and caused no marked changes in leaf biochemistry. Gas exchange and C¹⁸O¹⁶O discrimination measurements revealed that targeted expression of SiPIP2;7 enhanced the conductance to CO₂ diffusion from the intercellular airspace to the mesophyll cytosol. Our results demonstrate that mesophyll conductance limits C₄ photosynthesis at low pCO₂ and that SiPIP2;7 is a functional CO₂ permeable aquaporin that can improve CO₂ diffusion at the airspace/mesophyll interface and enhance C₄ photosynthesis.

Development of Freeze-Thaw Stable Starch through Enzymatic Modification

The use of unmodified starch in frozen foods can cause extremely undesirable textural changes after the freeze-thaw process. In this study, using cyclodextrin glucanotransferase (CGTase) and branching enzymes, an amylopectin cluster with high freeze-thaw stability was produced, and was named CBAC. It was found to have a water solubility seven times higher, and a molecular weight 77 times lower, than corn starch. According to the results of a differential scanning calorimetry (DSC) analysis, dough containing 5% CBAC lost 19% less water than a control dough after three freeze-thaw cycles. During storage for 7 days at 4 °C, bread produced using CBAC-treated dough exhibited a 14% smaller retrogradation peak and 37% less hardness than a control dough, suggesting that CBAC could be a potential candidate for clean label starch, providing high-level food stability under repeated freeze-thaw conditions.

Preservation of non-Saccharomyces yeasts: Current technologies and challenges

There is a recent and growing interest in the study and application of non-Saccharomyces yeasts, mainly in fermented foods. Numerous publications and patents show the importance of these yeasts. However, a fundamental issue in studying and applying them is to ensure an appropriate preservation scheme that allows to the non-Saccharomyces yeasts conserve their characteristics and fermentative capabilities by long periods of time. The main objective of this review is to present and analyze the techniques available to preserve these yeasts (by conventional and non-conventional methods), in small or large quantities for laboratory or industrial applications, respectively. Wine fermentation is one of the few industrial applications of non-Saccharomyces yeasts, but the preservation stage has been a major obstacle to achieve a wider application of these yeasts. This review considers the preservation techniques, and clearly defines parameters such as culturability, viability, vitality and robustness. Several conservation strategies published in research articles as well as patents are analyzed, and the advantages and disadvantages of each technique used are discussed. Another important issue during conservation processes is the stress to which yeasts are subjected at the time of preservation (mainly oxidative stress). There is little published information on the subject for non-Saccharomyces yeast, but it is a fundamental point to consider when designing a preservation strategy.

Phosphorylation influences water and ion channel function of AtPIP2;1

The phosphorylation state of two serine residues within the C-terminal domain of AtPIP2;1 (S280, S283) regulates its plasma membrane localization in response to salt and osmotic stress. Here, we investigated whether the phosphorylation state of S280 and S283 also influence AtPIP2;1 facilitated water and cation transport. A series of single and double S280 and S283 phosphomimic and phosphonull AtPIP2;1 mutants were tested in heterologous systems. In Xenopus laevis oocytes, phosphomimic mutants AtPIP2;1 S280D, S283D, and S280D/S283D had significantly greater ion conductance for Na⁺ and K⁺ , whereas the S280A single phosphonull mutant had greater water permeability. We observed a phosphorylation-dependent inverse relationship between AtPIP2;1 water and ion transport with a 10-fold change in both. The results revealed that phosphorylation of S280 and S283 influences the preferential facilitation of ion or water transport by AtPIP2;1. The results also hint that other regulatory sites play roles that are yet to be elucidated. Expression of the AtPIP2;1 phosphorylation mutants in Saccharomyces cerevisiae confirmed that phosphorylation influences plasma membrane localization, and revealed higher Na⁺ accumulation for S280A and S283D mutants. Collectively, the results show that phosphorylation in the C-terminal domain of AtPIP2;1 influences its subcellular localization and cation transport capacity.

Frozen-dough baking potential of psychrotolerant Saccharomyces species and derived hybrids

Despite Saccharomyces cerevisiae being a synonym for baker's yeast, the species does not perform well in all baking-related conditions. In particular, dough fermentation, or proofing, is compromised by the species' sensitivity to the low and freezing temperatures that are often used in modern bakeries. Here, screening trials that included representatives of all known Saccharomyces species, showed that S. cerevisiae was generally the most sensitive member of the genus with respect to cold and freezing conditions. We hypothesized therefore that the superior cold tolerance of the non-S. cerevisiae yeast would enable their use as frozen-dough baking strains. To test this, the different yeast species were incorporated into doughs, flash frozen and kept in a frozen state for 14 days. During the proofing stage, dough development was lower in doughs that had been frozen, relative to fresh doughs. This reduction in fermentation performance was however most pronounced with S. cerevisiae. The psychrotolerant yeasts S. eubayanus, S. jurei and S. arboricola showed a strong capacity for post-freeze proofing in terms of dough development and duration of lag phase prior to fermentation. The superior proofing power of these species resulted in breads that were significantly softer and less dense than those prepared with S. cerevisiae. A sensory panel could distinguish the S. cerevisiae and non-S. cerevisiae breads based on their physical properties, but aroma and taste were unaffected by the species employed. To further improve frozen dough baking properties, S. eubayanus, S. jurei and S. arboricola were crossed with baker's yeast through rare mating, and hybrids with improved proofing capacities in both fresh and frozen doughs relative to the parents were created. The use of S. jurei and S. arboricola in baking represents the first potential technological application of these species.

Cryopreservation and the Freeze-Thaw Stress Response in Yeast

The ability of yeast to survive freezing and thawing is most frequently considered in the context of cryopreservation, a practical step in both industrial and research applications of these organisms. However, it also relates to an evolved ability to withstand freeze-thaw stress that is integrated with a larger network of survival responses. These responses vary between different strains and species of yeast according to the environments to which they are adapted, and the basis of this adaptation appears to be both conditioned and genetic in origin. This review article briefly touches upon common yeast cryopreservation methods and describes in detail what is known about the biochemical and genetic determinants of cell viability following freeze-thaw stress. While we focus on the budding yeast Saccharomyces cerevisiae, in which the freeze-thaw stress response is best understood, we also highlight the emerging diversity of yeast freeze-thaw responses as a manifestation of biodiversity among these organisms.

Plasma Membrane Aquaporin Members PIPs Act in Concert to Regulate Cold Acclimation and Freezing Tolerance Responses in Arabidopsis thaliana

Aquaporins play a major role in plant water uptake at both optimal and environmentally stressed conditions. However, the functional specificity of aquaporins under cold remains obscure. To get a better insight to the role of aquaporins in cold acclimation and freezing tolerance, we took an integrated approach of physiology, transcript profiling and cell biology in Arabidopsis thaliana. Cold acclimation resulted in specific upregulation of PIP1;4 and PIP2;5 aquaporin (plasma membrane intrinsic proteins) expression, and immunoblotting analysis confirmed the increase in amount of PIP2;5 protein and total amount of PIPs during cold acclimation, suggesting that PIP2;5 plays a major role in tackling the cold milieu. Although single mutants of pip1;4 and pip2;5 or their double mutant showed no phenotypic changes in freezing tolerance, they were more sensitive in root elongation and cell survival response under freezing stress conditions compared with the wild type. Consistently, a single mutation in either PIP1;4 or PIP2;5 altered the expression of a number of aquaporins both at the transcriptional and translational levels. Collectively, our results suggest that aquaporin members including PIP1;4 and PIP2;5 function in concert to regulate cold acclimation and freezing tolerance responses.

Genome-Wide Association Study Identifies Single Nucleotide Polymorphism Markers Associated with Mycelial Growth (at 15, 20, and 25°C), Mefenoxam Resistance, and Mating Type in Phytophthora infestans

Phenotypic diversity among individuals defines the potential for evolutionary selection in a species. Phytophthora infestans epidemics are generally thought to be favored by moderate to low temperatures, but temperatures in many locations worldwide are expected to rise as a result of global climate change. Thus, we investigated variation among individuals of P. infestans for relative growth at different temperatures. Isolates of P. infestans came from three collections: (i) individual genotypes recently dominant in the United States, (ii) recently collected individuals from Central Mexico, and (iii) progeny of a recent sexual recombination event in the northeastern United States. In general, these isolates had optimal mycelial growth rates at 15 or 20°C. However, two individuals from Central Mexico grew better at higher temperatures than did most others and two individuals grew relatively less at higher temperatures than did most others. The isolates were also assessed for mefenoxam sensitivity and mating type. Each collection contained individuals of diverse sensitivities to mefenoxam and individuals of the A1 and A2 mating type. We then searched for genomic regions associated with phenotypic diversity using genotyping-by-sequencing. We found one single nucleotide polymorphism (SNP) associated with variability in mycelial growth at 20°C, two associated with variability in mycelial growth at 25°C, two associated with sensitivity to mefenoxam, and one associated with mating type. Interestingly, the SNPs associated with mefenoxam sensitivity were found in a gene-sparse region, whereas the SNPs associated with growth at the two temperatures and mating type were found both at more gene-dense regions.

Plant Aquaporins: Diversity, Evolution and Biotechnological Applications

The plasma membrane forms a permeable barrier that separates the cytoplasm from the external environment, defining the physical and chemical limits in each cell in all organisms. The movement of molecules and ions into and out of cells is controlled by the plasma membrane as a critical process for cell stability and survival, maintaining essential differences between the composition of the extracellular fluid and the cytosol. In this process aquaporins (AQPs) figure as important actors, comprising highly conserved membrane proteins that carry water, glycerol and other hydrophilic molecules through biomembranes, including the cell wall and membranes of cytoplasmic organelles. While mammals have 15 types of AQPs described so far (displaying 18 paralogs), a single plant species can present more than 120 isoforms, providing transport of different types of solutes. Such aquaporins may be present in the whole plant or can be associated with different tissues or situations, including biotic and especially abiotic stresses, such as drought, salinity or tolerance to soils rich in heavy metals, for instance. The present review addresses several aspects of plant aquaporins, from their structure, classification, and function, to in silico methodologies for their analysis and identification in transcriptomes and genomes. Aspects of evolution and diversification of AQPs (with a focus on plants) are approached for the first time with the aid of the LCA (Last Common Ancestor) analysis. Finally, the main practical applications involving the use of AQPs are discussed, including patents and future perspectives involving this important protein family.

Higher Expression Levels of Aquaporin Family of Proteins in the Kidneys of Arid-Desert Living Lepus yarkandensis

Lepus yarkandensis specifically lives in arid climate with rare precipitation of Tarim Basin in western China. Aquaporins (AQPs) are a family of channel proteins that facilitate water transportation across cell membranes. Kidney AQPs play vital roles in renal tubule water permeability and maintenance of body water homeostasis. This study aimed to investigate whether kidney AQPs exhibit higher expression in arid-desert living animals. Immunohistochemistry results revealed localization of AQP1 to the capillary endothelial cells in glomerulus and epithelial cells in proximal tubule and descending thin limbs, AQP2 to the apical plasma membrane of principal cells in the cortical collecting duct (CCD), outer medullary collecting duct (OMCD), and IMCD cells in the initial inner medullary collecting duct (IMCD1) and middle IMCD (IMCD2), and AQP3 and AQP4 to the basolateral plasma membrane of principal cells and IMCD cells in CCD, OMCD, IMCD1, and IMCD2 in L. yarkandensis kidneys. Quantitative real-time PCR analysis showed higher mRNA levels of AQP1, AQP2, AQP3, and AQP4 in L. yarkandensis kidneys compared with Oryctolagus cuniculus. Similar results were obtained by western blotting. Our results suggested that higher expression levels of AQP1, AQP2, AQP3, and AQP4 in L. yarkandensis kidneys favored for drawing more water from the tubular fluid.

Quantitative Trait Nucleotides Impacting the Technological Performances of Industrial Saccharomyces cerevisiae Strains

The budding yeast Saccharomyces cerevisiae is certainly the prime industrial microorganism and is related to many biotechnological applications including food fermentations, biofuel production, green chemistry, and drug production. A noteworthy characteristic of this species is the existence of subgroups well adapted to specific processes with some individuals showing optimal technological traits. In the last 20 years, many studies have established a link between quantitative traits and single-nucleotide polymorphisms found in hundreds of genes. These natural variations constitute a pool of QTNs (quantitative trait nucleotides) that modulate yeast traits of economic interest for industry. By selecting a subset of genes functionally validated, a total of 284 QTNs were inventoried. Their distribution across pan and core genome and their frequency within the 1,011 Saccharomyces cerevisiae genomes were analyzed. We found that 150 of the 284 QTNs have a frequency lower than 5%, meaning that these variants would be undetectable by genome-wide association studies (GWAS). This analysis also suggests that most of the functional variants are private to a subpopulation, possibly due to their adaptive role to specific industrial environment. In this review, we provide a literature survey of their phenotypic impact and discuss the opportunities and the limits of their use for industrial strain selection.

Aquaporin1 regulates development, secondary metabolism and stress responses in Fusarium graminearum

The Ascomycete fungus Fusarium graminearum, the causal agent of Fusarium head blight of wheat and barley, has become a predominant model organism for the study of fungal phytopathogens. Aquaporins (AQPs) have been implicated in the transport of water, glycerol, and a variety of other small molecules in yeast, plants and animals. However, the role of these proteins in phytopathogenic fungi is not well understood. Here, we identified and attempted to elucidate the function of the five aquaporin genes in F. graminearum. The phylogenetic analysis revealed that FgAQPs are divided into two clades, with FgAQP1 in the first clade. The ∆AQP1 mutant formed whitish colonies with longer aerial hyphae and reduced conidiation and perithecium formation. The ∆AQP1 mutant conidia were morphologically abnormal and appeared to undergo abnormal germination. The ∆AQP1 mutant and the wild type strain were equally pathogenic, while the mutant produced significantly higher quantities of deoxynivalenol (DON). The ∆AQP1 mutant also exhibited increased resistance to osmotic and oxidative stress as well as cell-wall perturbing agents. Using FgAQP1-GFP and DAPI staining, we found that FgAQP1 is localized to the nuclear membrane in conidia. Importantly, deletion of FgAQP1 increased the severity of conidium autophagy. Taken together, these results suggest that FgAQP1 is involved in hyphal development, stress responses, secondary metabolism, and sexual and asexual reproduction in F. graminearum. Unlike the ∆AQP1 mutant, the ∆AQP2, ∆AQP3, ∆AQP4 and ∆AQP5 mutants had no variable phenotypes.

Functional relevance of water and glycerol channels in Saccharomyces cerevisiae

Our understanding of the functional relevance of orthodox aquaporins and aquaglyceroporins in Saccharomyces cerevisiae is essentially based on phenotypic variations obtained by expression/overexpression/deletion of these major intrinsic proteins in selected strains. These water/glycerol channels are considered crucial during various life-cycle phases, such as sporulation and mating and in some life processes such as rapid freeze-thaw tolerance, osmoregulation and phenomena associated with cell surface. Despite their putative functional roles not only as channels but also as sensors, their underlying mechanisms and their regulation are still poorly understood. In the present review, we summarize and discuss the physiological relevance of S. cerevisiae aquaporins (Aqy1 and Aqy2) and aquaglyceroporins (Fps1 and Yfl054c). In particular, the fact that most S. cerevisiae laboratory strains harbor genes coding for non-functional aquaporins, while wild and industrial strains possess at least one functional aquaporin, suggests that aquaporin activity is required for cell survival under more harsh conditions.

Temperature dependence of protein-water interactions in a gated yeast aquaporin

Regulation of aquaporins is a key process of living organisms to counteract sudden osmotic changes. Aqy1, which is a water transporting aquaporin of the yeast Pichia pastoris, is suggested to be gated by chemo-mechanical stimuli as a protective regulatory-response against rapid freezing. Here, we tested the influence of temperature by determining the X-ray structure of Aqy1 at room temperature (RT) at 1.3 Å resolution, and by exploring the structural dynamics of Aqy1 during freezing through molecular dynamics simulations. At ambient temperature and in a lipid bilayer, Aqy1 adopts a closed conformation that is globally better described by the RT than by the low-temperature (LT) crystal structure. Locally, for the blocking-residue Tyr31 and the water molecules inside the pore, both LT and RT data sets are consistent with the positions observed in the simulations at room-temperature. Moreover, as the temperature was lowered, Tyr31 adopted a conformation that more effectively blocked the channel, and its motion was accompanied by a temperature-driven rearrangement of the water molecules inside the channel. We therefore speculate that temperature drives Aqy1 from a loosely- to a tightly-blocked state. This analysis provides high-resolution structural evidence of the influence of temperature on membrane-transport channels.

The Saccharomyces cerevisiae Cdk8 Mediator Represses AQY1 Transcription by Inhibiting Set1p-Dependent Histone Methylation

In the budding yeast Saccharomyces cerevisiae, nutrient depletion induces massive transcriptional reprogramming that relies upon communication between transcription factors, post-translational histone modifications, and the RNA polymerase II holoenzyme complex. Histone H3Lys4 methylation (H3Lys4 me), regulated by the Set1p-containing COMPASS methyltransferase complex and Jhd2p demethylase, is one of the most well-studied histone modifications. We previously demonstrated that the RNA polymerase II mediator components cyclin C-Cdk8p inhibit locus-specific H3Lys4 3me independently of Jhd2p. Here, we identify loci subject to cyclin C- and Jhd2p-dependent histone H3Lys4 3me inhibition using chromatin immunoprecipitation (ChIP)-seq. We further characterized the independent and combined roles of cyclin C and Jhd2p in controlling H3Lys4 3me and transcription in response to fermentable and nonfermentable carbon at multiple loci. These experiments suggest that H3Lys4 3me alone is insufficient to induce transcription. Interestingly, we identified an unexpected role for cyclin C-Cdk8p in repressing AQY1 transcription, an aquaporin whose expression is normally induced during nutrient deprivation. These experiments, combined with previous work in other labs, support a two-step model in which cyclin C-Cdk8p mediate AQY1 transcriptional repression by stimulating transcription factor proteolysis and preventing Set1p recruitment to the AQY1 locus.

Road to the future of systems biotechnology: CRISPR-Cas-mediated metabolic engineering for recombinant protein production

The rising potential for CRISPR-Cas-mediated genome editing has revolutionized our strategies in basic and practical bioengineering research. It provides a predictable and precise method for genome modification in a robust and reproducible fashion. Emergence of systems biotechnology and synthetic biology approaches coupled with CRISPR-Cas technology could change the future of cell factories to possess some new features which have not been found naturally. We have discussed the possibility and versatile potentials of CRISPR-Cas technology for metabolic engineering of a recombinant host for heterologous protein production. We describe the mechanisms involved in this metabolic engineering approach and present the diverse features of its application in biotechnology and protein production.

Comparative transcriptome analysis revealing dormant conidia and germination associated genes in Aspergillus species: an essential role for AtfA in conidial dormancy

Background

Fungal conidia are usually dormant unless the extracellular conditions are right for germination. Despite the importance of dormancy, little is known about the molecular mechanism underlying entry to, maintenance of, and exit from dormancy. To gain comprehensive and inter-species insights, transcriptome analyses were conducted across Aspergillus fumigatus, Aspergillus niger, and Aspergillus oryzae.

Results

We found transcripts of 687, 694, and 812 genes were enriched in the resting conidia compared with hyphae in A. fumigatus, A. niger, and A. oryzae, respectively (conidia-associated genes). Similarly, transcripts of 766, 1,241, and 749 genes were increased in the 1 h-cultured conidia compared with the resting conidia (germination-associated genes). Among the three Aspergillus species, we identified orthologous 6,172 genes, 91 and 391 of which are common conidia- and germination-associated genes, respectively. A variety of stress-related genes, including the catalase genes, were found in the common conidia-associated gene set, and ribosome-related genes were significantly enriched among the germination-associated genes. Among the germination-associated genes, we found that calA-family genes encoding a thaumatin-like protein were extraordinary expressed in early germination stage in all Aspergillus species tested here. In A. fumigatus 63 % of the common conidia-associated genes were expressed in a bZIP-type transcriptional regulator AtfA-dependent manner, indicating that AtfA plays a pivotal role in the maintenance of resting conidial physiology. Unexpectedly, the precocious expression of the germination-associated calA and an abnormal metabolic activity were detected in the resting conidia of the atfA mutant, suggesting that AtfA was involved in the retention of conidial dormancy.

Conclusions

A comparison among transcriptomes of hyphae, resting conidia, and 1 h-grown conidia in the three Aspergillus species revealed likely common factors involved in conidial dormancy. AtfA positively regulates conidial stress-related genes and negatively mediates the gene expressions related to germination, suggesting a major role for AtfA in Aspergillus conidial dormancy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2689-z) contains supplementary material, which is available to authorized users.

The Role of Aquaporins in pH-Dependent Germination of Rhizopus delemar Spores

Rhizopus delemar and associated species attack a wide range of fruit and vegetables after harvest. Host nutrients and acidic pH are required for optimal germination of R. delemar, and we studied how this process is triggered. Glucose induced spore swelling in an acidic environment, expressed by an up to 3-fold increase in spore diameter, whereas spore diameter was smaller in a neutral environment. When suspended in an acidic environment, the spores started to float, indicating a change in their density. Treatment of the spores with HgCl2, an aquaporin blocker, prevented floating and inhibited spore swelling and germ-tube emergence, indicating the importance of water uptake at the early stages of germination. Two putative candidate aquaporin-encoding genes-RdAQP1 and RdAQP2-were identified in the R. delemar genome. Both presented the conserved NPA motif and six-transmembrane domain topology. Expressing RdAQP1 and RdAQP2 in Arabidopsis protoplasts increased the cells' osmotic water permeability coefficient (Pf) compared to controls, indicating their role as water channels. A decrease in R. delemar aquaporin activity with increasing external pH suggested pH regulation of these proteins. Substitution of two histidine (His) residues, positioned on two loops facing the outer side of the cell, with alanine eliminated the pH sensing resulting in similar Pf values under acidic and basic conditions. Since hydration is critical for spore switching from the resting to activate state, we suggest that pH regulation of the aquaporins can regulate the initial phase of R. delemar spore germination, followed by germ-tube elongation and host-tissue infection.

Aquaporin8 regulates cellular development and reactive oxygen species production, a critical component of virulence in Botrytis cinerea

Aquaporins (AQPs) are ubiquitous in nearly all organisms, mediating selective and rapid flux of water across biological membranes. The role of AQPs in phytopathogenic fungi is poorly understood. Orthologs of AQP genes in Botrytis cinerea were identified and knocked out. The effects of AQPs on hyphal growth and conidiation, formation of infection structures and virulence on plant hosts were examined. The role of AQP8 in reactive oxygen species (ROS) production, distribution and transport were further determined. Among eight AQPs, only AQP8 was essential for the ability of B. cinerea to infect plants. AQP8 was demonstrated to be an intrinsic plasma membrane protein, which may function as a channel and mediate hydrogen peroxide uptake. Deletion of AQP8 in B. cinerea completely inhibited the development of conidia and infection structures, and significantly affected noxR expression. Further observations revealed that both AQP8 and noxR impacted ROS distribution in the hyphal tips of B. cinerea. Moreover, AQP8 affected the expression of a mitochondrial protein, NQO1. A knockout mutant of NQO1 was observed to display reduced virulence. These data lead to a better understanding of the important role of AQP8 in the development and pathogenesis of plant pathogens.

Water Transport in Yeasts

Water moves across membranes through the lipid bilayer and through aquaporins, in this case in a regulated manner. Aquaporins belong to the MIP superfamily and two subfamilies are represented in yeasts: orthodox aquaporins considered to be specific water channels and aquaglyceroporins (heterodox aquaporins). In Saccharomyces cerevisiae genome, four aquaporin isoforms were identified, two of which are genetically close to orthodox aquaporins (ScAqy1 and ScAqy2) and the other two are more closely related to the aquaglyceroporins (ScFps1 and ScAqy3). Advances in the establishment of water channels structure are reviewed in this chapter in relation with the mechanisms of selectivity, conductance and gating. Aquaporins are important for key aspects of yeast physiology. They have been shown to be involved in sporulation, rapid freeze-thaw tolerance, osmo-sensitivity, and modulation of cell surface properties and colony morphology, although the underlying exact mechanisms are still unknown.

Can Stabilization and Inhibition of Aquaporins Contribute to Future Development of Biomimetic Membranes?

In recent years, the use of biomimetic membranes that incorporate membrane proteins, i.e., biomimetic-hybrid membranes, has increased almost exponentially. Key membrane proteins in these systems have been aquaporins, which selectively permeabilize cellular membranes to water. Aquaporins may be incorporated into synthetic lipid bilayers or to more stable structures made of block copolymers or solid-state nanopores. However, translocation of aquaporins to these alien environments has adverse consequences in terms of performance and stability. Aquaporins incorporated in biomimetic membranes for use in water purification and desalination should also withstand the harsh environment that may prevail in these conditions, such as high pressure, and presence of salt or other chemicals. In this respect, modified aquaporins that can be adapted to these new environments should be developed. Another challenge is that biomimetic membranes that incorporate high densities of aquaporin should be defect-free, and this can only be efficiently ascertained with the availability of completely inactive mutants that behave otherwise like the wild type aquaporin, or with effective non-toxic water channel inhibitors that are so far inexistent. In this review, we describe approaches that can potentially be used to overcome these challenges.

Diversity and adaptive evolution of Saccharomyces wine yeast: a review

Saccharomyces cerevisiae and related species, the main workhorses of wine fermentation, have been exposed to stressful conditions for millennia, potentially resulting in adaptive differentiation. As a result, wine yeasts have recently attracted considerable interest for studying the evolutionary effects of domestication. The widespread use of whole-genome sequencing during the last decade has provided new insights into the biodiversity, population structure, phylogeography and evolutionary history of wine yeasts. Comparisons between S. cerevisiae isolates from various origins have indicated that a variety of mechanisms, including heterozygosity, nucleotide and structural variations, introgressions, horizontal gene transfer and hybridization, contribute to the genetic and phenotypic diversity of S. cerevisiae. This review will summarize the current knowledge on the diversity and evolutionary history of wine yeasts, focusing on the domestication fingerprints identified in these strains.

This review summarizes current knowledge and recent advances on the diversity and evolutionary history of Saccharomyces cerevisiae wine yeasts, focusing on the domestication fingerprints identified in these strains.

A generic high-throughput assay to detect aquaporin functional mutants: Potential application to discovery of aquaporin inhibitors

BACKGROUND

The discovery of stable, yet functional, protein mutants is a limiting factor in the development of biotechnological applications, structural studies or in drug discovery. Rapid detection of functional mutants is especially challenging for water channel aquaporins, as they do not have a directly measurable enzymatic or binding activity. Current methods available are time consuming and only applicable to specific aquaporins.

METHODS

Herein we describe an assay based on the protective effect of aquaporins on yeast S. cerevisiae in response to rapid freezing.

RESULTS

Yeast overexpressing a functional water-permeable aquaporin of choice are rescued after the challenge, while inactive or blocked aquaporins confer no protection and lead to cell death. The potential of this assay is shown by screening a small number of E. coli aquaporin Z (AQPZ) mutants. Additionally, a library of ~10,000 drug-like compounds was tested against human AQP1 (hAQP1).

CONCLUSIONS

Since rescue is only dependent on transmembrane water flux, the assay is applicable to water-permeable aquaporins of any origin.

GENERAL SIGNIFICANCE

Mapping of permissive mutations on the aquaporin structure can help delineate the minimal requirements for effective water transport. Alternatively, the assay can be potentially used to discover compounds that inhibit aquaporin water transport. When additionally screened for thermostability, functional aquaporin mutants can be useful in the development of biomimetic membranes for water purification, or to improve the likelihood of producing well-diffracting crystals, enabling rational design of much needed aquaporin inhibitors.

Fungal aquaporins: cellular functions and ecophysiological perspectives

Three aspects have to be taken into consideration when discussing cellular water and solute permeability of fungal cells: cell wall properties, membrane permeability, and transport through proteinaceous pores (the main focus of this review). Yet, characterized major intrinsic proteins (MIPs) can be grouped into three functional categories: (mainly) water transporting aquaporins, aquaglyceroporins that confer preferentially solute permeability (e.g., glycerol and ammonia), and bifunctional aquaglyceroporins that can facilitate efficient water and solute transfer. Two ancestor proteins, a water (orthodox aquaporin) and a solute facilitator (aquaglyceroporin), are supposed to give rise to today's MIPs. Based on primary sequences of fungal MIPs, orthodox aquaporins/X-intrinsic proteins (XIPs) and FPS1-like/Yfl054-like/other aquaglyceroporins are supposed to be respective sister groups. However, at least within the fungal kingdom, no easy functional conclusion can be drawn from the phylogenetic position of a given protein within the MIP pedigree. In consequence, ecophysiological prediction of MIP relevance is not feasible without detailed functional analysis of the respective protein and expression studies. To illuminate the diverse MIP implications in fungal lifestyle, our current knowledge about protein function in two organisms, baker's yeast and the Basidiomycotic Laccaria bicolor, an ectomycorrhizal model fungus, was exemplarily summarized in this review. MIP function has been investigated in such a depth in Saccharomyces cerevisiae that a system-wide view is possible. Yeast lifestyle, however, is special in many circumstances. Therefore, L. bicolor as filamentous Basidiomycete was added and allows insight into a very different way of life. Special emphasis was laid in this review onto ecophysiological interpretation of MIP function.

Characterization of new polyol/H+ symporters in Debaryomyces hansenii

Debaryomyces hansenii is a halotolerant yeast that produces and assimilates a wide variety of polyols. In this work we evaluate polyol transport in D. hansenii CBS 767, detecting the occurrence of polyol/H⁺ (and sugar/H⁺) symporter activity, through the transient extracellular alkalinization of unbuffered starved cell suspensions. From the D. hansenii genome database, we selected nine ORFs encoding putative transporter proteins to clone in a centromeric plasmid with C-terminal GFP tagging and screened for polyol/H⁺ symporters by heterologous expression in Saccharomyces cerevisiae. Five distinct D. hansenii polyol/H⁺ symporters were identified and characterized, with different specificities and affinities for polyols, namely one glycerol-specific (DhStl1), one D-galactitol-specific (DhSgl1, Symporter galactitol/H⁺ 1), one D-(+)-chiro-inositol-specific (DhSyi1, Symporter D-(+)-chiro-inositol/H⁺ 1), one for D-sorbitol/D-mannitol/ribitol/D-arabitol/D-galactitol (DhSyl1, Symporter Polyols 1) and another for D-sorbitol/D-mannitol/ribitol/D-arabitol (DhSyl2, Symporter Polyols 2). This work contributed to the annotation of new yeast polyol transporters, including two specific for uncommon substrates as galactitol and D-(+)-chiro-inositol.

Transcriptome analysis of poplar rust telia reveals overwintering adaptation and tightly coordinated karyogamy and meiosis processes

Most rust fungi have a complex life cycle involving up to five different spore-producing stages. The telial stage that produces melanized overwintering teliospores is one of these and plays a fundamental role for generating genetic diversity as karyogamy and meiosis occur at that stage. Despite the importance of telia for the rust life cycle, almost nothing is known about the fungal genetic programs that are activated in this overwintering structure. In the present study, the transcriptome of telia produced by the poplar rust fungus Melampsora larici-populina has been investigated using whole genome exon oligoarrays and RT-qPCR. Comparative expression profiling at the telial and uredinial stages identifies genes specifically expressed or up-regulated in telia including osmotins/thaumatin-like proteins (TLPs) and aquaporins that may reflect specific adaptation to overwintering as well numerous lytic enzymes acting on plant cell wall, reflecting extensive cell wall remodeling at that stage. The temporal dynamics of karyogamy was followed using combined RT-qPCR and DAPI-staining approaches. This reveals that fusion of nuclei and induction of karyogamy-related genes occur simultaneously between the 25 and 39 days post inoculation time frame. Transcript profiling of conserved meiosis genes indicates a preferential induction right after karyogamy and corroborates that meiosis begins prior to overwintering and is interrupted in Meiosis I (prophase I, diplonema stage) until teliospore germination in early spring.

Yeast reveals unexpected roles and regulatory features of aquaporins and aquaglyceroporins

BACKGROUND

The yeast Saccharomyces cerevisiae provides unique opportunities to study roles and regulation of aqua/glyceroporins using frontline tools of genetics and genomics as well as molecular cell and systems biology.

SCOPE OF REVIEW

S. cerevisiae has two similar orthodox aquaporins. Based on phenotypes mediated by gene deletion or overexpression as well as on their expression pattern, the yeast aquaporins play important roles in key aspects of yeast biology: establishment of freeze tolerance, during spore formation as well as determination of cell surface properties for substrate adhesion and colony formation. Exactly how the aquaporins perform those roles and the mechanisms that regulate their function under such conditions remain to be elucidated. S. cerevisiae also has two different aquaglyceroporins. While the role of one of them, Yfl054c, remains to be determined, Fps1 plays critical roles in osmoregulation by controlling the accumulation of the osmolyte glycerol. Fps1 communicates with two osmo-sensing MAPK signalling pathways to perform its functions but the details of Fps1 regulation remain to be determined.

MAJOR CONCLUSIONS

Several phenotypes associated with aqua/glyceroporin function in yeasts have been established. However, how water and glycerol transport contribute to the observed effects is not understood in detail. Also many of the basic principles of regulation of yeast aqua/glyceroporins remain to be elucidated.

GENERAL SIGNIFICANCE

Studying the yeast aquaporins and aquaglyceroporins offers rich insight into the life style, evolution and adaptive responses of yeast and rewards us with discoveries of unexpected roles and regulatory mechanisms of members of this ancient protein family. This article is part of a Special Issue entitled Aquaporins.

Molecular dynamics simulation and bioinformatics study on yeast aquaporin Aqy1 from Pichia pastoris

In the present study, an equilibrated system for the Aqy1 tetramer was developed, and molecular biophysics modeling showed that the Aqy1 channel was blocked by Tyr-31 in the N-terminus, which was also supported by the free energy profiles. However, bioinformatics analysis of the amino acid sequence of Aqy1 indicated this Tyr-31 is not conserved across all fungi. Analysis of the equilibrated structure showed that the central pore along the four-fold axis of the tetramers is formed with hydrophobic amino acid residues. In particular, Phe-90, Trp-198, and Phe-202 form the narrowest part of the pore. Therefore, water molecules are not expected to translocate through the central pore, a hypothesis that we confirmed by molecular dynamics simulations. Each monomer of the Aqy1 tetramers forms a channel whose walls consist mostly of hydrophilic residues, transporting through the selectivity filter containing Arg-227, His-212, Phe-92, and Ala-221, and the two conserved Asn-Pro-Ala (NPA) motifs containing asparagines 224 and 112. In summary, not all fungal aquaporins share the same gating mechanism by a tyrosine residue in the N-terminus, and the structural analysis in the present study should aid our understanding of aquaporin structure and its functional implications.

Genome-wide analytical approaches for reverse metabolic engineering of industrially relevant phenotypes in yeast

Successful reverse engineering of mutants that have been obtained by nontargeted strain improvement has long presented a major challenge in yeast biotechnology. This paper reviews the use of genome-wide approaches for analysis of Saccharomyces cerevisiae strains originating from evolutionary engineering or random mutagenesis. On the basis of an evaluation of the strengths and weaknesses of different methods, we conclude that for the initial identification of relevant genetic changes, whole genome sequencing is superior to other analytical techniques, such as transcriptome, metabolome, proteome, or array-based genome analysis. Key advantages of this technique over gene expression analysis include the independency of genome sequences on experimental context and the possibility to directly and precisely reproduce the identified changes in naive strains. The predictive value of genome-wide analysis of strains with industrially relevant characteristics can be further improved by classical genetics or simultaneous analysis of strains derived from parallel, independent strain improvement lineages.

Aquaporins in yeasts and filamentous fungi

Recently, genome sequences from different fungi have become available. This information reveals that yeasts and filamentous fungi possess up to five aquaporins. Functional analyses have mainly been performed in budding yeast, Saccharomyces cerevisiae, which has two orthodox aquaporins and two aquaglyceroporins. Whereas Aqy1 is a spore-specific water channel, Aqy2 is only expressed in proliferating cells and controlled by osmotic signals. Fungal aquaglyceroporins often have long, poorly conserved terminal extensions and differ in the otherwise highly conserved NPA motifs, being NPX and NXA respectively. Three subgroups can be distinguished. Fps1-like proteins seem to be restricted to yeasts. Fps1, the osmogated glycerol export channel in S. cerevisiae, plays a central role in osmoregulation and determination of intracellular glycerol levels. Sequences important for gating have been identified within its termini. Another type of aquaglyceroporin, resembling S. cerevisiae Yfl054, has a long N-terminal extension and its physiological role is currently unknown. The third group of aquaglyceroporins, only found in filamentous fungi, have extensions of variable size. Taken together, yeasts and filamentous fungi are a fruitful resource to study the function, evolution, role and regulation of aquaporins, and the possibility to compare orthologous sequences from a large number of different organisms facilitates functional and structural studies.

Functional Annotation Analytics of Rhodopseudomonas palustris Genomes

Rhodopseudomonas palustris, a nonsulphur purple photosynthetic bacteria, has been extensively investigated for its metabolic versatility including ability to produce hydrogen gas from sunlight and biomass. The availability of the finished genome sequences of six R. palustris strains (BisA53, BisB18, BisB5, CGA009, HaA2 and TIE-1) combined with online bioinformatics software for integrated analysis presents new opportunities to determine the genomic basis of metabolic versatility and ecological lifestyles of the bacteria species. The purpose of this investigation was to compare the functional annotations available for multiple R. palustris genomes to identify annotations that can be further investigated for strain-specific or uniquely shared phenotypic characteristics. A total of 2,355 protein family Pfam domain annotations were clustered based on presence or absence in the six genomes. The clustering process identified groups of functional annotations including those that could be verified as strain-specific or uniquely shared phenotypes. For example, genes encoding water/glycerol transport were present in the genome sequences of strains CGA009 and BisB5, but absent in strains BisA53, BisB18, HaA2 and TIE-1. Protein structural homology modeling predicted that the two orthologous 240 aa R. palustris aquaporins have water-specific transport function. Based on observations in other microbes, the presence of aquaporin in R. palustris strains may improve freeze tolerance in natural conditions of rapid freezing such as nitrogen fixation at low temperatures where access to liquid water is a limiting factor for nitrogenase activation. In the case of adaptive loss of aquaporin genes, strains may be better adapted to survive in conditions of high-sugar content such as fermentation of biomass for biohydrogen production. Finally, web-based resources were developed to allow for interactive, user-defined selection of the relationship between protein family annotations and the R. palustris genomes.

Endo-Porter-mediated delivery of phosphorodiamidate morpholino oligos (PMOs) in erythrocyte suspension cultures from Cope's gray treefrog Hyla chrysoscelis

Cope's gray treefrog, Hyla chrysoscelis, is a freeze-tolerant anuran that accumulates cryoprotective glycerol during cold acclimation. H. chrysoscelis erythrocytes express the aquaglyceroporin HC-3, which facilitates transmembrane glycerol and water movement. Aquaglyceroporins have no pharmacological inhibitors, and no genetic knockout tools currently exist for H. chrysoscelis. A phosphorodiamidate morpholino oligo (PMO)-mediated expression knockdown approach was therefore pursued to provide a model for testing the role of HC-3. We describe a novel procedure optimized for specific, efficient knockdown of HC-3 expression in amphibian erythrocyte suspensions cultured at nonmammalian physiological temperatures using Endo-Porter. Our protocol includes three critical components: pre-incubation at 37°C, two rounds of Endo-Porter and HC-3 PMO administration at ~23°C, and continuous shaking at 190 rpm. This combination of steps resulted in 94% reduction in HC-3 protein expression (Western blot), substantial decrease in HC-3 expression in >65% of erythrocytes, and no detectable expression in an additional 30% of cells (immunocytochemistry).

Function and immuno-localization of aquaporins in the Antarctic midge Belgica antarctica

Aquaporin (AQP) water channel proteins play key roles in water movement across cell membranes. Extending previous reports of cryoprotective functions in insects, this study examines roles of AQPs in response to dehydration, rehydration, and freezing, and their distribution in specific tissues of the Antarctic midge, Belgica antarctica (Diptera, Chironomidae). When AQPs were blocked using mercuric chloride, tissue dehydration tolerance increased in response to hypertonic challenge, and susceptibility to overhydration decreased in a hypotonic solution. Blocking AQPs decreased the ability of tissues from the midgut and Malpighian tubules to tolerate freezing, but only minimal changes were noted in cellular viability of the fat body. Immuno-localization revealed that a DRIP-like protein (a Drosophila aquaporin), AQP2- and AQP3 (aquaglyceroporin)-like proteins were present in most larval tissues. DRIP- and AQP2-like proteins were also present in the gut of adult midges, but AQP4-like protein was not detectable in any tissues we examined. Western blotting indicated that larval AQP2-like protein levels were increased in response to dehydration, rehydration and freezing, whereas, in adults DRIP-, AQP2-, and AQP3-like proteins were elevated by dehydration. These results imply a vital role for aquaporin/aquaglyceroporins in water relations and freezing tolerance in B. antarctica.

Dynamic regulation of aquaglyceroporin expression in erythrocyte cultures from cold- and warm-acclimated Cope's gray treefrog, Hyla chrysoscelis

Cope's gray treefrog, Hyla chrysoscelis,is a freeze-tolerant anuran which accumulates and distributes glycerol as a cryoprotectant before freezing. We hypothesize that HC-3, an aquaglyceroporin member of the MIP family of water pores, may play an important role in the process of freeze tolerance by mediating transmembrane passage of glycerol and water during cold-acclimation. The objectives of this study were two-fold: to examine HC-3 protein abundance and cellular localization in erythrocytes from cold- and warm-acclimated frogs and to develop and characterize an erythrocyte cell culture system for examining HC-3 gene regulation. Compared with warm-acclimated frogs, erythrocytes from cold-acclimated frogs had higher HC-3 protein expression and enhanced plasma membrane localization. Furthermore, erythrocytes from cold- and warm-acclimated frogs maintained in culture at 4 and 20°C exhibited time- and temperature-dependent regulation of HC-3 expression and an increase in the abundance of high molecular weight immunoreactive species within 24 hr of culture at 20°C. Deglycosylation of erythrocyte proteins resulted in the disappearance of the high molecular weight species, indicating that HC-3 is post-translationally modified by N-linked glycosylation. Erythrocytes cultured in media containing glycerol also showed an increased abundance of the high molecular weight bands and enhanced plasma membrane localization of HC-3, suggesting a role for glycerol in regulating HC-3 subcellular trafficking. Thus, the development of this erythrocyte cell culture system from H. chrysoscelis opened an opportunity to study the properties of cells with changing expression of an aquaglyceroporin, HC-3, and to explore the factors regulating that expression.

The aquaporin gene family of the ectomycorrhizal fungus Laccaria bicolor: lessons for symbiotic functions

Soil humidity and bulk water transport are essential for nutrient mobilization. Ectomycorrhizal fungi, bridging soil and fine roots of woody plants, are capable of modulating both by being integrated into water movement driven by plant transpiration and the nocturnal hydraulic lift. Aquaporins are integral membrane proteins that function as gradient-driven water and/or solute channels. Seven aquaporins were identified in the genome of the ectomycorrhizal basidiomycete Laccaria bicolor and their role in fungal transfer processes was analyzed. Heterologous expression in Xenopus laevis oocytes revealed relevant water permeabilities for three aquaporins. In fungal mycelia, expression of the corresponding genes was high compared with other members of the gene family, indicating the significance of the respective proteins for plasma membrane water permeability. As growth temperature and ectomycorrhiza formation modified gene expression profiles of these water-conducting aquaporins, specific roles in those aspects of fungal physiology are suggested. Two aquaporins, which were highly expressed in ectomycorrhizas, conferred plasma membrane ammonia permeability in yeast. This indicates that these proteins are an integral part of ectomycorrhizal fungus-based plant nitrogen nutrition in symbiosis.

Design and characterization of genetically engineered zebrafish aquaporin-3 mutants highly permeable to the cryoprotectant ethylene glycol

Background

Increasing cell membrane permeability to water and cryoprotectants is critical for the successful cryopreservation of cells with large volumes. Artificial expression of water-selective aquaporins or aquaglyceroporins (GLPs), such as mammalian aquaporin-3 (AQP3), enhances cell permeability to water and cryoprotectants, but it is known that AQP3-mediated water and solute permeation is limited and pH dependent. To exploit further the possibilities of using aquaporins in cryobiology, we investigated the functional properties of zebrafish (Danio rerio) GLPs.

Results

Water, glycerol, propylene glycol and ethylene glycol permeability of zebrafish Aqp3a, -3b, -7, -9a, -9b, -10a and -10b, and human AQP3, was examined. Expression in Xenopus laevis oocytes indicated that the permeability of DrAqp3a and -3b to ethylene glycol was higher than for glycerol or propylene glycol under isotonic conditions, unlike other zebrafish GLPs and human AQP3, which were more permeable to glycerol. In addition, dose-response experiments and radiolabeled ethylene glycol uptake assays suggested that oocytes expressing DrAqp3b were permeated by this cryoprotectant more efficiently than those expressing AQP3. Water and ethylene glycol transport through DrAqp3a and -3b were, however, highest at pH 8.5 and completely abolished at pH 6.0. Point mutations in the DrAqp3b amino acid sequence rendered two constructs, DrAqp3b-T85A showing higher water and ethylene glycol permeability at neutral and alkaline pH, and DrAqp3b-H53A/G54H/T85A, no longer inhibited at acidic pH but less permeable than the wild type. Finally, calculation of permeability coefficients for ethylene glycol under concentration gradients confirmed that the two DrAqp3b mutants were more permeable than wild-type DrAqp3b and/or AQP3 at neutral pH, resulting in a 2.6- to 4-fold increase in the oocyte intracellular concentration of ethylene glycol.

Conclusion

By single or triple point mutations in the DrAqp3b amino acid sequence, we constructed one mutant with enhanced ethylene glycol permeability and another with reduced pH sensitivity. The DrAqp3b and the two mutant constructs may be useful for application in cryobiology.