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Keywords:

  • Yeast;
  • Saccharomyces cerevisiae;
  • Glycerol metabolism;
  • Osmoregulation

Abstract

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Glycerol metabolism in non-stressed cells
  5. 3The role of glycerol during osmoregulation
  6. 4The intracellular glycerol content in response to osmotic stress
  7. 5Signal transduction
  8. 6Conclusions
  9. Acknowledgements
  10. References

Glycerol is the main compatible solute in Saccharomyces cerevisiae. It is accumulated intracellularly when cells are exposed to decreased extracellular water activity. In general, increased intracellular accumulation of a solute may be caused by enhanced production, restricted dissimilation, increased retention by the plasma membrane and increased uptake from the medium. In this review, we evaluate current knowledge concerning mechanisms leading to the accumulation of glycerol in osmotically stressed cells of S. cerevisiae at the molecular and metabolic levels. An overview of glycerol metabolism in S. cerevisiae is provided.


1Introduction

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Glycerol metabolism in non-stressed cells
  5. 3The role of glycerol during osmoregulation
  6. 4The intracellular glycerol content in response to osmotic stress
  7. 5Signal transduction
  8. 6Conclusions
  9. Acknowledgements
  10. References

Exposing yeast cells to a hypertonic medium leads to a massive initial efflux of cellular water into the medium. This process is driven by the different water activities in the cell and the environment. Osmoregulation is understood to be the cellular response directed at restoring and maintaining volume, turgor pressure and normal biological activities of the cell.

A general mechanism which microorganisms use to counteract the outflow of water molecules during growth in decreased external water activities is the enhanced intracellular accumulation of one or more specific solutes. These compatible solutes can be accumulated in high concentrations without appreciable enzyme inhibition or inactivation [1, 2]. Thus, cellular processes can continue in spite of low intracellular water activities. In general, only a few compounds are used as compatible solutes. They can be grouped as follows: (i) ions, (ii) amino acids and (iii) polyhydroxy compounds. Glycerol is the most prominent compatible solute in Saccharomyces cerevisiae as in many other yeasts [3–6].

Glycerol, due to its role in osmoregulation, its metabolic pathways for production and consumption, as well as its mechanisms for intracellular accumulation has attracted increasing attention during the last few years. The use of modern tools in yeast genetics and molecular biology has resulted in much new and interesting information concerning both response to osmotic stress and glycerol metabolism in yeasts [7, 8]. This review focuses specifically on the yeast S. cerevisiae and the way it accumulates glycerol during hyperosmotic stress. We will first provide an overview of glycerol metabolism in S. cerevisiae and its regulation under non-stress conditions. This is the basis for discussing yeast's glycerol metabolism in osmotically stressed cells.

2Glycerol metabolism in non-stressed cells

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Glycerol metabolism in non-stressed cells
  5. 3The role of glycerol during osmoregulation
  6. 4The intracellular glycerol content in response to osmotic stress
  7. 5Signal transduction
  8. 6Conclusions
  9. Acknowledgements
  10. References

Glycerol is involved in carbon metabolism of S. cerevisiae in different ways (Fig. 1). On the one hand, it can be utilized as a sole carbon source under aerobic conditions [9]. On the other hand, glycerol is a by-product when glucose or other easily fermentable sugars are converted to ethanol. The production of ethanol from glucose is a redox-neutral process. The role of NADH-consuming glycerol formation is thought to be that it maintains cytosolic redox balance, compensating for cellular reactions which produce NADH [10]. The production of glycerol seems to be absolutely essential for balancing redox potential in the absence of oxygen; a mutant defective in glycerol production is not able grow under anaerobic conditions [11]. In addition to its role in reoxidizing NADH, it has been proposed that glycerol formation is important for recycling inorganic phosphate used in glycolysis [12]. Furthermore, glycerol 3-phosphate, the intermediate in the glycerol formation pathway, is a key metabolite for synthesis of glyceride lipids [13].

image

Figure 1. Important pathways of glycerol metabolism in S. cerevisiae.

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2.1Utilization of glycerol

The glycerol catabolic pathway includes phosphorylation by a glycerol kinase and a following oxidation by a FAD-dependent glycerol 3-phosphate dehydrogenase (mtGPD) located on the outer surface of the mitochondrial inner membrane [14, 15]. The dihydroxyacetone phosphate formed enters the glycolytic pathway.

Synthesis of glycerol kinase and mtGPD is mainly controlled by glucose repression. In contrast to the results reported for Candida utilis[14], there is no marked induction of these enzymes by glycerol [16]. The structural genes for glycerol kinase (GUT1) and mtGPD (GUT2) have been cloned by functional complementation of the corresponding gut1 and gut2 mutants. Authors have confirmed the repression of glycerol utilizing enzymes by glucose at the transcriptional level [17, 18].

A few yeast species have an alternative pathway for dissimilating glycerol which involves a NAD dependent glycerol dehydrogenase and a dihydroxyacetone kinase [19, 20]. It has recently been shown that these enzymes are also found in S. cerevisiae[21]. However, this pathway seems to be of less importance, at least for the utilization of glycerol in this species, since mutants defective in the glycerol kinase and mtGPD (phosphorylative pathway) are not able to grow with glycerol as the sole carbon source [16].

2.2Production of glycerol

Glycerol is synthesized by reducing dihydroxyacetone phosphate to glycerol 3-phosphate which is catalyzed by a NAD dependent cytosolic G3P dehydrogenase (ctGPD), followed by dephosphorylation of glycerol 3-phosphate by a specific phosphatase (GPP) [14]. This pathway seems to be the only route for producing glycerol in S. cerevisiae. Mutants lacking ctGPD do not produce any glycerol [11]. Dihydroxyacetone phosphate, the substrate for the glycerol formation pathway, can be provided either by the glycolytic degradation of sugars or by the gluconeogenic pathway when non-fermentable carbon sources are used.

It has been shown that both enzymes directly involved in the production of glycerol are found in two homologous isoforms in S. cerevisiae. As far as ctGPD is concerned, the product of the GPD1 gene (Gpd1p) is the most relevant form [22–24]. Mutants defective in the GPD1 gene have been shown to exhibit low residual activity of ctGPD which is due to the other isoform (Gpd2p) encoded by the GPD2 gene [25]. With regard to the other enzyme, glycerol 3-phosphatase, two isoforms have been recently purified and subsequently linked to two highly conserved isogenes (GPP1 and GPP2) of previously unknown function [26]. The product of GPP1 is the predominant form of glycerol 3-phosphatase under non-stress conditions.

The role of glycerol forming enzymes and their regulation during hyperosmotic stress will be discussed in Section 4.1. Here, we want to look at other kinds of regulation concerning ctGPD and GPP. In fact, glycerol production is higher in glucose medium than in non-fermentable carbon sources [27]. However, the two glycerol forming enzymes seem to be differently regulated by glucose. Concerning the glycerol 3-phosphatase, a higher enzyme activity has been reported when glucose is present [14]. This regulation could match the higher glycerol production under this conditions. In contrast to glycerol 3-phosphatase, it has been shown that ctGPD is repressed by glucose [27–29]. This seems to be paradoxical. However, one has to take into account that mtGPD (Section 2.1) and the respiratory system [30]are derepressed in the absence of glucose. Hence, there might be significant competition for glycerol 3-phosphate between the glycerol 3-phosphatase which produces glycerol and the mtGPD which produces dihydroxyacetone phosphate and transports reducing equivalents to the respiratory chain. Thus, the higher activity of ctGPD under non-repressive conditions could reflect an increased activity of the so-called glycerol 3-phosphate/dihydroxyacetone phosphate shuttle which reoxidizes cytosolic NADH associated with the generation of ATP without concomitant formation of glycerol [31]. An alternative reason for increased activity of ctGPD during growth on non-repressing carbon sources might be that a distinct intracellular level of glycerol 3-phosphate has to be balanced for synthesis of glyceride lipids.

Glucose repression of ctGPD has been shown to act at the level of transcription of GPD1 but not of GPD2[23, 25, 29, 32]. No data are yet available showing whether the induction of GPP by glucose also occurs at the transcriptional level.

It has already been mentioned that glycerol formation plays an important role in maintaining cytosolic redox balance. In fact, conditions leading to depletion of cytosolic NAD seem to cause an increase in glycerol production [10]. Recently, it has been found that transcription of GPD2 but not of GPD1 is increased both in the absence of oxygen and after addition of bisulfite which causes depletion of NAD [11]. This result suggests a novel, oxygen independent mechanism for regulating cellular redox balance. Moreover, it has become clear that the two isogenes encoding ctGPD are regulated independently and have entirely different physiological roles (see also Section 4.1).

3The role of glycerol during osmoregulation

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Glycerol metabolism in non-stressed cells
  5. 3The role of glycerol during osmoregulation
  6. 4The intracellular glycerol content in response to osmotic stress
  7. 5Signal transduction
  8. 6Conclusions
  9. Acknowledgements
  10. References

The significance of polyols as compatible solutes in the osmoregulation of yeast is a well-known fact [2]. A growing amount of evidence indicates that the intracellular level of glycerol is adjusted to external water activities. The commonly used experimental osmolyte to cause hyperosmotic stress is sodium chloride (NaCl). It should, however, be mentioned that other solutes used to decrease extracellular water activity (e.g. saccharose) have similar effects on the content of glycerol within the cell [6]. The importance of glycerol as the major compatible solute in exponentially growing, salt stressed S. cerevisiae has been demonstrated e.g. by natural abundance 13C nuclear magnetic resonance (NMR) spectroscopy [3–5]. Reed et al. [3]calculated the actual osmotic volume of the cell and concluded that intracellularly accumulated glycerol could counterbalance up to 95% of the external osmotic pressure caused by the addition of NaCl. These authors have also shown that the intracellular level of glycerol increased parallel to the external concentration of sodium chloride. The role of glycerol as an osmoregulator in S. cerevisiae has also been demonstrated by Blomberg and Adler [33]. These authors could show that accumulated glycerol of salt conditioned cells can be removed by washing. The glycerol concentration retained in the cell was proportional to the salt concentration of the washing solution.

The role of glycerol as a compatible solute is less clear in osmotically stressed cells grown on non-repressing sugars or non-fermentable carbon sources. In fact, the intracellular glycerol concentration in media with ethanol [27]or galactose [34]is about 30% compared to cells grown on glucose. Furthermore, it has been shown that osmotically stressed cells growing on galactose strongly accumulate trehalose [34]. This is in contrast to cells growing on glucose. These findings suggest that trehalose partly assumes the role of glycerol as a compatible solute under metabolic conditions suboptimal for glycerol production. Furthermore, glycerol seems to lose its significance as a compatible solute when glucose growing salt stressed cells enter the stationary phase. It has been shown that the intracellular glycerol content of NaCl stressed cells greatly decreased at the end of the exponential growth phase [35]. Moreover, stationary-phase cells also had a higher content of trehalose [36].

Nevertheless, there is no question that glycerol is clearly involved in controlling intracellular water activity in osmotically stressed cells of S. cerevisiae. It is, however, more difficult to evaluate the role of glycerol both in surviving an osmotic upshock and during the actual adaptation process of cells to lower extracellular water activities. In fact, there cannot be significant protection from glycerol immediately after an osmotic shock, since osmotically non-stressed cells do not intracellularly accumulate any glycerol [33]. Latterich et al. [37]proposed that the yeast vacuole participates in an immediate osmoregulatory process permitting survival until osmoadaptive glycerol accumulation occurs. Glycerol accumulation seems to start immediately after exposing cells to hyperosmolar media [33, 29], but high levels of intracellular glycerol seem to be reached only after several hours of adaption [38, 39].

S. cerevisiae cells adapted to intermediate salt stress exhibit an increased capacity to survive a sudden exposure to high concentrations of NaCl [40]. However, this phenomenon does not seem to be exclusively based on glycerol accumulation. It has been shown that osmotically conditioned cells washed free of glycerol retain at least 50% of their capacity for colony formation on high salt concentration media [33, 41]. In summary, it seems that glycerol accumulation is not necessarily responsible for surviving osmotic stress, but is essential for restoring the normal biological activities of the cell during osmoregulation.

4The intracellular glycerol content in response to osmotic stress

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Glycerol metabolism in non-stressed cells
  5. 3The role of glycerol during osmoregulation
  6. 4The intracellular glycerol content in response to osmotic stress
  7. 5Signal transduction
  8. 6Conclusions
  9. Acknowledgements
  10. References

In general, an increase in the intracellular concentration of glycerol can be the result of (i) increased production, (ii) increased retention by the cytoplasmic membrane, (iii) decreased dissimilation or (iv) uptake of glycerol from the medium. Strategies for accumulating glycerol in response to osmotic stress differ greatly among yeasts [6]. Current knowledge concerning the regulation of osmotically induced glycerol accumulation in S. cerevisiae will be presented in this review.

4.1Enhanced production of glycerol

An important avenue for S. cerevisiae to accumulate glycerol during osmotic stress is an increase in its glycerol production rate. When cells were incubated in medium containing 0.7 M NaCl for 40 min, they produced about three times more glycerol compared to cells without osmotic upshock. The specific enzyme activity of ctGPD, the key enzyme in the formation of glycerol, was about six times higher after cells were conditioned in 0.7 M NaCl for 60 min [33]. Edgley and Brown [38]have even reported a 30-fold increase of ctGPD activity in cells adapted to 1.7 M NaCl. It has been shown that this shift in activity is due to de novo synthesis of this enzyme [27, 33]. The transcription of the GPD1 gene has been found to be strongly induced by increasing extracellular osmolarity [23, 25, 42, 43].

There is some discussion on how much increased ctGPD activity controls the production rate of glycerol. A relatively high flux control coefficient for ctGPD over the flux from glucose to glycerol (0.63) has been estimated by Blomberg and Adler [33]based on their observations in cells exposed to lower water activities. The theoretical basis for the flux control coefficient is an analysis of flux changes instigated by changes in the activity and/or amount of one enzyme. Although such a flux control coefficient is a good guideline to evaluate how much one enzyme can control the corresponding flux, it can easily differ from actual conditions if all the other components of the system are not kept constant. This is the case in osmotically stressed cells where many alterations in enzyme activities occur [44], including those which could instigate or increase the flux control of ctGPD over glycerol formation. Compelling evidence for the rate-limiting role ctGPD plays in the flux from glucose to glycerol has recently been provided by overexpressing the GPD1 gene in S. cerevisiae[45]. The rate of glycerol formation in YEPD medium was enhanced six- to sevenfold in a GPD1 multicopy transformant exhibiting a specific activity of ctGPD that was 20-fold higher than in the wild-type. This increase in glycerol formation rate could be found in both the presence and absence of oxygen. Furthermore, it was possible to increase the glycerol production of three different S. cerevisiae strains to the same extent by using the same approach (Nevoigt, unpublished data). These results would suggest that an increased activity of the ctGPD is, in principle, sufficient to increase glycerol production. In fact, induction of ctGPD is not the only enzyme activity that changes during osmotic stress. It has recently been shown that the amount of one of the two isoforms of glycerol 3-phosphatase also increases in lower water activities [26, 46]. An increase in glycerol formation requires an equimolar amount of cytoplasmic NADH. When cells are osmotically stressed, this requirement seems to be partially met by decreased reduction of acetaldehyde to ethanol on the one hand and an increased oxidation to acetate on the other [33]. The observed decrease in the synthesis of alcohol dehydrogenase [44, 46]as well as the increase of aldehyde dehydrogenase [47]in osmotically stressed cells could account for these alterations in flux. Several glycolytic enzymes have also been identified among enzymes whose activity changed after osmotic upshock. Glyceraldehyde 3-phosphate dehydrogenase and enolase activity have been shown to decrease in salt grown cells [46]. These activity changes in enzymes could reduce the lower part of glycolysis and re-route metabolic flux towards glycerol at the expense of ethanol production. Interestingly, genes encoding both a hexose transporter (HXT1) and a glucokinase (GLK1) have been identified as highly osmo-induced genes [43]. Enhancing the uptake of glucose during osmotic stress may also supply the increased flux to glycerol.

In contrast to glucose growing cells, the metabolic basis of osmotically induced glycerol production in cells growing on raffinose or ethanol as carbon sources is not yet clear because of the quite different fluxes of carbon catabolism leading to dihydroxyacetone phosphate as the precursor for glycerol production. In fact, glycerol production of cells stressed by NaCl growing on ethanol was only 10% of that in glucose medium, in spite of a comparably strong induction of ctGPD activity in both media [27].

4.2Glycerol dissimilation

Enzymes involved in glycerol dissimilation have also been investigated with respect to osmoregulation. Albertyn et al. [29]reported a twofold reduction of glycerol kinase in cells growing in decreased water activity. This could prevent reutilization of glycerol, thereby facilitating glycerol accumulation during osmotic stress. No significant change in mitochondrial glycerol 3-phosphate dehydrogenase was found in that study.

The enzymes of the other pathway for glycerol dissimilation in yeasts, the dihydroxyacetone pathway (see Section 2.1), have been clearly shown to be induced by osmotic stress in the yeast Zygosaccharomyces rouxii[48]and to some extent also in the yeast Debaryomyces hansenii[49]. Recently, an osmotic induction of the dihydroxyacetone pathway was also been found in S. cerevisiae suggesting that glycerol is metabolized via this pathway during osmotic stress [21]. It has been proposed that the dihydroxyacetone pathway is involved in fine regulation of intracellular glycerol content.

4.3Intracellular retention

It has already been mentioned that glycerol formed by S. cerevisiae under non-stress conditions is quantitatively released to the environment. Osmotically stressed cells have also been shown to lose considerable amounts of overproduced glycerol to the medium [38]. Nevertheless, there are a few findings which indicate that glycerol efflux is regulated by the plasma membrane to some extent in S. cerevisiae cells which are osmotically stressed. One of these observations is that cycloheximide treated cells which have no increase in glycerol production rate start to retain glycerol under osmotic stress [33]. No intracellular accumulation of other compounds such as ethanol or acetate could be detected. Other evidence for the ability of S. cerevisiae to retain glycerol within the cell during growth in hyperosmolar media was provided by Larsson et al. [22]. A mutant strain defective in GPD1 (osg1) is able to accumulate glycerol to levels similar to those of the wild-type in spite of its considerably lower glycerol production rate. In fact, the permeation of glycerol through lipid bilayers is markedly influenced by the fatty acid composition of phospholipids [50]. Thunblad-Johansson and Adler [51]therefore studied the effects of extracellular NaCl on the fatty acid composition of different yeasts including S. cerevisiae. A slight decrease in the proportion of C16 acids accompanied by a corresponding increase in the C18 acids has been found in osmotically stressed cells. These changes should cause a decrease in membrane permeability to polyols [50]. New results, however, suggest that passive diffusion is not the sole mechanism by which glycerol passes the plasma membrane of S. cerevisiae, but there is also a facilitated diffusion which is mediated by a membrane spanning protein [8, 52]. Membrane channels which selectively retain intracellularly produced glycerol during osmoregulation could be a further explanation for why glycerol is released in the absence of osmotic stress, but accumulated when cells are subjected to dehydration. Van Aelst et al. [53]isolated a gene of S. cerevisiae (FPS1). The Fps1p protein sequence deduced is very similar to that of a glycerol facilitator of Escherichia coli. There are a few findings suggesting that the S. cerevisiae protein Fps1p is involved in controlling glycerol accumulation during osmotic stress [52]. Mutants lacking Fps1p accumulate more glycerol intracellularly than the wild-type. When such mutant cells are released from saline medium into water, they lose their intracellularly accumulated glycerol considerably more slowly than wild-type cells. This result has been attributed to the fact that the Fps1p channel, which should be closed under osmotic stress, is absent in the mutant. However, facilitated diffusion of glycerol by Fps1p does not seem to be important for utilization of glycerol under non-stress conditions; growth of S. cerevisiae on glycerol as a sole carbon source was not impaired by deleting FPS1.

4.4Transport of extracellular glycerol into the cell

It has been reported that the yeast Z. rouxii actively accumulates glycerol from the environment during osmotic stress by inducing a specific transport system for glycerol [54]. There were no data suggesting that there is an active transport of glycerol into the cell in S. cerevisiae[6]. Proceeding from their recent findings with S. cerevisiae mutant strains defective in glycerol production, Luyten et al. [52], however, suggested that an uptake system for glycerol inducible by osmotic stress might possibly also exist in S. cerevisiae.

5Signal transduction

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Glycerol metabolism in non-stressed cells
  5. 3The role of glycerol during osmoregulation
  6. 4The intracellular glycerol content in response to osmotic stress
  7. 5Signal transduction
  8. 6Conclusions
  9. Acknowledgements
  10. References

This section focuses on questions such as how yeast cells detect alterations in extracellular water activity and how the signal is converted to induce glycerol accumulation. When extracellular osmolarity increases, the initial signal for the cell could be an alteration in turgor pressure or cell volume due to the water efflux. With regard to osmosensors on the cell surface, it has been suggested that two distinct transmembrane proteins participate in sensing osmotic stress: Sln1p and Sho1p [55–58]. It has been shown that both of these interact with a specific osmoresponsive signal transduction pathway, the HOG pathway (Fig. 2). This pathway has been found in S. cerevisiae by studying osmoregulation defective mutants [59]. In this context, two genes have been isolated which encode Hog1p, a mitogen activated protein kinase (MAP kinase), and Pbs2p, a MAP kinase kinase that phosphorylates Hog1p in an osmostress dependent manner. Maximal phosphorylation of Hog1p was reached within 1 min after the increase of the external osmolarity. The downstream elements of the pathway which interact between the Hog1p and promoter of target genes are still unknown. In fact, the osmo-induced transcription of a few target genes of the HOG pathway is mediated by specific promoter elements, the so-called stress responsive elements (STREs) [60]. The HOG pathway is clearly involved in osmostress induced glycerol synthesis: hog1 mutants failed to increase the GPD1 mRNA level in response to osmotic stress [23]. In this context, it should be mentioned that four STREs have been identified within the GPD1 promoter [61]. Besides the increased level of Gpd1p, we have discussed other prerequisites for osmo-induced glycerol accumulation in this review. The following proteins have also been shown to be at least partly regulated by the HOG pathway: Gpp2p (an isoform of glycerol 3-phosphatase), Hxt1p (a hexose transporter), Glk1p (a glucokinase), Ald6p (an isoform of aldehyde dehydrogenase) and Dak1p (a putative dihydroxyacetone kinase) [26, 43, 62]. However, the Fps1p channel, which has been proposed to be responsible for glycerol retention by the plasma membrane during osmotic stress, does not seem to be controlled by the HOG pathway [52]. In fact, hog1 mutants are able to increase their intracellular glycerol content during osmotic stress up to 40% of the wild-type level [23]. Fps1p might respond directly to alterations in membrane tension. In fact, there are such membrane channels in S. cerevisiae[63]. However, it has been reported that these channels transmit cations and anions and may permit ion fluxes when cells shrink after reducing extracellular water activity.

image

Figure 2. A model of the high osmolarity signal transduction pathway in S. cerevisiae[55–58]. There are at least two independent transmembrane osmosensors, Sln1p-Ypd1p-Ssk1p and Sho1p. The Sln1p-Ypd1p-Ssk1p has similarity to bacterial ‘two component’ signal transducers and uses a multistep phosphorelay mechanism to regulate the redundant MAPKKKs Ssk2p and Ssk22p. Activated Ssk2p or Ssk22p activates the Pbs2p/Hog1p kinase cascade by phosphorylation of Pbs2p. The second known osmosensor, Sho1p, seems to activate the Pbs2p/Hog1p kinase cascade at high osmolarity through the Ste11p MAPKKK.

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Among signal transduction pathways involved in the osmostress response of S. cerevisiae, the HOG pathway is currently the best understood. As it shares homology with known MAP kinase cascades in other eukaryotes, it has been the subject of much research [64, 65]. In fact, there is evidence for a Pbs2p/Hog1p independent osmosignalling pathway. Mutants lacking Pbs2p show a remaining significant induction of HOG target genes, e.g. GPD1[62]. There are a variety of different signal transduction pathways thought to be involved in the osmostress response of S. cerevisiae, e.g. the RAS-cAMP pathway, Ca2+/calmodulin/calcineurin and the PKC pathway [66]. The PKC pathway has been shown to respond to hypo-osmotic but not to hyper-osmotic stress [67]. Further research is necessary to understand the complex interplay of the different signal transduction pathways involved in glycerol accumulation.

6Conclusions

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Glycerol metabolism in non-stressed cells
  5. 3The role of glycerol during osmoregulation
  6. 4The intracellular glycerol content in response to osmotic stress
  7. 5Signal transduction
  8. 6Conclusions
  9. Acknowledgements
  10. References

When exposed to highly osmolar media, S. cerevisiae cells respond by rapid intracellular accumulation of glycerol to counteract their dehydration. Moreover, a finely tuned regulation of intracellular glycerol content seems to be present. It has been shown that the accumulation of glycerol is mainly the result of an enhanced rate of production and increased retention of glycerol by the plasma membrane. Enhanced uptake of glycerol from the medium during osmotic stress has not yet been demonstrated in S. cerevisiae. Most attention has been paid to the markedly enhanced production rate of glycerol during osmotic stress and its molecular basis. In fact, the key enzyme of glycerol formation, the cytosolic NAD dependent glycerol 3-phosphate dehydrogenase (ctGPD), is strongly induced at the transcriptional level during osmotic stress via a specific osmoresponsive signal transduction pathway (HOG pathway). Furthermore, there is evidence that an increase simply in the ctGPD level can provoke a marked increase in the production of glycerol, at least when cells are grown on glucose as a carbon source. Besides the induction of ctGPD, further activity alterations of enzymes involved in glycerol metabolism and carbon catabolism have been reported, which were, however, less dramatic. These may reflect both a general adjustment of yeast metabolism to increased glycerol production and a fine regulation of intracellular glycerol content during growth under hyperosmotic stress.

References

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Glycerol metabolism in non-stressed cells
  5. 3The role of glycerol during osmoregulation
  6. 4The intracellular glycerol content in response to osmotic stress
  7. 5Signal transduction
  8. 6Conclusions
  9. Acknowledgements
  10. References
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