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Glucose metabolism in S. mansoni, an overview
It was early realized that mammalian stages of S. mansoni absorb copious quantities of glucose. In fact, S. mansoni utilizes in 1 hour an amount of glucose equivalent to one-sixth to one-fifth of its dry weight. In addition, over 80% of the metabolized glucose is converted to lactic acid. The rates of glucose utilization and of lactic acid production by S. mansoni are the same under aerobic and anaerobic conditions. In consequence, S. mansoni parasites are known as homolactic fermenters (Bueding, 1950). Despite the possesion of a mouth and functional gut, glucose is taken up by the parasites across their tegument mainly through two glucose transporters (Skelly et al., 1998). Therefore, glucose transporters constitute the molecular basis of glucose uptake in S. mansoni. Likewise, it was proposed that the parasite’s tegument also constitutes a major route of lactic acid excretion (Faghiri et al., 2010).
Remarkably, while arobic glycolysis is a major metabolic pathway of mammalian stages of S. mansoni, the energetic metabolism of the free-living cercariae, is almost completly based on oxidative phosphorylation (OXPHOS). It was shown that early upon infection in the vertebrate host, S. mansoni schistosomula undergoe a metabolic switch toward glycolysis which is dependent on external glucose concentration and most likely occurs exclusively in presence of oxygen (Thompson et al., 1984; Horemans et al., 1992).
In this section, glucose transporters and the major pathways of glucose degradation and their enzymatic components will be described. These pathways include glycolysis, the tricarboxylic cycle and OXPHOS. The particularities of these processes in S. mansoni will also be presented. In addition, for their importance in the understanding of metabolic switches in eukaryotic cells, three concepts will be introduced: “Pasteur effect”, “Crabtree effect” and “Warburg effect”. We conclude that S. mansoni has an exceptional metabolic plasticity, which may have evolved as a response to their cumbersome and challenging life cycle.
Glucose transporters in S. mansoni
Few physiological parameters are more tightly and acutely regulated than blood glucose concentration. The transport of sugar across the cellular plasma membrane is performed by two families of transporters. Active transport occurs via symporters of the SLC5 gene family, while passive transport occurs via facilitative transporters of the SLC2 (GLUT) family (Vitavska and Wieczorek, 2013). A third family has been described and named the SLC45 gene family (Vitavska and Wieczorek, 2013).
From these three families, GLUT contains the main glucose transporters in human cells (Augustin, 2010). As shown in Figure 8, based on phylogenetic analysis three classes of GLUT can be distinguished: class I includes GLUT1-4 and GLUT14, class II includes GLUT5, 7, 9 and 11 and class III includes GLUT6, 8, 10, 12 and the proton driven myoinositol transporter GLUT13 (or HMIT) (Augustin, 2010). GLUT glucose transporters play specific and major roles in glucose homeostasis within the body. For instance, GLUT1 mediates glucose transport across the blood-brain barrier (Brockmann, 2009), the translocation of GLUT4 constitutes the rate limiting step in insulin controlled glucose uptake of skeletal and heart muscle as well as adipose tissue (Huang and Czech, 2007), and GLUT2 is a glucose sensor in insulin-secreting β-cells in the islets of Langerhans in the pancreas (Thorens, 2001).
At the molecular level, all 14 GLUT isoforms present the predicted 12 transmembrane helices (Augustin, 2010; Deng et al., 2014, 2015; Nomura et al., 2015). All family members harbor an N-linked glycosylation site. For class I and II family members, this is positioned in the first extracellular loop between transmembrane helices 1 and 2, whereas in class III family members the N-linked glycosylation site is located between transmembrane helices 9 and 10 (Augustin, 2010). Sequence comparisons between isoforms revealed conserved residues that have been termed sugar transporter signatures (Joost and Thorens, 2001). These residues participate in the substrate recognition by GLUT1 and other isoforms and include the QLS sequence in helix 7, the STSIF-motif in loop 7, tryptophan 388, and glutamine 161 positions in helix 5 (Augustin, 2010). These features are resumed in Figure 9.
Schistosoma mansoni lactate dehydrogenase
An LDH gene has been sequenced and characterized in S. mansoni (Guerra-Sá et al., 1998). The cDNA was isolated from a directional cDNA library constructed with mRNA of S. mansoni adult worms (Guerra-Sá et al., 1998). The deduced translation product of 333 amino acids has high homology (>65% identity) with other LDH family proteins. The majority of the residues responsible for the substrate, coenzyme binding and catalysis sites are well conserved in S. mansoni LDH, however several indels12 were observed when compared to other LDH enzymes, suggesting differences in activity (Guerra-Sá et al., 1998). In fact, measurements of the rate of reduction of pyruvate to lactate revealed differences in the effect of pH on the activity of the schistosome compared to mammalian LDH. The optimal activity of the worm enzyme was at pH 6.9 and that of the mammalian enzyme at pH 7.8 (Mansour and Bueding, 1953). Likewise, the optimal range for the oxidation of lactate to pyruvate was between pH 8.2 and 8.9 for the schistosome enzyme and between pH 9.0 and 9.5 for the mammalian enzyme (Mansour and Bueding, 1953). Dissociation constants and the optimal concentrations for lactate were identical for both enzymes. These two values were, however, six to twelve times higher for the worm enzymes in the presence of pyruvate (Mansour and Bueding, 1953). In addition, antibodies raised against rabbit LDH did not recognize or inhibit the activity of schistosome LDH (Mansour et al., 1954). The enzymatic activity of schistosoma LDH was also found to be different in susceptible vs. resistant strains of S. mansoni to hycanthone (Doong et al., 1987). Lactate dehydrogenase activity from the drug-resistant S. mansoni strains was not inhibited by hycanthone and showed three to five times greater Km values than those from the drug-sensitive worms which were also inhibitable by hycanthone (Doong et al., 1987).
Interestingly, transcripts encoding S. mansoni LDH were expressed in larval schistosomes at higher levels than in adult worms (Guerra-Sá et al., 1998), and cercariae possess an active LDH (Coles et al., 1973). However, upon infection of the vertebrate host, SGTP4 and LDH were found to be upregulated in schistosomula after 2 days and 3 days of infection (Dillon et al., 2006; Parker-Manuel et al., 2011), presumably reflecting an increased demand for glucose as well as the increase in lactate production observed in the mammalian stages of the parasite.
Table of contents :
CHAPTER I Introduction
Schistosoma.
Schistosomes, ancient and deadly parasites
The epidemiology and distribution of human schistosomiasis are dynamic processes.
Control of Schistosomiasis, the lack of alternative therapeutics
Taxonomy of S. mansoni
Phylogeny of Neodermata, the controversy continues.
Life cycle of S. mansoni
Glucose metabolism in Schistosoma mansoni
Glucose metabolism in S. mansoni, an overview
Glucose transporters in S. mansoni
Glycolysis in S. mansoni
Schistosoma mansoni lactate dehydrogenase
Mitochondrial metabolism in S. mansoni
Glucose energetic balance
The pioneers of the metabolic switch
Cancer cells and the metabolic shift
The aerobic GLY and the metabolic needs of proliferating cells
Epigenetic control of gene regulation in schistosomes.
Abstract
Introduction
Epigenetic mechanisms as drug targets
DNA methylation
DNA methylation in schistosomes
Micro-RNAs
Schistosome miRNAs
Post-translational modifications of histones
Schistosome histone modifications
Schistosome histone modifying enzymes: which are the best targets?
Development of selective inhibitors as drugs: the challenges
Conclusions
Schistosome sirtuins.
Abstract
Schistosomiasis: strategies for drug discovery
Why the boom in research on sirtuins?
Platyhelminthes present an evolutionarily conserved family of sirtuins
Sirtuin cellular functions
Sirtuin structure and catalysis
Sirtuins, cancer and schistosomes
Sirtuin inhibitors: scaffolds and strategies
Sirtuins as drug targets in parasites
Future perspectives
HYPOTHESIS AND OBJECTIVES
Hypothesis
Objectives
CHAPTER II Results
Schistosome glucose transporters.
Abstract
Glucose induces transcriptional changes in S. mansoni glucose transporter genes in schistosomula larvae
S. mansoni glucose transporters cluster separately and evolve at different rates
The rapidly-evolving glucose transporters SGTP2 and SGTP3 are encoded by intron-rich genes
S. mansoni glucose transporters show typical molecular signatures
The predicted tertiary structural conformations of S. mansoni glucose transporters are homologous to GLUT1 and XylE
Residue dynamics reveal how glucose affects invariant residues of glucose transporters involved in binding
Biophysical properties of S. mansoni glucose transporters provide insights into glucose affinity
Methods
Role of sirtuins in the mitochondrial metabolism of S. mansoni
Abstract
Introduction
Results
High glucose concentration induces a gene expression reprogramming associated with increase lactate production and mitochondrial activity.
High glucose activates mitochondrial activity after metabolic transformation
SmSirt1, but not SmSirt6, is upregulated upon metabolic transformation in schistosomula
SmSirt1 is a repressor of SPDK1
SmSirt1 is an activator of mitochondrial activity in S. mansoni
Discussion
Conclusions
Materials and methods
CHAPTER III General discussion
CONCLUSIONS
PERSPECTIVES
REFERENCES
ANNEXES
