Trypanosoma brucei Bloodstream Forms Depend upon Uptake of myo-Inositol for Golgi Complex Phosphatidylinositol Synthesis and Normal Cell Growth

The Role of Inorganic Phosphate Transporters in Highly Proliferative Cells: From Protozoan Parasites to Cancer Cells

In addition to their standard inorganic phosphate (Pi) nutritional function, Pi transporters have additional roles in several cells, including Pi sensing (the so-called transceptor) and a crucial role in Pi metabolism, where they control several phenotypes, such as virulence in pathogens and tumour aggressiveness in cancer cells. Thus, intracellular Pi concentration should be tightly regulated by the fine control of intake and storage in organelles. Pi transporters are classified into two groups: the Pi transporter (PiT) family, also known as the Pi:Na⁺ symporter family; and the Pi:H⁺ symporter (PHS) family. Highly proliferative cells, such as protozoan parasites and cancer cells, rely on aerobic glycolysis to support the rapid generation of biomass, which is equated with the well-known Warburg effect in cancer cells. In protozoan parasite cells, Pi transporters are strongly associated with cell proliferation, possibly through their action as intracellular Pi suppliers for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity. Similarly, the growth rate hypothesis (GRH) proposes that the high Pi demands of tumours when achieving accelerated proliferation are mainly due to increased allocation to P-rich nucleic acids. The purpose of this review was to highlight recent advances in understanding the role of Pi transporters in unicellular eukaryotes and tumorigenic cells, correlating these roles with metabolism in these cells.

The Inositol Pyrophosphate Biosynthetic Pathway of Trypanosoma cruzi

Inositol phosphates (IPs) are phosphorylated derivatives of myo-inositol involved in the regulation of several cellular processes through their interaction with specific proteins. Their synthesis relies on the activity of specific kinases that use ATP as phosphate donor. Here, we combined reverse genetics and liquid chromatography coupled to mass spectrometry (LC-MS) to dissect the inositol phosphate biosynthetic pathway and its metabolic intermediates in the main life cycle stages (epimastigotes, cell-derived trypomastigotes, and amastigotes) of Trypanosoma cruzi, the etiologic agent of Chagas disease. We found evidence of the presence of highly phosphorylated IPs, like inositol hexakisphosphate (IP6), inositol heptakisphosphate (IP7), and inositol octakisphosphate (IP8), that were not detected before by HPLC analyses of the products of radiolabeled exogenous inositol. The kinases involved in their synthesis (inositol polyphosphate multikinase (TcIPMK), inositol 5-phosphate kinase (TcIP5K), and inositol 6-phosphate kinase (TcIP6K)) were also identified. TcIPMK is dispensable in epimastigotes, important for the synthesis of polyphosphate, and critical for the virulence of the infective stages. TcIP5K is essential for normal epimastigote growth, while TcIP6K mutants displayed defects in epimastigote motility and growth. Our results demonstrate the relevance of highly phosphorylated IPs in the life cycle of T. cruzi.

Phosphatidylinositol synthesis, its selective salvage, and inter-regulation of anionic phospholipids in Toxoplasma gondii

Phosphatidylinositol (PtdIns) serves as an integral component of eukaryotic membranes; however, its biosynthesis in apicomplexan parasites remains poorly understood. Here we show that Toxoplasma gondii-a common intracellular pathogen of humans and animals-can import and co-utilize myo-inositol with the endogenous CDP-diacylglycerol to synthesize PtdIns. Equally, the parasite harbors a functional PtdIns synthase (PIS) containing a catalytically-vital CDP-diacylglycerol phosphotransferase motif in the Golgi apparatus. Auxin-induced depletion of PIS abrogated the lytic cycle of T. gondii in human cells due to defects in cell division, gliding motility, invasion, and egress. Isotope labeling of the PIS mutant in conjunction with lipidomics demonstrated de novo synthesis of specific PtdIns species, while revealing the salvage of other lipid species from the host cell. Not least, the mutant showed decline in phosphatidylthreonine, and elevation of selected phosphatidylserine and phosphatidylglycerol species, indicating a rerouting of CDP-diacylglycerol and homeostatic inter-regulation of anionic phospholipids upon knockdown of PIS. In conclusion, strategic allocation of own and host-derived PtdIns species to gratify its metabolic demand features as a notable adaptive trait of T. gondii. Conceivably, the dependence of T. gondii on de novo lipid synthesis and scavenging can be exploited to develop new anti-infectives.

ITPK1 mediates the lipid-independent synthesis of inositol phosphates controlled by metabolism

Inositol phosphates (IPs) are a class of signaling molecules regulating cell physiology. The best-characterized IP, the calcium release factor IP₃, is generated by phospholipase C hydrolysis of phosphoinositides lipids. For historical and technical reasons, IPs synthesis is believed to originate from the lipid-generated IP₃. While this is true in yeast, our work has demonstrated that other organisms use a “soluble” (nonlipid) route to synthesize IPs. This soluble pathway depends on the metabolic status of the cells, and is under the control of the kinase ITPK1, which phosphorylates inositol monophosphate likely generated from glucose. The data shed light on the evolutionary origin of IPs, signaling and tightening the link between these small molecules and basic metabolism.

Inositol phosphates (IPs) comprise a network of phosphorylated molecules that play multiple signaling roles in eukaryotes. IPs synthesis is believed to originate with IP₃ generated from PIP₂ by phospholipase C (PLC). Here, we report that in mammalian cells PLC-generated IPs are rapidly recycled to inositol, and uncover the enzymology behind an alternative “soluble” route to synthesis of IPs. Inositol tetrakisphosphate 1-kinase 1 (ITPK1)—found in Asgard archaea, social amoeba, plants, and animals—phosphorylates I(3)P₁ originating from glucose-6-phosphate, and I(1)P₁ generated from sphingolipids, to enable synthesis of IP₆. We also found using PAGE mass assay that metabolic blockage by phosphate starvation surprisingly increased IP₆ levels in a ITPK1-dependent manner, establishing a route to IP₆ controlled by cellular metabolic status, that is not detectable by traditional [³H]-inositol labeling. The presence of ITPK1 in archaeal clades thought to define eukaryogenesis indicates that IPs had functional roles before the appearance of the eukaryote.

Genome-wide identification and characterization of glucose transporter (glut) genes in spotted sea bass (Lateolabrax maculatus) and their regulated hepatic expression during short-term starvation

The glucose transporters (GLUTs) are well known for their essential roles in moving the key metabolites, glucose, galactose, fructose and a number of other important substrates in and out of cells. In this study, we identified a total of 21 glut genes in spotted sea bass (Lateolabrax maculatus) through extensive data mining of existing genomic and transcriptomic databases. Glut genes of spotted sea bass were classified into three subfamilies (Class I, Class II and Class III) according to the phylogenetic analysis. Glut genes of spotted sea bass were distributed in 15 out of 24 chromosomes. Deduced gene structure analysis including the secondary structure and the three-dimensional structures, as well as the syntenic analysis further supported their annotations and orthologies. Expression profile in healthy tissues indicated that 9 of 21 glut genes were expressed in liver of spotted sea bass. During short-term starvation, the mRNA expression levels of 3 glut genes (glut2, glut5, and glut10) were significantly up-regulated in liver (P < 0.05), indicating their potential roles in sugar transport and consumption. These findings in our study will facilitate the further evolutionary characterization of glut genes in fish species and provide a theoretical basis for their functional study.

Mitochondrial DNA is critical for longevity and metabolism of transmission stage Trypanosoma brucei

The sleeping sickness parasite Trypanosoma brucei has a complex life cycle, alternating between a mammalian host and the tsetse fly vector. A tightly controlled developmental programme ensures parasite transmission between hosts as well as survival within them and involves strict regulation of mitochondrial activities. In the glucose-rich bloodstream, the replicative 'slender' stage is thought to produce ATP exclusively via glycolysis and uses the mitochondrial F1FO-ATP synthase as an ATP hydrolysis-driven proton pump to generate the mitochondrial membrane potential (ΔΨm). The 'procyclic' stage in the glucose-poor tsetse midgut depends on mitochondrial catabolism of amino acids for energy production, which involves oxidative phosphorylation with ATP production via the F1FO-ATP synthase. Both modes of the F1FO enzyme critically depend on FO subunit a, which is encoded in the parasite's mitochondrial DNA (kinetoplast or kDNA). Comparatively little is known about mitochondrial function and the role of kDNA in non-replicative 'stumpy' bloodstream forms, a developmental stage essential for disease transmission. Here we show that the L262P mutation in the nuclear-encoded F1 subunit γ that permits survival of 'slender' bloodstream forms lacking kDNA ('akinetoplastic' forms), via FO-independent generation of ΔΨm, also permits their differentiation into stumpy forms. However, these akinetoplastic stumpy cells lack a ΔΨm and have a reduced lifespan in vitro and in mice, which significantly alters the within-host dynamics of the parasite. We further show that generation of ΔΨm in stumpy parasites and their ability to use α-ketoglutarate to sustain viability depend on F1-ATPase activity. Surprisingly, however, loss of ΔΨm does not reduce stumpy life span. We conclude that the L262P γ subunit mutation does not enable FO-independent generation of ΔΨm in stumpy cells, most likely as a consequence of mitochondrial ATP production in these cells. In addition, kDNA-encoded genes other than FO subunit a are important for stumpy form viability.

Do inositol supplements enhance phosphatidylinositol supply and thus support endoplasmic reticulum function?

This review attempts to explain why consuming extra myoinositol (Ins), an essential component of membrane phospholipids, is often beneficial for patients with conditions characterised by insulin resistance, non-alcoholic fatty liver disease and endoplasmic reticulum (ER) stress. For decades we assumed that most human diets provide an adequate Ins supply, but newer evidence suggests that increasing Ins intake ameliorates several disorders, including polycystic ovary syndrome, gestational diabetes, metabolic syndrome, poor sperm development and retinopathy of prematurity. Proposed explanations often suggest functional enhancement of minor facets of Ins Biology such as insulin signalling through putative inositol-containing 'mediators', but offer no explanation for this selectivity. It is more likely that eating extra Ins corrects a deficiency of an abundant Ins-containing cell constituent, probably phosphatidylinositol (PtdIns). Much of a cell's PtdIns is in ER membranes, and an increase in ER membrane synthesis, enhancing the ER's functional capacity, is often an important part of cell responses to ER stress. This review: (a) reinterprets historical information on Ins deficiency as describing a set of events involving a failure of cells adequately to adapt to ER stress; (b) proposes that in the conditions that respond to dietary Ins there is an overstretching of Ins reserves that limits the stressed ER's ability to make the 'extra' PtdIns needed for ER membrane expansion; and (c) suggests that eating Ins supplements increases the Ins supply to Ins-deficient and ER-stressed cells, allowing them to make more PtdIns and to expand the ER membrane system and sustain ER functions.

Squeezing for Life - Properties of Red Blood Cell Deformability

Deformability is an essential feature of blood cells (RBCs) that enables them to travel through even the smallest capillaries of the human body. Deformability is a function of (i) structural elements of cytoskeletal proteins, (ii) processes controlling intracellular ion and water handling and (iii) membrane surface-to-volume ratio. All these factors may be altered in various forms of hereditary hemolytic anemia, such as sickle cell disease, thalassemia, hereditary spherocytosis and hereditary xerocytosis. Although mutations are known as the primary causes of these congenital anemias, little is known about the resulting secondary processes that affect RBC deformability (such as secondary changes in RBC hydration, membrane protein phosphorylation, and RBC vesiculation). These secondary processes could, however, play an important role in the premature removal of the aberrant RBCs by the spleen. Altered RBC deformability could contribute to disease pathophysiology in various disorders of the RBC. Here we review the current knowledge on RBC deformability in different forms of hereditary hemolytic anemia and describe secondary mechanisms involved in RBC deformability.

Transporters of Trypanosoma brucei-phylogeny, physiology, pharmacology

Trypanosoma brucei comprise the causative agents of sleeping sickness, T. b. gambiense and T. b. rhodesiense, as well as the livestock-pathogenic T. b. brucei. The parasites are transmitted by the tsetse fly and occur exclusively in sub-Saharan Africa. T. brucei are not only lethal pathogens but have also become model organisms for molecular parasitology. We focus here on membrane transport proteins of T. brucei, their contribution to homeostasis and metabolism in the context of a parasitic lifestyle, and their pharmacological role as potential drug targets or routes of drug entry. Transporters and channels in the plasma membrane are attractive drug targets as they are accessible from the outside. Alternatively, they can be exploited to selectively deliver harmful substances into the trypanosome's interior. Both approaches require the targeted transporter to be essential: in the first case to kill the trypanosome, in the second case to prevent drug resistance due to loss of the transporter. By combining functional and phylogenetic analyses, we were mining the T. brucei predicted proteome for transporters of pharmacological significance. Here, we review recent progress in the identification of transporters of lipid precursors, amino acid permeases and ion channels in T. brucei.

Lipidomics and anti-trypanosomatid chemotherapy

BACKGROUND

Trypanosomatids such as Leishmania, Trypanosoma brucei and Trypanosoma cruzi belong to the order Kinetoplastida and are the source of many significant human and animal diseases. Current treatment is unsatisfactory and is compromised by the rising appearance of drug resistant parasites. Novel and more effective chemotherapeutics are urgently needed to treat and prevent these devastating diseases, which relies on the identification of essential, parasite specific targets that are absent in the host. Lipids constitute essential components of the cell and carry out multiple critical functions from building blocks of biological membranes to regulatory roles in signal transduction, organellar biogenesis, energy storage, and virulence. The recent technological advances of lipidomics has facilitated the broadening of our knowledge in the field of cellular lipid content, structure, functions, and metabolic pathways.

MAIN BODY

This review highlights the application of lipidomics (i) in the characterization of the lipidome of kinetoplastid parasites or of their subcellular structure(s), (ii) in the identification of unique lipid species or metabolic pathways that can be targeted for novel drug therapies, (iii) as an analytic tool to gain a deeper insight into the roles of specific enzymes in lipid metabolism using genetically modified microorganisms, and (iv) in deciphering the mechanism of action of anti-microbial drugs on lipid metabolism. Lastly, an outlook stating where the field is evolving is presented.

CONCLUSION

Lipidomics has contributed to the expanding knowledge related to lipid metabolism, mechanism of drug action and resistance, and pathogen-host interaction of trypanosomatids, which provides a solid basis for the development of better anti-parasitic pharmaceuticals.

H+-dependent inorganic phosphate uptake in Trypanosoma brucei is influenced by myo-inositol transporter

Trypanosoma brucei is an extracellular protozoan parasite that causes human African trypanosomiasis or "sleeping sickness". During the different phases of its life cycle, T. brucei depends on exogenous inorganic phosphate (P_i), but little is known about the transport of P_i in this organism. In the present study, we showed that the transport of ³²P_i across the plasma membrane follows Michaelis-Menten kinetics and is modulated by pH variation, with higher activity at acidic pH. Bloodstream forms presented lower P_i transport in comparison to procyclic forms, that displayed an apparent K_0.5 = 0.093 ± 0.008 mM. Additionally, FCCP (H⁺-ionophore), valinomycin (K⁺-ionophore) and SCH28080 (H⁺, K⁺-ATPase inhibitor) inhibited the P_i transport. Gene Tb11.02.3020, previously described to encode the parasite H⁺:myo-inositol transporter (TbHMIT), was hypothesized to be potentially involved in the H⁺:P_i cotransport because of its similarity with the Pho84 transporter described in S. cerevisiae and other trypanosomatids. Indeed, the RNAi mediated knockdown remarkably reduced TbHMIT gene expression, compromised cell growth and decreased P_i transport by half. In addition, P_i transport was inhibited when parasites were incubated in the presence of concentrations of myo-inositol that are above 300 μM. However, when expressed in Xenopus laevis oocytes, two-electrode voltage clamp experiments provided direct electrophysiological evidence that the protein encoded by TbHMIT is definitely a myo-inositol transporter that may be only marginally affected by the presence of P_i. These results confirmed the presence of a P_i carrier in T. brucei, similar to the H⁺-dependent inorganic phosphate system described in S. cerevisiae and other trypanosomatids. This transport system contributes to the acquisition of P_i and may be involved in the growth and survival of procyclic forms. In summary, this work presents the first description of a P_i transport system in T. brucei.

A biocatalytic approach towards the stereoselective synthesis of protected inositols

Immobilized TbINO1 produces >400 mg of chiral inositol 1-phosphate in a biocatalytic flow process.

Xenopus Oocytes: Optimized Methods for Microinjection, Removal of Follicular Cell Layers, and Fast Solution Changes in Electrophysiological Experiments

The Xenopus oocyte as a heterologous expression system for proteins, was first described by Gurdon et al.¹ and has been widely used since its discovery (References 2 - 3, and references therein). A characteristic that makes the oocyte attractive for foreign channel expression is the poor abundance of endogenous ion channels⁴. This expression system has proven useful for the characterization of many proteins, among them ligand-gated ion channels. The expression of GABAA receptors in Xenopus oocytes and their functional characterization is described here, including the isolation of oocytes, microinjections with cRNA, the removal of follicular cell layers, and fast solution changes in electrophysiological experiments. The procedures were optimized in this laboratory^5,6 and deviate from the ones routinely used^7-9. Traditionally, denuded oocytes are prepared with a prolonged collagenase treatment of ovary lobes at RT, and these denuded oocytes are microinjected with mRNA. Using the optimized methods, diverse membrane proteins have been expressed and studied with this system, such as recombinant GABAA receptors^10-12, human recombinant chloride channels¹³, Trypanosome potassium channels¹⁴, and a myo-inositol transporter^{15, 16}. The methods detailed here may be applied to the expression of any protein of choice in Xenopus oocytes, and the rapid solution change can be used to study other ligand-gated ion channels.

Functional Characterization of Pneumocystis carinii Inositol Transporter 1

Fungi in the genus Pneumocystis live in the lungs of mammals, where they can cause a fatal pneumonia (PCP [Pneumocystis pneumonia]) in hosts with compromised immune systems. The absence of a continuous in vitro culture system for any species of Pneumocystis has led to limited understanding of these fungi, especially for the discovery of new therapies. We recently reported that Pneumocystis carinii, Pneumocystis murina, and most significantly, Pneumocystis jirovecii lack both enzymes necessary for myo-inositol biosynthesis but contain genes with homologies to fungal myo-inositol transporters. Since myo-inositol is essential for eukaryotic viability, the primary transporter, ITR1, was functionally and structurally characterized in P. carinii The predicted structure of P. carinii ITR1 (PcITR1) contained 12 transmembrane alpha-helices with intracellular C and N termini, consistent with other inositol transporters. The apparent K_m was 0.94 ± 0.08 (mean ± standard deviation), suggesting that myo-inositol transport in P. carinii is likely through a low-affinity, highly selective transport system, as no other sugars or inositol stereoisomers were significant competitive inhibitors. Glucose transport was shown to use a different transport system. The myo-inositol transport was distinct from mammalian transporters, as it was not sodium dependent and was cytochalasin B resistant. Inositol transport in these fungi offers an attractive new drug target because of the reliance of the fungi on its transport, clear differences between the mammalian and fungal transporters, and the ability of the host to both synthesize and transport this critical nutrient, predicting low toxicity of potential inhibitors to the fungal transporter.

IMPORTANCE

myo-Inositol is a sugarlike nutrient that is essential for life in most organisms. Humans and microbes alike can obtain it by making it, which involves only 2 enzymes, by taking it from the environment by a transport process, or by recycling it from other cellular constituents. Inspection of the genomes of the pathogenic fungi of the genus Pneumocystis showed that these pneumonia-causing parasites could not make myo-inositol, as they lacked the 2 enzymes. Instead, we found evidence of inositol transporters, which would import the sugar from the lungs where the fungi reside. In the present report, we characterized the transport of myo-inositol in the fungus and found that the transporter was highly selective for myo-inositol and did not transport any other molecules. The transport was distinct from that in mammalian cells, and since mammals can both make and transport myo-inositol, while Pneumocystis fungi must transport it, this process offers a potential new drug target.

Lipid topogenesis--35years on

Glycerophospholipids are the principal fabric of cellular membranes. The pathways by which these lipids are synthesized were elucidated mainly through the work of Kennedy and colleagues in the late 1950s and early 1960s. Subsequently, attention turned to cell biological aspects of lipids: Where in the cell are lipids synthesized? How are lipids integrated into membranes to form a bilayer? How are they sorted and transported from their site of synthesis to other cellular destinations? These topics, collectively termed 'lipid topogenesis', were the subject of a review article in 1981 by Bell, Ballas and Coleman. We now assess what has been learned about early events of lipid topogenesis, i.e. "lipid synthesis, the integration of lipids into membranes, and lipid translocation across membranes", in the 35 years since the publication of this important review. We highlight the recent elucidation of the X-ray structures of key membrane enzymes of glycerophospholipid synthesis, progress on identifying lipid scramblase proteins needed to equilibrate lipids across membranes, and new complexities in the subcellular location and membrane topology of phosphatidylinositol synthesis revealed through a comparison of two unicellular model eukaryotes. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

The Glycerol-3-Phosphate Acyltransferase TbGAT is Dispensable for Viability and the Synthesis of Glycerolipids in Trypanosoma brucei

Glycerolipids are the main constituents of biological membranes in Trypanosoma brucei, which causes sleeping sickness in humans. Importantly, they occur as a structural component of the glycosylphosphatidylinositol lipid anchor of the abundant cell surface glycoproteins procyclin in procyclic forms and variant surface glycoprotein in bloodstream form, that play crucial roles for the development of the parasite in the insect vector and the mammalian host, respectively. The present work reports the characterization of the glycerol-3-phosphate acyltransferase TbGAT that initiates the biosynthesis of ester glycerolipids. TbGAT restored glycerol-3-phosphate acyltransferase activity when expressed in a Leishmania major deletion strain lacking this activity and exhibited preference for medium length, unsaturated fatty acyl-CoAs. TbGAT localized to the endoplasmic reticulum membrane with its N-terminal domain facing the cytosol. Despite that a TbGAT null mutant in T. brucei procyclic forms lacked glycerol-3-phosphate acyltransferase activity, it remained viable and exhibited similar growth rate as the wild type. TbGAT was dispensable for the biosynthesis of phosphatidylcholine, phosphatidylinositol, phosphatidylserine, and GPI-anchored protein procyclin. However, the null mutant exhibited a slight decrease in phosphatidylethanolamine biosynthesis that was compensated with a modest increase in production of ether phosphatidylcholine. Our data suggest that an alternative initial acyltransferase takes over TbGAT's function in its absence.