Leishmania are parasitic protozoa which infect humans and cause severe morbidity and mortality. Leishmania parasitise as extracellular promastigotes in the insect vector and as intracellular amastigotes in the mammalian host. Cycling between hosts involves implementation of stringent and co-ordinated responses to shifting environmental conditions. One of the key dynamic aspects of Leishmania biology is substrate acquisition and metabolism. Genomic analyses have revealed that Leishmania encode many putative membrane transporters, many of which are differentially expressed during the parasite life cycle. Only a small fraction of these transporters, however, have been functionally characterised. Currently, most information is available about nutrient transporters, mainly involved in carbohydrate, amino acid, nucleobase and nucleoside, cofactor, and ion acquisition. Several have apparent roles in Leishmania virulence and will be discussed in this perspective.

Introduction

Leishmaniasis constitutes an assembly of symptomatically diverse neglected infectious diseases in humans and animals that are caused by ∼20 species of genus Leishmania [1]. In addition to the number of unique morphological and genetic features that are characteristic of kinetoplastid parasites [27], Leishmania possess a highly adaptable metabolic machinery which enables the parasites to establish infection and thrive, despite the heterogeneous environmental challenges they encounter during development [812]. Metabolic adaptations that are important for virulence encompass energy metabolism, osmoregulation, proteostasis, redox equilibrium, and morphology. Membrane transporters are responsible for scavenging substrates for these pathways and are also implicated in environmental sensing, roles that are critical for virulence. Understanding the role of membrane transporters in Leishmania biology may reveal potential targets for control of leishmaniasis.

The Leishmania transportome

Transporters mediate the movement of solutes across membranes, conferring selective permeability on the lipid barriers that define the cell and organelles. Membrane transporters facilitate the acquisition of essential small molecules such as nutrients, delivering them to metabolic enzymes, and moving products within and out of the cell. In single-celled pathogens like Leishmania, transporters are responsible for scavenging nutrients from the host milieu to support parasite growth and metabolism and, consequently, some Leishmania transporters are virulence factors. Transport proteins have hydrophilic domains exposed at both faces of the membrane, linked by multiple hydrophobic domains that span the lipid bilayer. This characteristic structure can be recognised by hydropathy analysis of genome datasets, suggesting that Leishmania encode some 270 genes for potential transporters or ion channels (membranetransport.org). A small minority of these have been experimentally characterised, including transporters involved in uptake of key nutrients such as sugars, amino acids, folates, heme, polyamines, and purines and pyrimidines (Table 1). The majority of putative transporters remain to be studied and have unknown substrate specificities, subcellular localisations, and biological importance.

Table 1
Characterised nutrient transporters in Leishmania
Transporter References 
Amino acid metabolism 
 Arginine transporter, AAP3 [13
 Lysine transporter, AAP7 [14
 Proline/alanine transporter, AAP24 [15
Carbohydrate metabolism 
 Hexose transporters, GT1, GT2, GT3 [16
 Golgi GDP-mannose transporter, LPG2 [17
 Golgi UDP-galactose transporter, LPG5 [17
 Sucrose/H+ symporter [18
myo-inositol/H+ symporter, MIT [19
Nucleotide metabolism 
 Purine nucleoside/nucleobase transporters,   
 NT1, NT2, NT3, NT4, T1, and T2 [2026
 Pyrimidine transporter, U1 [27
Cofactor metabolism 
 Biopterin transporter, BT1 [28
 Folate transporter, FT1 [29
 Folate transporter, FT5 [30
 Heme transporter, LHR1 [31
 Iron transporter, LIT1 [32
 Mitochondrial iron transporter, MIT1 [33
Polyamine metabolism 
 Polyamine transporter, POT1 [34
Ion metabolism 
 Ca2+-transporting ATPase, Lmaa1 [35
Transporter References 
Amino acid metabolism 
 Arginine transporter, AAP3 [13
 Lysine transporter, AAP7 [14
 Proline/alanine transporter, AAP24 [15
Carbohydrate metabolism 
 Hexose transporters, GT1, GT2, GT3 [16
 Golgi GDP-mannose transporter, LPG2 [17
 Golgi UDP-galactose transporter, LPG5 [17
 Sucrose/H+ symporter [18
myo-inositol/H+ symporter, MIT [19
Nucleotide metabolism 
 Purine nucleoside/nucleobase transporters,   
 NT1, NT2, NT3, NT4, T1, and T2 [2026
 Pyrimidine transporter, U1 [27
Cofactor metabolism 
 Biopterin transporter, BT1 [28
 Folate transporter, FT1 [29
 Folate transporter, FT5 [30
 Heme transporter, LHR1 [31
 Iron transporter, LIT1 [32
 Mitochondrial iron transporter, MIT1 [33
Polyamine metabolism 
 Polyamine transporter, POT1 [34
Ion metabolism 
 Ca2+-transporting ATPase, Lmaa1 [35

Transport activity and parasite phenotype

Leishmania have a digenetic life cycle. Promastigotes are motile forms that develop in the alimentary tract of the sandfly vector, while amastigotes are non-motile, intracellular parasites of mammalian reticuloendothelial cells. These forms encounter contrasting habitats, not least in terms of nutrient availability, and an ability to obtain essential nutrients from the host milieu is critical for their survival. The nutrient content in the parasite niches depends on the insect diet or the phagocytic or endocytic activity of the macrophage. Leishmania monitor nutrient levels both intra- and extracellularly [3638] and elaborate transport systems with distinct kinetic and molecular properties, to operate optimally under the physiological conditions they encounter [8,39]. In vitro studies have shown that procyclic promastigotes rely on glucose and amino acid catabolism, while later stages in promastigote development increase amino acid utilisation [40]. To utilise all available carbohydrates, the differentiating promastigotes secret a variety of glycosidases and disaccharidases to predigest the more complex sugars in the fly gut contents [4143] to monosaccharides that are substrates for parasite transporters [16,44].

Upon phagocytosis by the macrophage host, metacyclic promastigotes differentiate into amastigotes [45], a morphological transformation that is accompanied by profound transcriptomic changes [40]. Nutrient availability in the Leishmania parasitophorous vacuole is poorly understood but, in contrast with the situation in the gut of the primarily phytophagous sandfly, free sugars are probably scarce [8]. Amino acids, amino sugars, and fatty acids seem to be important carbon sources, but amastigotes are difficult to obtain in sufficient quantities for transport studies and so their transport capacity is less understood compared with promastigotes.

Carbohydrate transport in the Leishmania life cycle

Leishmania hexose transport has been studied in relative detail and initial biochemical characterisation of transport activity has been augmented by molecular approaches. This discussion will focus on hexose transport in Leishmania mexicana, drawing on observations from other species and other transport systems where relevant.

The three permeases of L. mexicana designated GT1, GT2, and GT3 transport hexoses [16,44], ribose [46], and possibly amino sugars [47]. The three transporters are closely related, but they each display distinct functional and biological properties, including differential localisation, substrate specificity, and developmental stage expression [16,37,44,46,48]. GT1 has lower affinity towards hexoses compared with GT2 and GT3 [16], and as a flagellar membrane protein is proposed to act as a glucose sensor and signal transducer [37]. In glucose-sparse conditions, GT1 promotes transition from the growth to stationary phase [37] and thus is critical for promastigote adaptation and survival. The GT1 transporter, however, is not required for metacyclogenesis or infectivity to mammals [37]. GT2 and GT3 are high-affinity hexose permeases that are localised on the plasma membrane [16] (Figure 1). GT2 appears to be functionally the most active among the three transporters in promastigotes, which exhibit significantly higher glucose uptake capacity than amastigotes [39], while GT3 is expressed at a higher level in the amastigote stage [49] where it was shown to be essential [16]. GT null mutants (ΔLmGT) [16] are able to propagate continuously in cultures, albeit at a reduced rate [16], indicating that hexose transport is not essential for promastigotes in culture media with multiple carbon sources. Nevertheless, reduced virulence in sandflies and considerable reorganisation of cellular structure and metabolism observed in the ΔLmGT promastigotes [16,50,51] suggest that a lesion in hexose transport may have detrimental consequences in the insect vector [16]. In amastigotes, hexoses preferably serve as precursors for the synthesis of key anabolic intermediates [52]. Perturbation of hexose utilisation in the ΔLmGT amastigotes correlates with growth cessation and diminished ability to infect macrophages [16]. The non-infective phenotype of the ΔLmGT amastigotes may suggest that acquisition of sugars or amino sugars [47] from the parasitophorous vacuole is critical for Leishmania's survival and virulence, or that these transporters have other important, possibly intracellular, roles.

Characterised membrane transporters and their potential links to virulence in Leishmania.

Figure 1.
Characterised membrane transporters and their potential links to virulence in Leishmania.

1, flagellum; 2, flagellar pocket; 3, kinetoplast; 4, mitochondrion; 5, acidocalcisomes; 6, glycosomes; 7, multivesicular bodies; 8, endoplasmic reticulum; 9, nucleus; 10, β-mannan; 11, multivesicular tubule lysosome; 12, Golgi apparatus; 13, autophagosomes; 14, plasma membrane; 15, megasome. Abbreviations: AAP, amino acid permease; AAP3, arginine transporter; AAP7, lysine transporter; aPPG, amastigote PPG; Ca2+, calcium ion; Cd2+, cadmium ion; Co2+, cobalt ion; Fe3+, ferric ion; Fe2+, ferrous ion; fPPG, filamentous PPG; Gal, galactose; GDP, guanosine diphosphate; GILP, glycoinositolphospholipid; GP63, leishmanolysin; GT, glucose transporter; H+, hydrogen ion; K+, potassium ion; LIT1, Leishmania iron transporter; LFR1, Leishmania ferric reductase; LHR1, Leishmania heme response-1; Lmaa1, Ca2+-ATPase; LPG, lipophosphoglycan; Man, mannose; Mg2+, magnesium ion; MGT, magnesium transporter; MIT, myo-inositol transporter; Mn2+, manganese ion; mPPG, membrane Na+, sodium ion; NT, nucleobase/nucleoside transporter; POT1, polyamine transporter; PPG, proteophosphoglycan; ROS, reactive oxygen species; sAP, secreted acid phosphatase; SOD, superoxide dismutase; Zn2+, zinc ion.

Figure 1.
Characterised membrane transporters and their potential links to virulence in Leishmania.

1, flagellum; 2, flagellar pocket; 3, kinetoplast; 4, mitochondrion; 5, acidocalcisomes; 6, glycosomes; 7, multivesicular bodies; 8, endoplasmic reticulum; 9, nucleus; 10, β-mannan; 11, multivesicular tubule lysosome; 12, Golgi apparatus; 13, autophagosomes; 14, plasma membrane; 15, megasome. Abbreviations: AAP, amino acid permease; AAP3, arginine transporter; AAP7, lysine transporter; aPPG, amastigote PPG; Ca2+, calcium ion; Cd2+, cadmium ion; Co2+, cobalt ion; Fe3+, ferric ion; Fe2+, ferrous ion; fPPG, filamentous PPG; Gal, galactose; GDP, guanosine diphosphate; GILP, glycoinositolphospholipid; GP63, leishmanolysin; GT, glucose transporter; H+, hydrogen ion; K+, potassium ion; LIT1, Leishmania iron transporter; LFR1, Leishmania ferric reductase; LHR1, Leishmania heme response-1; Lmaa1, Ca2+-ATPase; LPG, lipophosphoglycan; Man, mannose; Mg2+, magnesium ion; MGT, magnesium transporter; MIT, myo-inositol transporter; Mn2+, manganese ion; mPPG, membrane Na+, sodium ion; NT, nucleobase/nucleoside transporter; POT1, polyamine transporter; PPG, proteophosphoglycan; ROS, reactive oxygen species; sAP, secreted acid phosphatase; SOD, superoxide dismutase; Zn2+, zinc ion.

Other characterised transporters that are involved in carbohydrate metabolism in Leishmania include the plasma membrane sucrose and myo-inositol H+-coupled symporters [18,19] and the Golgi guanosine diphosphate (GDP)-mannose (GDP-Man) transporter LPG2 [17] (Figure 1). Leishmania, similar to Saccharomyces cerevisiae [53], excrete sucrase to hydrolyse exogenous sucrose to monosaccharides [41] and, in parallel, express a sucrose transporter to internalise the disaccharide [18]. The coexistence of two mechanisms for utilisation of sucrose, along with the recent discovery that the sucrose transporter of the pathogen Ustilago maydis is a virulence factor [54], suggests that sucrose may have an important, yet unknown role in Leishmania metabolism. The MIT (myo-inositol transporter) mRNA is constitutively expressed and expression is not modulated by exogenous levels of inositol [19,55]. Inositol transport, however, is increased by ∼25-fold when the polyol is depleted from the culture medium [56], suggesting that inositol acquisition is most probably regulated at the post-transcriptional or/and protein level. Exclusion of inositol from the culture medium, additionally, does not alter glucose or adenosine transport [56]. Inositol phosphate metabolism, however, was shown to be down-regulated in the ΔLmGT promastigotes [51], thus suggesting two possibilities. First, that inositol transport may be co-regulated with hexose transport when the levels of these substrates fluctuate. Correspondingly, it has been illustrated that the expression of GT2, GT3, and MIT is governed by hexose and inositol availability and developmental stage [57], which corroborates a possible link between the ways hexoses and polyols are utilised throughout the life cycle. Furthermore, Leishmania may have distinct pools of exogenous and de novo synthesised inositol, as has been shown for trypanosomes [58]. The polyol level is increased in the ΔLmGT promastigotes but decreased in the spent media, while the first enzyme of the de novo synthesis, myo-inositol-1-phosphate synthase, is significantly down-regulated [51] (Figure 2). This suggests that the acquired and endogenously synthesised inositol may indeed be compartmentalised for use in different biosynthetic pathways in Leishmania. The LPG2 protein is part of a hexameric complex which can transport GDP-Man, GDP-arabinose (GDP-Ara), and GDP-fucose (GDP-Fuc) [17,59], which are discussed below.

Transporters and the Leishmania glycome.

Figure 2.
Transporters and the Leishmania glycome.

Abbreviations: myo-I, myo-inositol; myo-I1P, myo-inositol-1-phosphate; IPP, inositol phosphate phosphatase; I1PS, myo-inositol-1-phosphate synthase; Scr, sucrose; Suc, sucrase; Glc, glucose; Glc 1P, glucose 1-phosphate; Glc 6P, glucose 6-phsophate; Fru, fructose; Fru 6P, fructose 6-phosphate; Gal, galactose; Gal 1P, galactose 1-phsophate; UDP, uridine diphosphate; UDP-gal, UDP-galactose; UDP-glc, UDP-glucose; Man 1P, mannose 1-phosphate; Man 6P, mannose 6-phosphate; GDP, guanosine diphosphate; GDP-man, GDP-mannose; DP-man, dolichol-phosphate mannose; Ara, arabinose; Ara 1P, arabinose 1-phosphate; GDP-ara, GDP-arabinose; Fuc, fucose; Fuc 1P, fucose 1-phosphate; GDP-fuc, GDP-fucose; GDP-4d6dm, GDP-4-dehydro-6-deoxy-mannose; ADP-ribose, adenosine diphosphate ribose; RBK, ribokinase; HXK, hexokinase; G6PI, glucose-6-phsophate isomerase; PMI, phosphomannose isomerase; PMM, phosphomannomutase; GDPMP, guanosine diphosphate mannose pyrophosphorylase; DPMS, dolichol-phosphate mannose synthase; GMDS, GDP-mannose 4,6-dehydratase; GDPFS, GDP-fucose synthase; FPGT, fucose-1-phosphate guanylyltransferase; FK, fucokinase; GlcNAc6P, N-acetylglucosamine 6-phosphate; GlcN6P, glucosamine 6-phosphate; NAG6PD, N-acetylglucosamine-6-phosphate deacetylase; G6PD, glucosamine-6-phosphate deaminase; PGM, phosphoglucomutase; GALE, uridine diphopshate glucose 4′-epimerase; GALT, UDP-glucose—hexose 1-phosphate uridylyltransferase; GALK, galactokinase; GILP, glycoinositolphospholipid; GPI, glycosylphosphatidylinositol; LPG, lipophosphoglycan; PPG, proteophosphoglycan.

Figure 2.
Transporters and the Leishmania glycome.

Abbreviations: myo-I, myo-inositol; myo-I1P, myo-inositol-1-phosphate; IPP, inositol phosphate phosphatase; I1PS, myo-inositol-1-phosphate synthase; Scr, sucrose; Suc, sucrase; Glc, glucose; Glc 1P, glucose 1-phosphate; Glc 6P, glucose 6-phsophate; Fru, fructose; Fru 6P, fructose 6-phosphate; Gal, galactose; Gal 1P, galactose 1-phsophate; UDP, uridine diphosphate; UDP-gal, UDP-galactose; UDP-glc, UDP-glucose; Man 1P, mannose 1-phosphate; Man 6P, mannose 6-phosphate; GDP, guanosine diphosphate; GDP-man, GDP-mannose; DP-man, dolichol-phosphate mannose; Ara, arabinose; Ara 1P, arabinose 1-phosphate; GDP-ara, GDP-arabinose; Fuc, fucose; Fuc 1P, fucose 1-phosphate; GDP-fuc, GDP-fucose; GDP-4d6dm, GDP-4-dehydro-6-deoxy-mannose; ADP-ribose, adenosine diphosphate ribose; RBK, ribokinase; HXK, hexokinase; G6PI, glucose-6-phsophate isomerase; PMI, phosphomannose isomerase; PMM, phosphomannomutase; GDPMP, guanosine diphosphate mannose pyrophosphorylase; DPMS, dolichol-phosphate mannose synthase; GMDS, GDP-mannose 4,6-dehydratase; GDPFS, GDP-fucose synthase; FPGT, fucose-1-phosphate guanylyltransferase; FK, fucokinase; GlcNAc6P, N-acetylglucosamine 6-phosphate; GlcN6P, glucosamine 6-phosphate; NAG6PD, N-acetylglucosamine-6-phosphate deacetylase; G6PD, glucosamine-6-phosphate deaminase; PGM, phosphoglucomutase; GALE, uridine diphopshate glucose 4′-epimerase; GALT, UDP-glucose—hexose 1-phosphate uridylyltransferase; GALK, galactokinase; GILP, glycoinositolphospholipid; GPI, glycosylphosphatidylinositol; LPG, lipophosphoglycan; PPG, proteophosphoglycan.

Role of the glycome in virulence

All carbohydrate transporters characterised in Leishmania, to date, acquire substrates involved in macromolecule synthesis. Hexoses are converted into glycosyl donors such as GDP-Man, dolichol-phosphate mannose (Dol-P-Man), GDP-Ara, GDP-Fuc, uridine diphosphate (UDP)-Gal, and UDP-glucose (UDP-Glc), which are utilised in glycopolymer and glycoconjugate synthesis (Figure 2). myo-Inositol is required for phosphatidylinositol (PI) and phospholipid synthesis [60]. Ribose is phosphorylated to ribose 5-phosphate in the pentose phosphate pathway and then directed to nucleotide synthesis. Ribose, additionally, is converted into adenosine diphosphate (ADP)-ribose and used in ribosylation reactions to post-translationally modify proteins. The spectrum of surface-associated and secreted glycoconjugates and glycopolymers that Leishmania synthesise includes lipophosphoglycan (LPG), proteophosphoglycans (PPGs), glycosylphosphatidylinositol (GPI)-anchored proteins, glycoinositolphospholipids (GILPs), N-glycans, and the carbohydrate reserve material β-mannan [61,62] (Figure 2). LPG [63], GP63 (leishmanolysin) [64], and β-mannan [62] are virulence determinants in Leishmania, although some species-specific differences have been observed [63]. The capacity of ΔLmGT to synthesise LPG, membrane PPG (mPPG), GILP, GP63, secreted acid phosphatase (sAP), and β-mannan is impaired [50]. Similarly, mutants lacking the LPG2 gene are incapable of LPG and PPG glycosylation, and have reduced virulence as both promastigotes and amastigotes [65].

Non-carbohydrate transporters implicated in virulence

Arginine transporter

Amino acid permease 3 (AAP3) (Figure 1) is a high-affinity, arginine-specific transporter that salvages arginine, an essential amino acid for Leishmania, from the host milieu [10,13]. When starved for amino acids, Leishmania increase AAP3 mRNA and protein levels and arginine uptake, a response that involves sensing of both intra- and extracellular arginine levels, activation of a mitogen-activated protein kinase 2 (MPK2)-mediated signalling cascade, and control of the arginine transport by regulation of the transporter-coding mRNA levels [36,6668]. In amastigotes, arginine plays a key role in Leishmania survival and parasitism through co-ordinated polyamine and nitric oxide (NO) production and host immune component regulation [68]. Macrophages infected with Leishmania up-regulate the cationic amino acid transporter-2 (CAT-2)-mediated acquisition of arginine [68]. CAT-2 activity and arginine availability in the macrophages have an impact on arginase and inducible nitric oxide synthase (iNOS) expression, associated with polyamine and NO synthesis, respectively, and influence in Leishmania survival [68]. Arginine transport activity may alter the function of components of the immune system, such as T and B cells, and promote parasite survival in macrophages [68,69].

Lysine transporter

AAP7 (Figure 1) is a high-affinity, lysine-specific permease which retrieves this amino acid, essential for Leishmania viability, from exogenous sources [14]. Enzymes for lysine degradation are not present in Leishmania [70], and the amino acid thus does not represent a ketogenic substrate. The main role of lysine in Leishmania appears to be in protein synthesis. Protein lysine residues are targets for a variety of post-translational modifications, such as ubiquitination, acetylation, and methylation [71], and are thus possibly implicated in regulation of cellular processes such as transcription, translation, cell signalling, metabolism, and others [72,73]. Deletion of AAP7 is not possible [14], supporting the hypothesis that this transporter is important for Leishmania viability.

Purine transporters

Leishmania are unable to synthesise the purine ring that is required for adenylate and guanylate nucleotide synthesis [74] and are thus obliged to salvage purines from the host environment. In view of its importance, the purine salvage pathway has been regarded as a promising chemotherapeutic target and thus has been studied in considerable detail (reviewed in refs [75,76]). The salvage pathway involves transport of enzymatically predigested nucleobases and nucleosides by four permeases of the equilibrative nucleoside transporter (ENT) family, designated NT1 to NT4 (Figure 1) [2025]. Purine restriction studies have revealed that Leishmania up-regulate NT1, NT2, and NT3 tranporters and enzymes with exogenous roles in the breakdown of complex purine sources to nucleosides and nucleobases [77,78]. Prolonged starvation for more than 12 h results in increased expression of key enzymes of purine metabolism, while NT4 is observed up-regulated after 48 h [78]. Gene knockout and replacement studies, on the other hand, demonstrate that none of the four NT transporters are individually essential for Leishmania viability, while the phenotype of double null mutants for NT1/NT2 and NT3/NT4 indicates that the nucleobase transporters are of a greater importance for purine salvage [24,79].

Iron and heme transporters

Iron is an essential micronutrient which can be obtained by Leishmania in an inorganic form or as iron-containing molecules such as haemin and haemoglobin [80]. Inorganic iron acquisition is a two-step process which involves the NADPH-dependent reduction of Fe3+ to Fe2+ by a plasma membrane-associated ferric reductase, LFR1 [81], and transport of the generated ferrous ions into the cell by the Leishmania iron transporter LIT1 (Figure 1) [30]. LIT1 is capable of transporting divalent cations of manganese, cadmium, and zinc, but shows higher affinity for ferrous iron [30,82]. It has been detected on the surface of both amastigotes and promastigotes grown in iron-depleted medium [30,83]. Omission of the iron from the culture medium results in an increase in the LIT1 mRNA and protein abundance and iron-uptake activity. Withdrawal of exogenous iron also enhances iron-dependent superoxide dismutase (FeSOD) and ascorbate peroxidase (APX) activity, and triggers reactive oxygen species (ROS)-mediated differentiation of promastigotes into virulent amastigotes [83]. Deletion of the LIT1 gene in Leishmania amazonensis amastigotes resulted in the inability of the parasites to proliferate within macrophages and to cause lesions in mice [82]. Generation of null mutants of the Leishmania mitochondrial iron transporter, MIT1, was unsuccessful, further supporting an essential role of this transporter in Leishmania virulence [31]. In 2012, the group which characterised LIT1 reported the identification of the plasma membrane Leishmania heme response-1 (LHR1) transporter (Figure 1) [29]. Leishmania are auxotrophic for heme and they salvage it either via LHR1 [29] or by receptor-mediated endocytosis of haemoglobin through the flagellar pocket [84,85], Rab5- and Rab7-regulated transport to the lysosome for degradation of haemoglobin [86,87], and translocation of the heme thus generated to the cytosol by the ATP-binding cassette protein LABCG5 [88]. Starvation for heme increases LHR1 transcript levels and heme transport [29]. Deletion of one allele of the LHR1 gene results in promastigotes that are more sensitive to heme deficiency and reduced virulence [29]. Deletion of both LHR1 alleles was not achieved [29], suggesting that the LHR1 is essential for Leishmania survival and a promising drug target [89].

Ion transporters

Intracellular ion levels in Leishmania are different by several orders of magnitude compared with those in the extracellular milieu [8]. Ion homeostasis plays a critical role in Leishmania survival [8], and it is maintained by a heterogeneous group of plasma membrane and organellar transporters and ATP-driven pumps. H+, K+, and Na+ are translocated by cation (Mg2+/Ca2+)-dependent proton-ATPases and transporters which are involved in many essential processes, such as control of the electrochemical gradient across membranes, regulation of intracellular pH, substrate (e.g. amino acid, nucleobase/nucleoside, sucrose, and myo-inositol) transport, and energy generation [8,15,18,19,9093]. Recently, a Mg2+-dependent adenosine-translocating ecto-ATPase and two organellar Ca2+-transporting ATPases (Figure 1) have been identified as virulence determinants [35,94,95]. Similarly, two potential Mg2+ transporters of the two-transmembrane domain (2-TM-GxN) type family of transporters, designated MGT1 and MGT2 (Figure 1), have been shown to be essential for parasite development and pathogenesis in mice [96].

Leishmania lack the enzymatic capacity for de novo synthesis of a range of substrates that are critical for their survival. The parasites are adapted to scavenge these substrates from the host environment, in competition with host transport systems that may deplete molecules from the insect gut or the macrophage phagolysosome [97]. The essentiality of transport systems to Leishmania, together with differences in the transportome of parasite and host, highlights Leishmania transporters as potent and specific drug targets, with the additional advantage that cell surface transporters are directly accessible to exogenous inhibitors or toxic substrates. Nutrient transport in Leishmania, and in the clinically important amastigote stage in particular, is poorly understood. Recent transcriptomic studies highlight potential transporters that are up-regulated in amastigotes, and the recent development of high-throughput reverse genetic tools [98] and metabolomics approaches [51] present opportunities to dissect the importance of transport systems in Leishmania virulence.

Summary
  • Membrane transporters are critical for life, and thus for virulence in pathogens.

  • Leishmania fine-tune transporter expression to profit from the contrasting nutrient environments they encounter during their life cycle.

  • Transporters contribute also to environmental sensing and thus mediate metabolic adaptations to nutrient availability.

  • Leishmania encode a plethora of putative transporters, but few have been functionally characterised.

  • Recent developments in reverse genetic and metabolomic analysis of Leishmania may reveal the previously intractable amastigote transportome.

Abbreviations

     
  • AAP3

    amino acid permease 3

  •  
  • CAT-2

    cationic amino acid transporter-2

  •  
  • GDP

    guanosine diphosphate

  •  
  • GDP-Ara

    GDP-arabinose

  •  
  • GDP-Fuc

    GDP-fucose

  •  
  • GDP-Man

    GDP-mannose

  •  
  • GILPs

    glycoinositolphospholipids

  •  
  • LHR1

    Leishmania heme response-1

  •  
  • LIT1

    Leishmania iron transporter

  •  
  • LPG

    lipophosphoglycan

  •  
  • MIT

    myo-inositol transporter

  •  
  • NO

    nitric oxide

  •  
  • PPGs

    proteophosphoglycans

  •  
  • UDP

    uridine diphosphate

  •  
  • UDP-Glc

    UDP-glucose

Competing Interests

The Authors declare that there are no competing interests associated with the manuscript.

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