Parasite Polyamines as Pharmaceutical Targets

There is an urgent need for the identification and validation of new therapeutic targets in protozoan parasites because currently available drugs are limited in number and usefulness, and no vaccines are available. The discovery that alpha-difluoromethylornithine, an inhibitor of polyamine biosynthesis, is an efficacious treatment for African Sleeping Sickness caused by the protozoan parasite Trypanosoma brucei, has validated the polyamine pathway as a target in protozoan parasites. Polyamines are ubiquitous organic cations that play critical roles in key cellular processes such as growth, differentiation, and macromolecular biosynthesis. In recent years, remarkable progress has been made in the characterization of the polyamine pathway in a variety of protozoan parasites and this review will highlight surprising and unique features that could lead to new therapeutic strategies.

GMP reductase and genetic uncoupling of adenylate and guanylate metabolism in Leishmania donovani parasites

Purine acquisition is an essential nutritional process for Leishmania. Although purine salvage into adenylate nucleotides has been investigated in detail, little attention has been focused on the guanylate branch of the purine pathway. To characterize guanylate nucleotide metabolism in Leishmania and create a cell culture model in which the pathways for adenylate and guanylate nucleotide synthesis can be genetically uncoupled for functional studies in intact cells, we created and characterized null mutants of L. donovani that were deficient in either GMP reductase alone (Δgmpr) or in both GMP reductase and its paralog IMP dehydrogenase (Δgmpr/Δimpdh). Whereas wild type parasites were capable of utilizing virtually any purine nucleobase/nucleoside, the Δgmpr and Δgmpr/Δimpdh null lines exhibited highly restricted growth phenotypes. The Δgmpr single mutant could not grow in xanthine, guanine, or their corresponding nucleosides, while no purine on its own could support the growth of Δgmpr/Δimpdh cells. Permissive growth conditions for the Δgmpr/Δimpdh necessitated both xanthine, guanine, or the corresponding nucleosides, and additionally, a second purine that could serve as a source for adenylate nucleotide synthesis. Interestingly, GMPR, like its paralog IMPDH, is compartmentalized to the leishmanial glycosome, a process mediated by its COOH-terminal peroxisomal targeting signal. The restricted growth phenotypes displayed by the L. donovani Δgmpr and Δgmpr/Δimpdh null mutants confirms the importance of GMPR in the purine interconversion processes of this parasite.

A role for adenine nucleotides in the sensing mechanism to purine starvation in Leishmania donovani

Purine salvage by Leishmania is an obligatory nutritional process that impacts both cell viability and growth. Previously, we have demonstrated that the removal of purines in culture provokes significant metabolic changes that enable Leishmania to survive prolonged periods of purine starvation. In order to understand how Leishmania sense and respond to changes in their purine environment, we have exploited several purine pathway mutants, some in which adenine and guanine nucleotide metabolism is uncoupled. While wild type parasites grow in any one of a variety of naturally occurring purines, the proliferation of these purine pathway mutants requires specific types or combinations of exogenous purines. By culturing purine pathway mutants in high levels of extracellular purines that are either permissive or non-permissive for growth and monitoring for previously defined markers of the adaptive response to purine starvation, we determined that adaptation arises from a surveillance of intracellular purine nucleotide pools rather than from a direct sensing of the extracellular purine content of the environment. Specifically, our data suggest that perturbation of intracellular adenine-containing nucleotide pools provides a crucial signal for inducing the metabolic changes necessary for the long-term survival of Leishmania in a purine-scarce environment.

The Trypanosoma cruzi Diamine Transporter Is Essential for Robust Infection of Mammalian Cells

Trypanosoma cruzi is incapable of synthesizing putrescine or cadaverine de novo, and, therefore, salvage of polyamines from the host milieu is an obligatory nutritional function for the parasite. A high-affinity diamine transporter (TcPOT1) from T. cruzi has been identified previously that recognizes both putrescine and cadaverine as ligands. In order to assess the functional role of TcPOT1 in intact parasites, a Δtcpot1 null mutant was constructed by targeted gene replacement and characterized. The Δtcpot1 mutant lacked high-affinity putrescine-cadaverine transport capability but retained the capacity to transport diamines via a non-saturable, low-affinity mechanism. Transport of spermidine and arginine was not impacted by the Δtcpot1 lesion. The Δtcpot1 cell line exhibited a significant but not total defect in its ability to subsist in Vero cells, although initial infection rates were not affected by the lesion. These findings reveal that TcPOT1 is the sole high-affinity diamine permease in T. cruzi, that genetic obliteration of TcPOT1 impairs the ability of the parasite to maintain a robust infection in mammalian cells, and that a secondary low-affinity uptake mechanism for this key parasite nutrient is operative but insufficient for optimal infection.

The cystathionine-β-synthase domains on the guanosine 5''-monophosphate reductase and inosine 5'-monophosphate dehydrogenase enzymes from Leishmania regulate enzymatic activity in response to guanylate and adenylate nucleotide levels

The Leishmania guanosine 5'-monophosphate reductase (GMPR) and inosine 5'-monophosphate dehydrogenase (IMPDH) are purine metabolic enzymes that function maintaining the cellular adenylate and guanylate nucleotide. Interestingly, both enzymes contain a cystathionine-β-synthase domain (CBS). To investigate this metabolic regulation, the Leishmania GMPR was cloned and shown to be sufficient to complement the guaC (GMPR), but not the guaB (IMPDH), mutation in Escherichia coli. Kinetic studies confirmed that the Leishmania GMPR catalyzed a strict NADPH-dependent reductive deamination of GMP to produce IMP. Addition of GTP or high levels of GMP induced a marked increase in activity without altering the Km values for the substrates. In contrast, the binding of ATP decreased the GMPR activity and increased the GMP Km value 10-fold. These kinetic changes were correlated with changes in the GMPR quaternary structure, induced by the binding of GMP, GTP, or ATP to the GMPR CBS domain. The capacity of these CBS domains to mediate the catalytic activity of the IMPDH and GMPR provides a regulatory mechanism for balancing the intracellular adenylate and guanylate pools.

Integrating ribosomal promoter vectors that offer a choice of constitutive expression profiles in Leishmania donovani

We have designed a novel series of integrating ribosomal RNA promoter vectors with five incrementally different constitutive expression profiles, covering a 250-fold range. Differential expression was achieved by placing different combinations of synthetic or leishmanial DNA sequences upstream and downstream of the transgene coding sequence in order to modulate pre-mRNA processing efficiency and mRNA stability, respectively. All of the vectors have extensive multiple cloning sites, and versions are available for producing N- or C- terminal GFP fusions at each of the possible relative expression levels. In addition, the modular configuration of the vectors allows drug resistance cassettes and other components to be readily exchanged. In toto, these vectors should be useful additions to the toolkit available for molecular and genetic studies of Leishmania donovani.

A quantitative proteomic screen to identify potential drug resistance mechanism in α-difluoromethylornithine (DFMO) resistant Leishmania donovani

Visceral leishmaniasis (VL) caused by Leishmania donovani is a systemic protozoan disease that is fatal if left untreated. The promastigote form of L. donovani is sensitive to growth inhibition by dl-α-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase (ODC), the first enzyme of the polyamine biosynthetic pathway. Exposure of a wild type (DI700) cell population to gradually increasing concentrations of DFMO resulted in the selection of a strain of Leishmania (DFMO-16), which was capable of proliferating in 16mM DFMO. To elucidate the molecular basis for this resistance, we undertook a comparative proteomic analysis of DFMO-resistant/sensitive parasites using isobaric tagging for relative and absolute quantification (iTRAQ/LC-MS/MS). Out of the 101 proteins identified in at least 2 of the 3 independent experiments, 82 proteins are 1.5- to 44.0-fold more abundant in DFMO-resistant strain (DFMO-16) while 19 are 2- to 5.0-fold less abundant as compared to the wild-type (DI700) parasites. Proteins with 2-fold or greater abundance in the DFMO-resistant strain include free radical detoxification, polyamine and trypanothione metabolic proteins, proteins involved in metabolism, intracellular survival and proteolysis, elongation factors, signaling molecules and mitochondrial transporters, and many with no annotated function. Differentially modulated proteins contribute to our understanding of molecular mechanism of DFMO-resistance and have the potential to act as biomarkers.

BIOLOGICAL SIGNIFICANCE

This study will facilitate a deeper understanding of the phenomenon of acquired drug resistance and possible biomarkers in Leishmania against antiparasitic drug DFMO. Also it will provide information about the metabolic pathways modulated in resistant parasites as an adaptation mechanism to counter drugs. Studies like this are important to safeguard the efficacy of a limited repertoire of anti-parasitic drugs, and to lead the development of new drugs and drug combinations.

Pyrazolopyrimidine metabolism in parasitic protozoa

The pyrazolopyrimidines are purine analogs that are cytotoxic toward and metabolized by several genera of parasitic protozoa, including the Leishmania and the Trypanosoma. Examples of pyrazolopyrimidines that are selectively metabolized by these parasites include allopurinol, allopurinol riboside, 4-thiopurinol, 4-thiopurinol riboside, and formycin B. These pathogenic protozoa are capable of efficient conversion of the pyrazolopyrimidines to the nucleotide level. The pyrazolopyrimidine metabolites which are isomers of inosine monophosphate are subsequently aminated and incorporated as the adenylate analog into RNA. Mammalian cells are incapable of these metabolic transformations. The sulfur containing pyrazolopyrimidines, however, are neither aminated nor incorporated into nucleic acid. The selective metabolism of the pyrazolopyrimidines by the intracellular metabolic machinery of the parasites of the Trypanosomatidae family offers a rational approach to the chemotherapy of the diseases caused by these pathogenic hemoflagellates.

Substrate inhibition of uracil phosphoribosyltransferase by uracil can account for the uracil growth sensitivity of Leishmania donovani pyrimidine auxotrophs

The pathogenic protozoan parasite Leishmania donovani is capable of both de novo pyrimidine biosynthesis and salvage of pyrimidines from the host milieu. Genetic analysis has authenticated L. donovani uracil phosphoribosyltransferase (LdUPRT), an enzyme not found in mammalian cells, as the focal enzyme of pyrimidine salvage because all exogenous pyrimidines that can satisfy the requirement of the parasite for pyrimidine nucleotides are funneled to uracil and then phosphoribosylated to UMP in the parasite by LdUPRT. To characterize this unique parasite enzyme, LdUPRT was expressed in Escherichia coli, and the recombinant enzyme was purified to homogeneity. Kinetic analysis revealed apparent Km values of 20 and 99 μM for the natural substrates uracil and phosphoribosylpyrophosphate, respectively, as well as apparent Km values 6 and 7 μM for the pyrimidine analogs 5-fluorouracil and 4-thiouracil, respectively. Size exclusion chromatography revealed the native LdUPRT to be tetrameric and retained partial structure and activity in high concentrations of urea. L. donovani mutants deficient in de novo pyrimidine biosynthesis, which require functional LdUPRT for growth, are hypersensitive to high concentrations of uracil, 5-fluorouracil, and 4-thiouracil in the growth medium. This hypersensitivity can be explained by the observation that LdUPRT is substrate-inhibited by uracil and 4-thiouracil, but 5-fluorouracil toxicity transpires via an alternative mechanism. This substrate inhibition of LdUPRT provides a protective mechanism for the parasite by facilitating purine and pyrimidine nucleotide pool balance and by sparing phosphoribosylpyrophosphate for consumption by the nutritionally indispensable purine salvage process.

Adenine and adenosine salvage in Leishmania donovani

6-aminopurine metabolism in Leishmania is unique among trypanosomatid pathogens since this genus expresses two distinct routes for adenine salvage: adenine phosphoribosyltransferase (APRT) and adenine deaminase (AAH). To evaluate the relative contributions of APRT and AAH, adenine salvage was evaluated in Δaprt, Δaah, and Δaprt/Δaah null mutants of L. donovani. The data confirm that AAH plays the dominant role in adenine metabolism in L. donovani, although either enzyme alone is sufficient for salvage. Adenosine salvage was also evaluated in a cohort of null mutants. Adenosine is also primarily converted to hypoxanthine, either intracellularly or extracellularly, but can also be phosphorylated to the nucleotide level by adenosine kinase when the predominant pathways are genetically or pharmacologically blocked. These data provide genetic verification for the relative contributions of 6-aminopurine metabolizing pathways in L. donovani and demonstrate that all of the pathways can function under appropriate conditions of genetic or pharmacologic perturbation.

Crystal structure of arginase from Leishmania mexicana and implications for the inhibition of polyamine biosynthesis in parasitic infections

Arginase from parasitic protozoa belonging to the genus Leishmania is a potential drug target for the treatment of leishmaniasis because this binuclear manganese metalloenzyme catalyzes the first committed step in the biosynthesis of polyamines that enable cell growth and survival. The high resolution X-ray crystal structures of the unliganded form of Leishmania mexicana arginase (LmARG) and four inhibitor complexes are now reported. These complexes include the reactive substrate analogue 2(S)-amino-6-boronohexanoic acid (ABH) and the hydroxylated substrate analogue nor-N(ω)-hydroxy-l-arginine (nor-NOHA), which are the most potent arginase inhibitors known to date. Comparisons of the LmARG structure with that of the archetypal arginase, human arginase I, reveal that all residues important for substrate binding and catalysis are strictly conserved. However, three regions of tertiary structure differ between the parasitic enzyme and the human enzyme corresponding to the G62 - S71, L161 - C172, and I219 - V230 segments of LmARG. Additionally, variations are observed in salt link interactions that stabilize trimer assembly in LmARG. We also report biological studies in which we demonstrate that localization of LmARG to the glycosome, a unique subcellular organelle peculiar to Leishmania and related parasites, is essential for robust pathogenesis.

Adenylosuccinate synthetase and adenylosuccinate lyase deficiencies trigger growth and infectivity deficits in Leishmania donovani

Leishmania are auxotrophic for purines, and consequently purine acquisition from the host is a requisite nutritional function for the parasite. Both adenylosuccinate synthetase (ADSS) and adenylosuccinate lyase (ASL) have been identified as vital components of purine salvage in Leishmania donovani, and therefore Δadss and Δasl null mutants were constructed to test this hypothesis. Unlike wild type L. donovani, Δadss and Δasl parasites in culture exhibited a profoundly restricted growth phenotype in which the only permissive growth conditions were a 6-aminopurine source in the presence of 2'-deoxycoformycin, an inhibitor of adenine aminohydrolase activity. Although both knock-outs showed a diminished capacity to infect murine peritoneal macrophages, only the Δasl null mutant was profoundly incapacitated in its ability to infect mice. The enormous discrepancy in parasite loads observed in livers and spleens from mice infected with either Δadss or Δasl parasites can be explained by selective accumulation of adenylosuccinate in the Δasl knock-out and consequent starvation for guanylate nucleotides. Genetic complementation of a Δasl lesion in Escherichia coli implied that the L. donovani ASL could also recognize 5-aminoimidazole-(N-succinylocarboxamide) ribotide as a substrate, and purified recombinant ASL displayed an apparent Km of ∼24 μm for adenylosuccinate. Unlike many components of the purine salvage pathway of L. donovani, both ASL and ADSS are cytosolic enzymes. Overall, these data underscore the paramount importance of ASL to purine salvage by both life cycle stages of L. donovani and authenticate ASL as a potential drug target in Leishmania.

How Trypanosoma cruzi feasts upon its mammalian host

Trypanosoma cruzi has a complex relationship with its mammalian host in which parasite and host metabolic networks are intertwined. A genome-wide functional screen of T. cruzi infection in HeLa cells (Caradonna et al., 2013) divulges host metabolic functions and signaling pathways that impact intracellular parasite replication and reveals potential targets for therapeutic exploitation.

Purine salvage in Leishmania: complex or simple by design?

Purine nucleotides function in a variety of vital cellular and metabolic processes including energy production, cell signaling, synthesis of vitamin-derived cofactors and nucleic acids, and as determinants of cell fate. Unlike their mammalian and insect hosts, Leishmania cannot synthesize the purine ring de novo and are absolutely dependent upon them to meet their purine requirements. The obligatory nature of purine salvage in these parasites, therefore, offers an attractive paradigm for drug targeting and, consequently, the delineation of the pathway has been under scientific investigation for over 30 years. Here, we review recent developments that reveal how purines flux in Leishmania and offer a potential 'Achilles' heel' for future validation.

Effects of intravenous exenatide in type 2 diabetic patients with congestive heart failure: a double-blind, randomised controlled clinical trial of efficacy and safety

AIMS/HYPOTHESIS

The aim of this study was to determine whether exenatide improves haemodynamic function in patients with type 2 diabetes with congestive heart failure (CHF).

METHODS

The main eligibility criteria for inclusion were: male/female (18-80 years) with type 2 diabetes and CHF (ejection fraction ≤ 35%, and New York Heart Association functional class III or IV). Out of 237 patients screened, 20 male type 2 diabetic patients participated in this crossover trial design and were allocated (sequentially numbered) to i.v. infusions during two consecutive days with (1) exenatide (0.12 pmol/kg/min); and (2) placebo for 6 h followed by a washout period for 18 h, at Stockholm South Hospital, Sweden. Patients and researchers were blinded to the assignment. Cardiac haemodynamic variables were determined by right heart catheterisation. The primary endpoint was defined as an increase in cardiac index (CI) or a decrease in pulmonary capillary wedge pressure (PCWP) of ≥ 20%. Secondary endpoints were tolerability and safety of exenatide infusion.

RESULTS

CI increased at 3 and 6 h by 0.4 ± 0.1 (23%) and 0.33 ± 0.1 (17%) l min(-1) m(-2), during exenatide infusion vs -0.02 ± 0.1 (-1%) and -0.08 ± 0.1 (-5%) l min(-1) m(-2) during placebo (p = 0.003); and heart rate (HR) increased at 1, 3 and 6 h by 8 ± 3 (11%), 15 ± 4 (21%) and 21 ± 5 (29%) beats per min (bpm), during exenatide infusion vs -1 ± 2 (-2%), 1 ± 1 (2%) and 6 ± 2 (8%) bpm, during placebo (p = 0.006); and PCWP decreased at 1, 3 and 6 h by -1.3 ± 0.8 (-8%), -1.2 ± 1 (-8%) and -2.2 ± 0.9 (-15%) mmHg, during exenatide infusion vs 0.3 ± 0.5 (2%), 1 ± 0.6 (6%) and 1.4 ± 0.7 (8%) mmHg, during placebo (p = 0.001). No serious adverse event was observed. Adverse events were reported in nine patients (six, nausea; two, increased HR; one, increased systolic blood pressure).

CONCLUSIONS/INTERPRETATION

Infusion of exenatide in male type 2 diabetic patients with CHF increased the CI as a result of chronotropy, with concomitant favourable effects on PCWP and reasonable tolerability of the drug. The clinical implications of using exenatide in patients with CHF are still not clear and further studies are warranted.

TRIAL REGISTRATION

www.isrctn.org/ISRCTN47533126

Genetic dissection of pyrimidine biosynthesis and salvage in Leishmania donovani

Protozoan parasites of the Leishmania genus express the metabolic machinery to synthesize pyrimidine nucleotides via both de novo and salvage pathways. To evaluate the relative contributions of pyrimidine biosynthesis and salvage to pyrimidine homeostasis in both life cycle stages of Leishmania donovani, individual mutant lines deficient in either carbamoyl phosphate synthetase (CPS), the first enzyme in pyrimidine biosynthesis, uracil phosphoribosyltransferase (UPRT), a salvage enzyme, or both CPS and UPRT were constructed. The Δcps lesion conferred pyrimidine auxotrophy and a growth requirement for medium supplementation with one of a plethora of pyrimidine nucleosides or nucleobases, although only dihydroorotate or orotate could circumvent the pyrimidine auxotrophy of the Δcps/Δuprt double knockout. The Δuprt null mutant was prototrophic for pyrimidines but could not salvage uracil or any pyrimidine nucleoside. The capability of the Δcps parasites to infect mice was somewhat diminished but still robust, indicating active pyrimidine salvage by the amastigote form of the parasite, but the Δcps/Δuprt mutant was completely attenuated with no persistent parasites detected after a 4-week infection. Complementation of the Δcps/Δuprt clone with either CPS or UPRT restored infectivity. These data establish that an intact pyrimidine biosynthesis pathway is essential for the growth of the promastigote form of L. donovani in culture, that all uracil and pyrimidine nucleoside salvage in the parasite is mediated by UPRT, and that both the biosynthetic and salvage pathways contribute to a robust infection of the mammalian host by the amastigote. These findings impact potential therapeutic design and vaccine strategies for visceral leishmaniasis.

Adenine aminohydrolase from Leishmania donovani: unique enzyme in parasite purine metabolism

Adenine aminohydrolase (AAH) is an enzyme that is not present in mammalian cells and is found exclusively in Leishmania among the protozoan parasites that infect humans. AAH plays a paramount role in purine metabolism in this genus by steering 6-aminopurines into 6-oxypurines. Leishmania donovani AAH is 38 and 23% identical to Saccharomyces cerevisiae AAH and human adenosine deaminase enzymes, respectively, catalyzes adenine deamination to hypoxanthine with an apparent K(m) of 15.4 μM, and does not recognize adenosine as a substrate. Western blot analysis established that AAH is expressed in both life cycle stages of L. donovani, whereas subcellular fractionation and immunofluorescence studies confirmed that AAH is localized to the parasite cytosol. Deletion of the AAH locus in intact parasites established that AAH is not an essential gene and that Δaah cells are capable of salvaging the same range of purine nucleobases and nucleosides as wild type L. donovani. The Δaah null mutant was able to infect murine macrophages in vitro and in mice, although the parasite loads in both model systems were modestly reduced compared with wild type infections. The Δaah lesion was also introduced into a conditionally lethal Δhgprt/Δxprt mutant in which viability was dependent on pharmacologic ablation of AAH by 2'-deoxycoformycin. The Δaah/Δhgprt/Δxprt triple knock-out no longer required 2'-deoxycoformycin for growth and was avirulent in mice with no persistence after a 4-week infection. These genetic studies underscore the paramount importance of AAH to purine salvage by L. donovani.

IMP dehydrogenase deficiency in Leishmania donovani causes a restrictive growth phenotype in promastigotes but is not essential for infection in mice

Leishmania cannot synthesize purines de novo and therefore must scavenge purines from its host for survival and growth. Biochemical and genomic analyses have indicated that Leishmania species express three potential routes for the synthesis of guanylate nucleotides: (1) a two-step pathway that converts IMP to GMP; (2) a three-step pathway that starts with the deamination of guanine to xanthine, followed by phosphoribosylation to XMP and then conversion to GMP; or (3) direct guanine phosphoribosylation by HGPRT. To determine the role of the first of these pathways to guanylate nucleotide synthesis, an L. donovani line deficient in IMP dehydrogenase (IMPDH), the first step in the IMP to GMP pathway, was constructed by targeted gene replacement. The Δimpdh lesion triggered a highly restrictive growth phenotype in promastigotes in culture but did not impact parasitemias in mice. The dispensability of IMPDH in vivo is the first definitive demonstration that intracellular L. donovani amastigotes have access to a sufficient pool of guanine, xanthine, or guanylate precursors from the host.

The Leishmania donovani UMP synthase is essential for promastigote viability and has an unusual tetrameric structure that exhibits substrate-controlled oligomerization

The final two steps of de novo uridine 5'-monophosphate (UMP) biosynthesis are catalyzed by orotate phosphoribosyltransferase (OPRT) and orotidine 5'-monophosphate decarboxylase (OMPDC). In most prokaryotes and simple eukaryotes these two enzymes are encoded by separate genes, whereas in mammals they are expressed as a bifunctional gene product called UMP synthase (UMPS), with OPRT at the N terminus and OMPDC at the C terminus. Leishmania and some closely related organisms also express a bifunctional enzyme for these two steps, but the domain order is reversed relative to mammalian UMPS. In this work we demonstrate that L. donovani UMPS (LdUMPS) is an essential enzyme in promastigotes and that it is sequestered in the parasite glycosome. We also present the crystal structure of the LdUMPS in complex with its product, UMP. This structure reveals an unusual tetramer with two head to head and two tail to tail interactions, resulting in two dimeric OMPDC and two dimeric OPRT functional domains. In addition, we provide structural and biochemical evidence that oligomerization of LdUMPS is controlled by product binding at the OPRT active site. We propose a model for the assembly of the catalytically relevant LdUMPS tetramer and discuss the implications for the structure of mammalian UMPS.

Adaptive responses to purine starvation in Leishmania donovani

Starvation of Leishmania donovani parasites for purines leads to a rapid amplification in purine nucleobase and nucleoside transport. Studies with nucleoside transport-deficient L. donovani indicate that this phenomenon is mediated by the nucleoside transporters LdNT1 and LdNT2, as well as by the purine nucleobase transporter LdNT3. The escalation in nucleoside transport cannot be ascribed to an increase in either LdNT1 or LdNT2 mRNA. However, Western analyses on parasites expressing epitope-tagged LdNT2 revealed a marked upregulation in transporter protein at the cell surface. Kinetic investigations of LdNT1 and LdNT2 activities from purine-replete and purine-starved cells indicated that both transporters exhibited significant increases in V(max) for their ligands under conditions of purine-depletion, although neither transporter displayed an altered affinity for its respective ligands. Concomitant with the increase in purine nucleoside and nucleobase transport, the purine salvage enzymes HGPRT, XPRT and APRT were also upregulated, suggesting that under conditions where purines are limiting, Leishmania parasites remodel their purine metabolic pathway to maximize salvage. Moreover, qRT-PCR analyses coupled with cycloheximide inhibition studies suggest that the underlying molecular mechanism for this augmentation in purine salvage occurs post-transcriptionally and is reliant on de novo protein synthesis.

Inhibition profile of Leishmania mexicana arginase reveals differences with human arginase I

Arginase (ARG), the enzyme that catalyzes the conversion of arginine to ornithine and urea, is the first and committed step in polyamine biosynthesis in Leishmania. The creation of a conditionally lethal Δarg null mutant in Leishmania mexicana has established that ARG is an essential enzyme for the promastigote form of the parasite and that the enzyme provides an important defense mechanism for parasite survival in the eukaryotic host. Furthermore, human ARGI (HsARGI) has also been implicated as a key factor in parasite proliferation. Thus, inhibitors of ARG offer a rational paradigm for drug design. To initiate a search for inhibitors of the L. mexicana ARG (LmARG), recombinant LmARG and HsARGI enzymes were purified from Escherichia coli. Both LmARG and HsARGI were specific for l-arginine and exhibited no activity with either d-arginine or agmatine as possible substrates. LmARG exhibited a K(m) of 25±4mM for l-arginine, a pH optimum ∼9.0, and was dependent upon the presence of a divalent cation, preferentially manganese. A K(m) of 13.5 ± 2mM for l-arginine was calculated for the HsARGI. A collection of 37 compounds was evaluated against both enzymes. Twelve of these compounds were identified as being either strong inhibitors of both LmARG and HsARGI or differential inhibitors between the two enzymes. Of the 12 compounds, six were selected for further analysis and the type and extent of inhibition determined.

Genetic and biochemical analysis of protozoal polyamine transporters

Polyamines are aliphatic polycations that function in key cellular processes such as growth, differentiation, and macromolecular biosynthesis. Intracellular polyamines pools are maintained from de novo synthesis and from transport of polyamines from the extracellular milieu. This acquisition of exogenous polyamines is mediated by cell surface transporter proteins. Protozoan parasites are the etiologic agents of a plethora of devastating and often fatal diseases in humans and their domestic animals. These pathogens accommodate de novo and/or salvage mechanisms for polyamine acquisition. Because of its therapeutic relevance, the polyamine biosynthetic pathway has been thoroughly investigated in many genera of protozoan parasites, but the polyamine permeation pathways have generally been ignored. Our group has now identified at the molecular level polyamine transporters from two species of protozoan parasites, Leishmania major and Trypanosoma cruzi, characterized these polytopic proteins with respect to ligand specificities and affinities, and determined the subcellular environments in which these transporters reside.

Oral putrescine restores virulence of ornithine decarboxylase-deficient Leishmania donovani in mice

Administration of putrescine as a 1% solution in the drinking water ameliorated the profound loss of virulence exhibited by ornithine decarboxylase (ODC) deficient Leishmania donovani in mice. Furthermore, supplying α-difluoromethylornithine, an ODC inhibitor, at 2% in the drinking water reduced but did not eliminate infection with wild type L. donovani in the mouse model. Taken collectively, these findings: (1) demonstrate that oral putrescine can access the phagolysosome of macrophages in which the parasite resides in mice; (2) establish that the loss of virulence due to the Δodc lesion is a consequence of the inability of the mutant parasite to synthesize sufficient polyamines de novo; (3) imply that the L. donovani amastigote cannot access host polyamines in sufficient amounts for survival and growth; (4) and validate ODC as a drug target, although oral administration of DFMO is an unlikely therapeutic paradigm for visceral leishmaniasis.

A rapid, efficient and economical method for generating leishmanial gene targeting constructs

Targeted gene replacement is a powerful tool in Leishmania genetics that can be time-consuming to implement. One tedious aspect that delays progress is the multi-step construction of gene targeting vectors. To accelerate this process, we developed a streamlined method that allows the assembly of a complete targeting vector from all its constituent parts in a single-step multi-fragment ligation. The individual components to be assembled are flanked by sites for the restriction endonuclease SfiI that generates non-identical, non-palindromic three base 3'-overhangs designed to allow annealing and ligation of the parts only in the proper order. The method was optimized by generating constructs for targeting the Leishmania donovani inosine monophosphate dehydrogenase gene (LdIMPDH) encoding six different drug resistance markers, and was found to be rapid and efficient. These constructs were successfully employed to generate heterozygous LdIMPDH gene replacement mutants. This method is adaptable for generating targeting vectors for a variety of species.

Amplification of adenine phosphoribosyltransferase suppresses the conditionally lethal growth and virulence phenotype of Leishmania donovani mutants lacking both hypoxanthine-guanine and xanthine phosphoribosyltransferases

Leishmania donovani cannot synthesize purines de novo and obligatorily scavenge purines from the host. Previously, we described a conditional lethal Deltahgprt/Deltaxprt mutant of L. donovani (Boitz, J. M., and Ullman, B. (2006) J. Biol. Chem. 281, 16084-16089) that establishes that L. donovani salvages purines primarily through hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and xanthine phosphoribosyltransferase (XPRT). Unlike wild type L. donovani, the Deltahgprt/Deltaxprt knock-out cannot grow on 6-oxypurines and displays an absolute requirement for adenine or adenosine and 2'-deoxycoformycin, an inhibitor of parasite adenine aminohydrolase activity. Here, we demonstrate that the ability of Deltahgprt/Deltaxprt parasites to infect mice was profoundly compromised. Surprisingly, mutant parasites that survived the initial passage through mice partially regained their virulence properties, exhibiting a >10-fold increase in parasite burden in a subsequent mouse infection. To dissect the mechanism by which Deltahgprt/Deltaxprt parasites persisted in vivo, suppressor strains that had regained their capacity to grow under restrictive conditions were cloned from cultured Deltahgprt/Deltaxprt parasites. The ability of these suppressor clones to grow in and metabolize 6-oxypurines could be ascribed to a marked amplification and overexpression of the adenine phosphoribosyltransferase (APRT) gene. Moreover, transfection of Deltahgprt/Deltaxprt cells with an APRT episome recapitulated the suppressor phenotype in vitro and enabled growth on 6-oxypurines. Biochemical studies further showed that hypoxanthine, unexpectedly, was an inefficient substrate for APRT, evidence that could account for the ability of the suppressors to metabolize hypoxanthine. Subsequent analysis implied that APRT amplification was also a potential contributory mechanism by which Deltahgprt/Deltaxprt parasites displayed persistence and increased virulence in mice.

A high-affinity putrescine-cadaverine transporter from Trypanosoma cruzi

Whereas mammalian cells and most other organisms can synthesize polyamines from basic amino acids, the protozoan parasite Trypanosoma cruzi is incapable of polyamine biosynthesis de novo and therefore obligatorily relies upon putrescine acquisition from the host to meet its nutritional requirements. The cell surface proteins that mediate polyamine transport into T. cruzi, as well as most eukaryotes, however, have by-in-large eluded discovery at the molecular level. Here we report the identification and functional characterization of two polyamine transporters, TcPOT1.1 and TcPOT1.2, encoded by alleles from two T. cruzi haplotypes. Overexpression of the TcPOT1.1 and TcPOT1.2 genes in T. cruzi epimastigotes revealed that TcPOT1.1 and TcPOT1.2 were high-affinity transporters that recognized both putrescine and cadaverine but not spermidine or spermine. Furthermore, the activities and subcellular locations of both TcPOT1.1 and TcPOT1.2 in intact parasites were profoundly influenced by extracellular putrescine availability. These results establish TcPOT1.1 and TcPOT1.2 as key components of the T. cruzi polyamine transport pathway, an indispensable nutritional function for the parasite that may be amenable to therapeutic manipulation.

Role of transmembrane domain 4 in ligand permeation by Crithidia fasciculata equilibrative nucleoside transporter 2 (CfNT2)

Equilibrative nucleoside transporters play essential roles in nutrient uptake, cardiovascular and renal function, and purine analog drug chemotherapies. Limited structural information is available for this family of transporters; however, residues in transmembrane domains 1, 2, 4, and 5 appear to be important for ligand and inhibitor binding. In order to identify regions of the transporter that are important for ligand specificity, a genetic selection for mutants of the inosine-guanosine-specific Crithidia fasciculata nucleoside transporter 2 (CfNT2) that had gained the ability to transport adenosine was carried out in the yeast Saccharomyces cerevisiae. Nearly all positive clones from the genetic selection carried mutations at lysine 155 in transmembrane domain 4, highlighting lysine 155 as a pivotal residue governing the ligand specificity of CfNT2. Mutation of lysine 155 to asparagine conferred affinity for adenosine on the mutant transporter at the expense of inosine and guanosine affinity due to weakened contacts to the purine ring of the ligand. Following systematic cysteine-scanning mutagenesis, thiol-specific modification of several positions within transmembrane domain 4 was found to interfere with inosine transport capability, indicating that this helix lines the water-filled ligand translocation channel. Additionally, the pattern of modification of transmembrane domain 4 suggested that it may deviate from helicity in the vicinity of residue 155. Position 155 was also protected from modification in the presence of ligand, suggesting that lysine 155 is in or near the ligand binding site. Transmembrane domain 4 and particularly lysine 155 appear to play key roles in ligand discrimination and translocation by CfNT2.

Arginine homeostasis and transport in the human pathogen Leishmania donovani

Arginine is an essential amino acid for the human pathogen Leishmania but not to its host. Thus, the mechanism by which this protozoan parasite regulates cellular homeostasis of arginine is critical for its survival and virulence. In a previous study, we cloned and functionally characterized a high affinity arginine-specific transporter, LdAAP3, from Leishmania donovani. In this investigation, we have characterized the relationship between arginine transport via LdAAP3 and amino acid availability. Starving promastigotes for amino acids decreased the cellular level of most amino acids including arginine but also increased the abundance of both LdAAP3 mRNA and protein and up-regulated arginine transport activity. Genetic obliteration of the polyamine biosynthesis pathway for which arginine is the sole precursor caused a significant decrease in the rate of arginine transport. Cumulatively, we established that LdAAP3 expression and activity changed whenever the cellular level of arginine changed. Our findings have led to the hypothesis that L. donovani promastigotes have a signaling pathway that senses cellular concentrations of arginine and subsequently activates a mechanism that regulates LdAAP3 expression and activity. Interestingly, this response of LdAAP3 to amino acid availability in L. donovani is identical to that of the mammalian cation amino acid transporter 1. Thus, we conjecture that Leishmania mimics the host response to amino acid availability to improve virulence.

Purine and pyrimidine metabolism in Leishmania

Purines and pyrimidines are indispensable to all life, performing many vital functions for cells: ATP serves as the universal currency of cellular energy, cAMP and cGMP are key second messenger molecules, purine and pyrimidine nucleotides are precursors for activated forms of both carbohydrates and lipids, nucleotide derivatives of vitamins are essential cofactors in metabolic processes, and nucleoside triphosphates are the immediate precursors for DNA and RNA synthesis. Unlike their mammalian and insect hosts, Leishmania lack the metabolic machinery to make purine nucleotides de novo and must rely on their host for preformed purines. The obligatory nature of purine salvage offers, therefore, a plethora of potential targets for drug targeting, and the pathway has consequently been the focus of considerable scientific investigation. In contrast, Leishmania are prototrophic for pyrimidines and also express a small complement of pyrimidine salvage enzymes. Because the pyrimidine nucleotide biosynthetic pathways of Leishmania and humans are similar, pyrimidine metabolism in Leishmania has generally been considered less amenable to therapeutic manipulation than the purine salvage pathway. However, evidence garnered from a variety of parasitic protozoa suggests that the selective inhibition of pyrimidine biosynthetic enzymes offers a rational therapeutic paradigm. In this chapter, we present an overview of the purine and pyrimidine pathways in Leishmania, make comparisons to the equivalent pathways in their mammalian host, and explore how these pathways might be amenable to selective therapeutic targeting.

Genetic evidence for the essential role of PfNT1 in the transport and utilization of xanthine, guanine, guanosine and adenine by Plasmodium falciparum

The malaria parasite, Plasmodium falciparum, is unable to synthesize the purine ring de novo and is therefore wholly dependent upon purine salvage from the host for survival. Previous studies have indicated that a P. falciparum strain in which the purine transporter PfNT1 had been disrupted was unable to grow on physiological concentrations of adenosine, inosine and hypoxanthine. We have now used an episomally complemented pfnt1Delta knockout parasite strain to confirm genetically the functional role of PfNT1 in P. falciparum purine uptake and utilization. Episomal complementation by PfNT1 restored the ability of pfnt1Delta parasites to transport and utilize adenosine, inosine and hypoxanthine as purine sources. The ability of wild-type and pfnt1Delta knockout parasites to transport and utilize the other physiologically relevant purines adenine, guanine, guanosine and xanthine was also examined. Unlike wild-type and complemented P. falciparum parasites, pfnt1Delta parasites could not proliferate on guanine, guanosine or xanthine as purine sources, and no significant transport of these substrates could be detected in isolated parasites. Interestingly, whereas isolated pfnt1Delta parasites were still capable of adenine transport, these parasites grew only when adenine was provided at high, non-physiological concentrations. Taken together these results demonstrate that, in addition to hypoxanthine, inosine and adenosine, PfNT1 is essential for the transport and utilization of xanthine, guanine and guanosine.

An effect of parasite-encoded arginase on the outcome of murine cutaneous leishmaniasis

Classical activation of macrophages infected with Leishmania species results in expression and activation of inducible NO synthase (iNOS) leading to intracellular parasite killing. Macrophages can contrastingly undergo alternative activation with increased arginase activity, metabolism of arginine along the polyamine pathway, and consequent parasite survival. An active role for parasite-encoded arginase in host microbicidal responses has not previously been documented. To test the hypothesis that parasite-encoded arginase can influence macrophage responses to intracellular Leishmania, a comparative genetic approach featuring arginase-deficient mutants of L. mexicana lacking both alleles of the gene encoding arginase (Deltaarg), as well as wild-type and complemented Deltaarg controls (Deltaarg[pArg]), was implemented. The studies showed: 1) the absence of parasite arginase resulted in a significantly attenuated infection of mice (p<0.05); 2) poorer survival of Deltaarg in mouse macrophages than controls correlated with greater NO generation; 3) the difference between Deltaarg or control intracellular survival was abrogated in iNOS-deficient macrophages, suggesting iNOS activity was responsible for increased Deltaarg killing; 4) consistently, immunohistochemistry showed enhanced nitrotyrosine modifications in tissues of mice infected with Deltaarg compared with control parasites. Furthermore, 5) in the face of decreased parasite survival, lymph node cells draining cutaneous lesions of Deltaarg parasites produced more IFN-gamma and less IL-4 and IL-10 than controls. These data intimate that parasite-encoded arginase of Leishmania mexicana subverts macrophage microbicidal activity by diverting arginine away from iNOS.

Genetic selection for a highly functional cysteine-less membrane protein using site saturation mutagenesis

We describe an efficient method for generating highly functional membrane proteins with variant amino acids at defined positions that couples a modified site saturation strategy with functional genetic selection. We applied this method to the production of a cysteine-less variant of the Crithidia fasciculata inosine-guanosine permease CfNT2 to facilitate biochemical studies using thiol-specific modifying reagents. Of 10 endogenous cysteine residues in CfNT2, two cannot be replaced with serine or alanine without loss of function. High-quality single- and double-mutant libraries were produced by combining a previously reported site saturation mutagenesis scheme based on the Stratagene Quikchange method with a novel gel purification step that effectively eliminated template DNA from the products. Following selection for functional complementation in Saccharomyces cerevisiae cells auxotrophic for purines, several highly functional noncysteine substitutions were efficiently identified at each desired position, allowing the construction of cysteine-less variants of CfNT2 that retained wild-type affinity for inosine. This combination of an improved site saturation mutagenesis technique and positive genetic selection provides a simple and efficient means to identify functional and perhaps unexpected amino acid variants at a desired position.

Leishmania donovani polyamine biosynthetic enzyme overproducers as tools to investigate the mode of action of cytotoxic polyamine analogs

A number of anticancer and antiparasitic drugs are postulated to target the polyamine biosynthetic pathway and polyamine function, but the exact mode of action of these compounds is still being elucidated. To establish whether polyamine analogs specifically target enzymes of the polyamine pathway, a model was developed using strains of the protozoan parasite Leishmania donovani that overproduce each of the polyamine biosynthetic enzymes. Promastigotes overexpressing episomal constructs encoding ornithine decarboxylase (ODC), S-adenosylmethionine decarboxylase (ADOMETDC), or spermidine synthase (SPDSYN) revealed robust overproduction of the corresponding polyamine biosynthetic enzyme. Polyamine pools, however, were either unchanged or only marginally affected, implying that regulatory mechanisms must exist. The ODC, ADOMETDC, and SPDSYN overproducer strains exhibited a high level of resistance to difluoromethylornithine, 5'-{[(Z)-4-amino-2-butenyl]methylamino}-5'-deoxyadenosine, and n-butylamine, respectively, confirming previous observations that these agents specifically target polyamine enzymes. Conversely, augmented levels of polyamine biosynthetic enzymes did not affect the sensitivity of L. donovani promastigotes to pentamidine, berenil, and mitoguazone, drugs that were postulated to target the polyamine pathway, implying alternative and/or additional targets for these agents. The sensitivities of wild-type and overproducing parasites to a variety of polyamine analogs were also tested. The polyamine enzyme-overproducing lines offer a rapid cell-based screen for assessing whether synthetic polyamine analogs exert their mechanism of action predominantly on the polyamine biosynthetic pathway in L. donovani. Furthermore, the drug resistance engendered by the amplification of target genes and the overproduction of the encoded protein offers a general strategy for evaluating and developing therapeutic agents that target specific proteins in Leishmania.

Functional characterization of nucleoside transporter gene replacements in Leishmania donovani

Leishmania donovani express two nucleoside transporters of non-overlapping ligand selectivity. To evaluate the physiological role of nucleoside transporters in L. donovani, homozygous null mutants of the genes encoding the LdNT1 adenosine-pyrimidine nucleoside transporter and the LdNT2 inosine-guanosine transporter were created singly and in combination by single targeted gene replacement followed by selection for loss-of-heterozygosity. The mutant alleles were verified by Southern blotting, and the effects of gene replacement on transport phenotype were evaluated by rapid sampling transport measurements and by drug resistance profiles. The Deltaldnt1, Deltaldnt2, and Deltaldnt1/Deltaldnt2 mutants were all capable of proliferation in defined culture medium supplemented with any of a spectrum of purine nucleobases or nucleosides, except that a Deltaldnt2 lesion conferred an inability to efficiently salvage exogenous xanthosine, a newly discovered ligand of LdNT2. Each of the three knockout strains was viable as promastigotes and axenic amastigotes and capable of maintaining an infection in J774 and bone marrow-derived murine macrophages. These genetic studies demonstrate: (1) that L. donovani promastigotes, axenic amastigotes, and tissue amastigotes are viable in the absence of nucleoside transport; (2) that nucleoside transporters are not essential for sustaining an infection in mammalian host cells; (3) that the phagolysosome of macrophages is likely to contain purines that are not LdNT1 or LdNT2 ligands, i.e., nucleobases. Furthermore, the Deltaldnt1, Deltaldnt2, and Deltaldnt1/Deltaldnt2 knockouts offer a unique genetically defined null background for the biochemical and genetic characterization of nucleoside transporter genes and cDNAs from phylogenetically diverse species and of genetically manipulated LdNT1 and LdNT2 constructs.

Leishmania donovani singly deficient in HGPRT, APRT or XPRT are viable in vitro and within mammalian macrophages

Leishmania species express three phosphoribosyltransferase enzymes, hypoxanthine-guanine phosphoribosyltransferase (HGPRT), adenine phosphoribosyltransferase (APRT), and xanthine phosphoribosyltransferase (XPRT), which enable this genus to acquire purine nutrients from their hosts. To test whether any of these enzymes is essential for viability, transformation into amastigotes, and infectivity and proliferation within mammalian macrophages, Deltahgprt, Deltaaprt, and Deltaxprt null mutants were created by targeted gene replacement within a virulent background of Leishmania donovani. Each of the three knockout strains was viable as promastigotes and axenic amastigotes and capable of maintaining an infection in bone marrow-derived murine macrophages. These data support the hypothesis that none of the three phosphoribosyltransferases is essential for purine salvage or viability by itself and that purine salvage occurs through multiple anabolic routes in both parasite life cycle stages. In addition these studies revealed the presence of an adenine aminohydrolase enzyme in L. donovani axenic amastigotes, an activity previously thought to be restricted to promastigotes.

Adenosine kinase from Cryptosporidium parvum

Analysis of the Cryptosporidium parvum genome demonstrates that the parasite cannot synthesize purines de novo and reveals that the sole route for purine salvage by the parasite is via adenosine kinase (CpAK). In order to initiate a biochemical characterization of CpAK and ultimately validate this apparently essential enzyme as a therapeutic target, the CpAK gene was redesigned for optimum codon usage, overexpressed in Escherichia coli, and the recombinant protein purified to homogeneity and characterized. CpAK appears to be specific for adenosine among the naturally occurring nucleosides but can utilize ATP, GTP, UTP and CTP as the phosphate donor. The enzyme exhibits K(m) values of 1.4microM for adenosine and 41microM for ATP, has a pH optimum approximately 7.0, and is dependent upon the presence of a divalent cation. Structure-activity data intimate that catalysis requires contacts between residues on CpAK with the six-position of the purine ring and the O2' and O3' hydroxyls of the ribose sugar. Additionally, 4-nitro-6-benzylthioinosine, a compound that demonstrates therapeutic promise against the related parasite Toxoplasma gondii, also inhibits adenosine phosphorylation by CpAK. The overproduction and purification of CpAK now enables a thorough evaluation of its potential as a drug target.

Comprehensive examination of charged intramembrane residues in a nucleoside transporter

Permeases of the equilibrative nucleoside transporter family mediate the uptake of nucleosides and/or nucleobases in a diverse array of eukaryotes and transport a host of drugs used for treatment of cancer, heart disease, AIDS, and parasitic infections. To identify residues that play central roles in transport function, we have systematically substituted by site-directed mutagenesis all the charged residues located within predicted transmembrane domains of the Leishmania donovani nucleoside transporter 1.1, LdNT1.1, which transports adenosine and the pyrimidine nucleosides. Substitution of three of these ten residues by uncharged amino acids resulted in loss of >95% transport activity, and we hence designated them "key" residues. These amino acids were Glu94, Lys153, and Arg404 located in transmembrane domains 2, 4, and 9, respectively. In addition, previous studies on the related LdNT2 inosine/guanosine transporter identified the highly conserved Asp389 and Arg393 (equivalent to Asp374 and Arg378 in LdNT1.1) in transmembrane domain 8 as key residues. Among these residues, the mutants in Arg393 (LdNT2) and Arg404 were strongly impaired in trafficking to the plasma membrane, but the other mutants were expressed with high to moderate efficiency at the cell surface, indicating that their mutation impaired transport activity per se. A conservative K153R substitution exhibited a change in substrate specificity, acquiring the ability to transport inosine, a nucleoside that is not a substrate for the wild-type LdNT1.1 permease. These results imply that the Glu94, Lys153, and Asp374 residues may play central roles in the mechanism of substrate translocation in LdNT1.1.

The plasma membrane permease PfNT1 is essential for purine salvage in the human malaria parasite Plasmodium falciparum

The human malaria parasite Plasmodium falciparum relies on the acquisition of host purines for its survival within human erythrocytes. Purine salvage by the parasite requires specialized transporters at the parasite plasma membrane (PPM), but the exact mechanism of purine entry into the infected erythrocyte, and the primary purine source used by the parasite, remain unknown. Here, we report that transgenic parasites lacking the PPM transporter PfNT1 (P. falciparum nucleoside transporter 1) are auxotrophic for hypoxanthine, inosine, and adenosine under physiological conditions and are viable only if these normally essential nutrients are provided at excess concentrations. Transport measurements across the PPM revealed a severe reduction in hypoxanthine uptake in the knockout, whereas adenosine and inosine transport were only partially affected. These data provide compelling evidence for a sequential pathway for exogenous purine conversion into hypoxanthine using host enzymes followed by PfNT1-mediated transport into the parasite. The phenotype of the conditionally lethal mutant establishes PfNT1 as a critical component of purine salvage in P. falciparum and validates PfNT1 as a potential therapeutic target.

A Conditional Mutant Deficient in Hypoxanthine-guanine Phosphoribosyltransferase and Xanthine Phosphoribosyltransferase Validates the Purine Salvage Pathway of Leishmania donovani

Leishmania donovani cannot synthesize purines de novo and express a multiplicity of enzymes that enable them to salvage purines from their hosts. Previous efforts to generate an L. donovani strain deficient in both hypoxanthine-guanine phosphoribosyl-transferase (HGPRT) and xanthine phosphoribosyltransferase (XPRT) using gene replacement approaches were not successful, lending indirect support to the hypothesis that either HGPRT or XPRT is crucial for purine salvage by the parasite. We now report the genetic confirmation of this hypothesis through the construction of a conditional delta hgprt/delta xprt mutant strain that exhibits an absolute requirement for 2'-deoxycoformycin, an inhibitor of the leishmanial adenine aminohydrolase enzyme, and either adenine or adenosine as a source of purine. Unlike wild type parasites, the delta hgprt/delta xprt strain cannot proliferate indefinitely without 2'-deoxycoformycin or with hypoxanthine, guanine, xanthine, guanosine, inosine, or xanthosine as the sole purine nutrient. The delta hgprt/delta xprt mutant infects murine bone marrow-derived macrophages <5% as effectively as wild type parasites and cannot sustain an infection. These data establish genetically that either HGPRT or XPRT is absolutely essential for purine acquisition, parasite viability, and parasite infectivity of mouse macrophages, that all exogenous purines are funneled to hypoxanthine and/or xanthine by L. donovani, and that the purine sources within the macrophage to which the parasites have access are HGPRT or XPRT substrates.

Point mutations within the LdNT2 nucleoside transporter gene from Leishmania donovani confer drug resistance and transport deficiency

Leishmania donovani, a protozoan parasite, expresses an unusual inosine/guanosine-specific transporter, LdNT2, the gene for which was cloned by functional rescue of a drug-resistant, LdNT2-deficient (FBD5) strain. In this investigation, we have uncovered and characterized the mutations within the LdNT2 open reading frame that are the basis for the drug-resistance and transport-incompetent phenotype of the FBD5 line. The FBD5 cells were shown to be compound heterozygotes in which both mutant ldnt2 alleles harbor discrete point mutations, each of which impaired transport function and conferred resistance to formycin B, the drug to which the clonal FBD5 line was selected. One of the mutant ldnt2 alleles encoded an S189L alteration in predicted transmembrane domain 5, while the second allele accommodated a null mutation at codon 376, which truncated the transporter just prior to transmembrane domain 8. In addition to the null transport phenotype, very little S189L ldnt2 mutant transporter targeted to the surface of the parasite. The bulk of the truncated ldnt2 appeared to be sequestered internally, possibly within the endoplasmic reticulum, but some of the truncated transporter seemed to be cell surface exposed. The ability to dissect mutations within a viable parasite offers LdNT2 as an attractive model for implementing a thorough forward genetic dissection of transporter function in a eukaryotic cell.

Differential localization of alternatively spliced hypoxanthine-xanthine-guanine phosphoribosyltransferase isoforms in Toxoplasma gondii

A unique feature of the Toxoplasma gondii purine salvage pathway is the expression of two isoforms of the hypoxanthine-xanthine-guanine phosophoribosyltransferase (HXGPRT) of the parasite encoded by a single genetic locus. These isoforms differ in the presence or absence of a 49-amino acid insertion (which is specified by a single differentially spliced exon) but exhibit similar substrate specificity, kinetic characteristics, and temporal expression patterns. To examine possible functional differences between the two HXGPRT isoforms, fluorescent protein fusions were expressed in parasites lacking the endogenous hxgprt gene. Immunoblot analysis of fractionated cell extracts and fluorescence microscopy indicated that HXGPRT-I (which lacks the 49-amino acid insertion) is found in the cytosol, whereas HXGPRT-II (which contains the insertion) localizes to the inner membrane complex (IMC) of the parasite. Simultaneous expression of both isoforms resulted in the formation of hetero-oligomers, which distributed between the cytosol and IMC. Chimeric constructs expressing N-terminal peptides from either isoform I (11 amino acids) or isoform II (60 amino acids) fused to a chloramphenicol acetyl transferase (CAT) reporter demonstrated that the N-terminal domain of isoform II is both necessary and sufficient for membrane association. Metabolic labeling experiments with transgenic parasites showed that isoform II or an isoform II-CAT fusion protein (but not isoform I or isoform I-CAT) incorporate [(3)H]palmitate. Mutation of three adjacent cysteine residues within the isoform II-targeting domain to serines blocked both palmitate incorporation and IMC attachment without affecting enzyme activity, demonstrating that acylation of N-terminal isoform II cysteine residues is responsible for the association of HXGPRT-II with the IMC.

Identification and Characterization of a Polyamine Permease from the Protozoan Parasite Leishmania major

The proteins that mediate polyamine translocation into eukaryotic cells have not been identified at the molecular level. To define the polyamine transport pathways in eukaryotic cells we have cloned a gene, LmPOT1, that encodes a polyamine transporter from the protozoan pathogen, Leishmania major. Sequence analysis of LmPOT1 predicted an unusual 803-residue polytopic protein with 9-12 transmembrane domains. Expression of LmPOT1 cRNA in Xenopus laevis oocytes revealed LmPOT1 to be a high affinity transporter for both putrescine and spermidine, whereas expression of LmPOT1 in Trypanosoma brucei stimulated putrescine uptake that was sensitive to inhibition by pentamidine and proton ionophores. Immunoblot analysis established that LmPOT1 was expressed predominantly in the insect vector form of L. major, and immunofluorescence demonstrated that LmPOT1 was localized predominantly to the parasite plasma membrane. To our knowledge this is the first molecular identification and characterization of a cell surface polyamine transporter in eukaryotic cells.

Identification and characterization of purine nucleoside transporters from Crithidia fasciculata

To initiate a molecular dissection into the mechanism by which purine transport is up-regulated in Crithidia, genes encoding nucleoside transporters from Crithidia fasciculata were cloned and functionally characterized. Sequence analysis revealed CfNT1 and CfNT2 to be members of the equilibrative nucleoside transporter family, and the genes isolated encompassed polypeptides of 497 and 502 amino acids, respectively, each with 11 predicted membrane-spanning domains. Heterologous expression of CfNT1 cRNA in Xenopus laevis oocytes or CfNT2 in nucleoside transport-deficient Leishmania donovani demonstrated that CfNT1 is a novel high affinity adenosine transporter that also recognizes inosine, hypoxanthine, and pyrimidine nucleosides, while CfNT2 is a high affinity permease specific for inosine and guanosine. Southern blot analysis revealed that CfNT2 is present as a single copy within the C. fasciculata genome. Starvation of parasites for purines increased CfNT2 transport activity by an order of magnitude, although Northern blot analysis indicated CfNT2 transcript levels increased by <2-fold. These data imply that this metabolic adaptation can mainly be ascribed to post-transcriptional events. Conversely, Southern analysis of CfNT1 suggests that it is a member of a highly homologous multi-copy gene family, indicating that adenosine transport by C. fasciculata is more complex than previously thought.

Second-site suppression of a nonfunctional mutation within the Leishmania donovani inosine-guanosine transporter

LdNT2 is a member of the equilibrative nucleoside transporter family, which possesses several conserved residues located mainly within transmembrane domains. One of these residues, Asp(389) within LdNT2, was shown previously to be critical for transporter function without affecting ligand affinity or plasma membrane targeting. To further delineate the role of Asp(389) in LdNT2 function, second-site suppressors of the ldnt2-D389N null mutation were selected in yeast deficient in purine nucleoside transport and incapable of purine biosynthesis. A library of random mutants within the ldnt2-D389N background was screened in yeast for restoration of growth on inosine. Twelve different clones were obtained, each containing secondary mutations enabling inosine transport. One mutation, N175I, occurred in four clones and conferred augmented inosine transport capability compared with LdNT2 in yeast. N175I was subsequently introduced into an ldnt2-D389N construct tagged with green fluorescent protein and transfected into a Deltaldnt1/Deltaldnt2 Leishmania donovani knockout. GFP-N175I/D389N significantly suppressed the D389N phenotype and targeted properly to the plasma membrane and flagellum. Most interestingly, N175I increased the inosine K(m) by 10-fold within the D389N background relative to wild type GFP-LdNT2. Additional substitutions introduced at Asn(175) established that only large, nonpolar amino acids suppressed the D389N phenotype, indicating that suppression by Asn(175) has a specific size and charge requirement. Because multiple suppressor mutations alleviate the constraint imparted by the D389N mutation, these data suggest that Asp(389) is a conformationally sensitive residue. To impart spatial information to the clustering of second-site mutations, a three-dimensional model was constructed based upon members of the major facilitator superfamily using threading analysis. The model indicates that Asn(175) and Asp(389) lie in close proximity and that the second-site suppressor mutations cluster to one region of the transporter.