Functional role of highly conserved residues of the N-terminal tail and first transmembrane segment of a P4-ATPase.

The P4 family of P-type ATPases (P4-ATPases) plays an important role in maintaining phospholipid asymmetry in eukaryotic cell membranes. Leishmania miltefosine transporter (LMT) is a plasma membrane (PM) P4-ATPase that catalyses translocation into the parasite of the leishmanicidal drug miltefosine as well as phosphatidylcholine and phosphatidylethanolamine analogues. In the present study, we analysed the role, in LMT, of a series of highly conserved amino acids previously undescribed in the N-terminal region of P4-ATPases. Seven residues were identified and, according to an LMT structural model, five were located in the cytosolic N-terminal tail (Asn58, Ile60, Lys64, Tyr65 and Phe70) and the other two (Pro72 and Phe79) in the first transmembrane segment (TM1). Alanine-scanning mutagenesis analysis showed that N58A, Y65A and F79A mutations caused a considerable reduction in the LMT translocase activity. These mutations did not affect protein expression levels. We generated additional mutations in these three residues to assess the influence of the conservation degree on LMT translocase activity. Some of these mutations reduced expression levels without affecting the interaction between LMT and its CDC50 subunit, LRos3. Conserved and non-conserved mutations in the invariant residue Asn58 drastically reduced the translocase activity. Consequently, Asn58 may be necessary to achieve optimal catalytic LMT activity as previously described for the potentially equivalent Asn39 of the sarco/endoplasmic reticulum Ca2+-ATPase isoform 1a (SERCA1a). Additionally, conservation of a hydrophobic residue at position 79 is crucial for LMT stability.

Trypanosomatid parasites rescue heme from endocytosed hemoglobin through lysosomal HRG transporters.

Pathogenic trypanosomatid parasites are auxotrophic for heme and they must scavenge it from their human host. Trypanosoma brucei (responsible for sleeping sickness) and Leishmania (leishmaniasis) can fulfill heme requirement by receptor-mediated endocytosis of host hemoglobin. However, the mechanism used to transfer hemoglobin-derived heme from the lysosome to the cytosol remains unknown. Here we provide strong evidence that HRG transporters mediate this essential step. In bloodstream T. brucei, TbHRG localizes to the endolysosomal compartment where endocytosed hemoglobin is known to be trafficked. TbHRG overexpression increases cytosolic heme levels whereas its downregulation is lethal for the parasites unless they express the Leishmania orthologue LmHR1. LmHR1, known to be an essential plasma membrane protein responsible for the uptake of free heme in Leishmania, is also present in its acidic compartments which colocalize with endocytosed hemoglobin. Moreover, LmHR1 levels modulated by its overexpression or the abrogation of an LmHR1 allele correlate with the mitochondrial bioavailability of heme from lysosomal hemoglobin. In addition, using heme auxotrophic yeasts we show that TbHRG and LmHR1 transport hemoglobin-derived heme from the digestive vacuole to the cytosol. Collectively, these results show that trypanosomatid parasites rescue heme from endocytosed hemoglobin through endolysosomal HRG transporters, which could constitute novel drug targets.

Genomic and Molecular Characterization of Miltefosine Resistance in Leishmania infantum Strains with Either Natural or Acquired Resistance through Experimental Selection of Intracellular Amastigotes.

During the last decade miltefosine (MIL) has been used as first-line treatment for visceral leishmaniasis in endemic areas with antimonial resistance, but a decline in clinical effectiveness is now being reported. While only two MIL-resistant Leishmania infantum strains from HIV co-infected patients have been documented, phenotypic MIL-resistance for L. donovani has not yet been identified in the laboratory. Hence, a better understanding of the factors contributing to increased MIL-treatment failure is necessary. Given the paucity of defined MIL-resistant L. donovani clinical isolates, this study used an experimental amastigote-selected MIL-resistant L. infantum isolate (LEM3323). In-depth exploration of the MIL-resistant phenotype was performed by coupling genomic with phenotypic data to gain insight into gene function and the mutant phenotype. A naturally MIL-resistant L. infantum clinical isolate (LEM5159) was included to compare both datasets. Phenotypically, resistance was evaluated by determining intracellular amastigote susceptibility in vitro and actual MIL-uptake. Genomic analysis provided supportive evidence that the resistance selection model on intracellular amastigotes can be a good proxy for the in vivo field situation since both resistant strains showed mutations in the same inward transporter system responsible for the acquired MIL-resistant phenotype. In line with previous literature findings in promastigotes, our data confirm a defective import machinery through inactivation of the LiMT/LiRos3 protein complex as the main mechanism for MIL-resistance also in intracellular amastigotes. Whole genome sequencing analysis of LEM3323 revealed a 2 base pair deletion in the LiMT gene that led to the formation an early stop codon and a truncation of the LiMT protein. Interestingly, LEM5159 revealed mutations in both the LiMT and LiRos3 genes, resulting in an aberrant expression of the LiMT protein. To verify that these mutations were indeed accountable for the acquired resistance, transfection experiments were performed to re-establish MIL-susceptibility. In LEM3323, susceptibility was restored upon expression of a LiMT wild-type gene, whereas the MIL-susceptibility of LEM5159 could be reversed after expression of the LiRos3 wild-type gene. The aberrant expression profile of the LiMT protein could be restored upon rescue of the LiRos3 gene both in the LEM5159 clinical isolate and a ΔLiRos3 strain, showing that expression of LdMT is dependent on LdRos3 expression. The present findings clearly corroborate the pivotal role of the LiMT/LiRos3 complex in resistance towards MIL.

Optimization by Molecular Fine Tuning of Dihydro-ß-agarofuran Sesquiterpenoids as Reversers of P-Glycoprotein-Mediated Multidrug Resistance.

P-glycoprotein (P-gp) plays a crucial role in the development of multidrug resistance (MDR), a major obstacle for successful chemotherapy in cancer. Herein, we report on the development of a natural-product-based library of 81 dihydro-ß-agarofuran sesquiterpenes (2-82) by optimization of the lead compound 1. The compound library was evaluated for its ability to inhibit P-gp-mediated daunomycin efflux in MDR cells. Selected analogues were further analyzed for their P-gp inhibition constant, intrinsic toxicity, and potency to reverse daunomycin and vinblastine resistances. Analogues 6, 24, 28, 59, and 66 were identified as having higher potency than compound 1 and verapamil, a first-generation P-gp modulator. SAR analysis revealed the size of the aliphatic chains and presence of nitrogen atoms are important structural characteristics to modulate reversal activity. The present study highlights the potential of these analogues as modulators of P-gp mediated MDR in cancer cells.

LmABCB3, an atypical mitochondrial ABC transporter essential for Leishmania major virulence, acts in heme and cytosolic iron/sulfur clusters biogenesis.

BACKGROUND: Mitochondria play essential biological functions including the synthesis and trafficking of porphyrins and iron/sulfur clusters (ISC), processes that in mammals involve the mitochondrial ATP-Binding Cassette (ABC) transporters ABCB6 and ABCB7, respectively. The mitochondrion of pathogenic protozoan parasites such as Leishmania is a promising goal for new therapeutic approaches. Leishmania infects human macrophages producing the neglected tropical disease known as leishmaniasis. Like most trypanosomatid parasites, Leishmania is auxotrophous for heme and must acquire porphyrins from the host. METHODS: LmABCB3, a new Leishmania major protein with significant sequence similarity to human ABCB6/ABCB7, was identified and characterized using bioinformatic tools. Fluorescent microscopy was used to determine its cellular localization, and its level of expression was modulated by molecular genetic techniques. Intracellular in vitro assays were used to demonstrate its role in amastigotes replication, and an in vivo mouse model was used to analyze its role in virulence. Functional characterization of LmABCB3 was carried out in Leishmania promastigotes and Saccharomyces cerevisiae. Structural analysis of LmABCB3 was performed using molecular modeling software. RESULTS: LmABCB3 is an atypical ABC half-transporter that has a unique N-terminal extension not found in any other known ABC protein. This extension is required to target LmABCB3 to the mitochondrion and includes a potential metal-binding domain. We have shown that LmABCB3 interacts with porphyrins and is required for the mitochondrial synthesis of heme from a host precursor. We also present data supporting a role for LmABCB3 in the biogenesis of cytosolic ISC, essential cofactors for cell viability in all three kingdoms of life. LmABCB3 fully complemented the severe growth defect shown in yeast lacking ATM1, an orthologue of human ABCB7 involved in exporting from the mitochondria a gluthatione-containing compound required for the generation of cytosolic ISC. Indeed, docking analyzes performed with a LmABCB3 structural model using trypanothione, the main thiol in this parasite, as a ligand showed how both, LmABCB3 and yeast ATM1, contain a similar thiol-binding pocket. Additionally, we show solid evidence suggesting that LmABCB3 is an essential gene as dominant negative inhibition of LmABCB3 is lethal for the parasite. Moreover, the abrogation of only one allele of the gene did not impede promastigote growth in axenic culture but prevented the replication of intracellular amastigotes and the virulence of the parasites in a mouse model of cutaneous leishmaniasis. CONCLUSIONS: Altogether our results present the previously undescribed LmABCB3 as an unusual mitochondrial ABC transporter essential for Leishmania survival through its role in the generation of heme and cytosolic ISC. Hence, LmABCB3 could represent a novel target to combat leishmaniasis.

Restoration of Chemosensitivity in P-Glycoprotein-Dependent Multidrug-Resistant Cells by Dihydro-ß-agarofuran Sesquiterpenes from Celastrus vulcanicola.

Multidrug resistance (MDR) caused by the overexpression of ABC drug transporters is a major obstacle in clinical cancer chemotherapy and underlines the urgent need for the development of new, potent, and safe reversal agents. Toward this goal, reported herein are the structure elucidation and biological activity of nine new (1-9) and four known (10-13) dihydro-ß-agarofuran sesquiterpenes, isolated from the leaves of Celastrus vulcanicola, as reversers of MDR mediated by human P-glycoprotein expression. The structures of these compounds were elucidated by extensive NMR spectroscopic and mass spectrometric analysis, and their absolute configurations were determined by circular dichroism studies, chemical correlations (1a, 8a, and 8b), and biogenetic means. Four compounds from this series were discovered as potent chemosensitizers for MDR1-G185 NIH-3T3 murine cells (3, 4, 6, and 7), showing higher efficacies than the classical P-glycoprotein inhibitor verapamil, a first-generation chemosensitizer, when reversing resistance to daunomycin and vinblastine at the lowest concentration tested of 1 µM.

Functional role of evolutionarily highly conserved residues, N-glycosylation level and domains of the Leishmania miltefosine transporter-Cdc50 subunit.

Cdc50 (cell-cycle control protein 50) is a family of conserved eukaryotic proteins that interact with P4-ATPases (phospholipid translocases). Cdc50 association is essential for the endoplasmic reticulum export of P4-ATPases and proper translocase activity. In the present study, we analysed the role of Leishmania infantum LiRos3, the Cdc50 subunit of the P4-ATPase MLF (miltefosine) transporter [LiMT (L. infantum MLF transporter)], on trafficking and complex functionality using site-directed mutagenesis and domain substitution. We identified 22 invariant residues in the Cdc50 proteins from L. infantum, human and yeast. Seven of these residues are found in the extracellular domain of LiRos3, the conservation of which is critical for ensuring that LiMT arrives at the plasma membrane. The substitution of other invariant residues affects complex trafficking to a lesser extent. Furthermore, invariant residues located in the N-terminal cytosolic domain play a role in the transport activity. Partial N-glycosylation of LiRos3 reduces MLF transport and total N-deglycosylation completely inhibits LiMT trafficking to the plasma membrane. One of the N-glycosylation residues is invariant along the Cdc50 family. The transmembrane and exoplasmic domains are not interchangeable with the other two L. infantum Cdc50 proteins to maintain LiMT interaction. Taken together, these findings indicate that both invariant and N-glycosylated residues of LiRos3 are implicated in LiMT trafficking and transport activity.

Overcoming human P-glycoprotein-dependent multidrug resistance with novel dihydro-ß-agarofuran sesquiterpenes.

Sixteen (1-16) dihydro-ß-agarofuran sesquiterpenes were isolated from the fruits of Maytenus jelskii and evaluated against mammalian cells with a multidrug resistance phenotype mediated by the overexpression of the human P-glycoprotein. Their stereostructures have been elucidated on the basis of spectroscopic analysis, including 1D and 2D NMR techniques, CD studies, chemical correlations and biogenetic means. Eight compounds from this series were discovered as potent chemosensitizers (1, 2, 4, 6, 8, 9, 11 and 14), showing similar effectiveness to or higher than the classical P-glycoprotein reversal agent verapamil, a first-generation chemosensitizer, when reversing resistance to daunomycin and vinblastine. Detailed structure-activity relationships revealed that aromatic substituents at the 6 and 9-position of the sesquiterpene scaffold were able to modulate the intensity of inhibition.

Disruption of the lipid-transporting LdMT-LdRos3 complex in Leishmania donovani affects membrane lipid asymmetry but not host cell invasion.

Maintenance and regulation of the asymmetric lipid distribution across eukaryotic plasma membranes is governed by the concerted action of specific membrane proteins controlling lipid movement across the bilayer. Here, we show that the miltefosine transporter (LdMT), a member of the P4-ATPase subfamily in Leishmania donovani, and the Cdc50-like protein LdRos3 form a stable complex that plays an essential role in maintaining phospholipid asymmetry in the parasite plasma membrane. Loss of either LdMT or LdRos3 abolishes ATP-dependent transport of NBD-labelled phosphatidylethanolamine (PE) and phosphatidylcholine from the outer to the inner plasma membrane leaflet and results in an increased cell surface exposure of endogenous PE. We also find that promastigotes of L. donovani lack any detectable amount of phosphatidylserine (PS) but retain their infectivity in THP-1-derived macrophages. Likewise, infectivity was unchanged for parasites without LdMT-LdRos3 complexes. We conclude that exposure of PS and PE to the exoplasmic leaflet is not crucial for the infectivity of L. donovani promastigotes.

Low plasma membrane expression of the miltefosine transport complex renders Leishmania braziliensis refractory to the drug.

Miltefosine (hexadecylphosphocholine, MLF) is the first oral drug with recognized efficacy against both visceral and cutaneous leishmaniasis. However, some clinical studies have suggested that MLF shows significantly less efficiency against the cutaneous leishmaniasis caused by Leishmania braziliensis. In this work, we have determined the cellular and molecular basis for the natural MLF resistance observed in L. braziliensis. Four independent L. braziliensis clinical isolates showed a marked decrease in MLF sensitivity that was due to their inability to internalize the drug. MLF internalization in the highly sensitive L. donovani species requires at least two proteins in the plasma membrane, LdMT, a P-type ATPase involved in phospholipid translocation, and its beta subunit, LdRos3. Strikingly, L. braziliensis parasites showed highly reduced levels of this MLF translocation machinery at the plasma membrane, mainly because of the low expression levels of the beta subunit, LbRos3. Overexpression of LbRos3 induces increased MLF sensitivity not only in L. braziliensis promastigotes but also in intracellular amastigotes. These results further highlight the importance of the MLF translocation machinery in determining MLF potency and point toward the development of protocols to routinely monitor MLF susceptibility in geographic areas where L. braziliensis might be prevalent.

Inactivation of the miltefosine transporter, LdMT, causes miltefosine resistance that is conferred to the amastigote stage of Leishmania donovani and persists in vivo.

Miltefosine (hexadecylphosphocholine) is the first oral antileishmanial drug. In this study, we addressed the question whether miltefosine-resistant Leishmania donovani promastigotes transform to miltefosine-resistant amastigotes. A promastigote line, M-mutR, showed defective internalisation of miltefosine owing to mutations in LdMT, similar to previously described resistant lines. M-mutR parasites were infective to macrophages in vitro as well as in BALB/c mice in vivo. There was good correlation of in vitro resistance indices between promastigotes and intracellular amastigotes. Most importantly, M-mutR parasites retained the resistant phenotype in vivo, with no decrease of hepatic burden in BALB/c mice following miltefosine treatment up to 30 mg/kg (ca. 90% inhibition in wild-type infections). No cross-resistance to other antileishmanial drugs was observed in M-mutR amastigotes.

Mechanisms of experimental resistance of Leishmania to miltefosine: Implications for clinical use.

Miltefosine (hexadecylphosphocholine, MIL), registered as Impavido((R)), has become the first oral drug for the treatment of visceral and cutaneous leishmanasis. MIL is a simple molecule, very stable, relatively safe and highly efficient in clinical trials. However, MIL requires a long treatment course (28 days) and has a long half-life (around 150h), which might accelerate the emergence of drug resistance in case of inadequate use. The mechanisms of MIL resistance have been studied in vitro with experimental resistant lines. Resistance was shown to develop quickly in Leishmania promastigotes. Interestingly, a decreased MIL accumulation has always accounted for the resistance phenotype. The lower MIL accumulation can be achieved by two independent mechanisms: (i) an increase in drug efflux, mediated by the overexpression of the ABC transporter P-glycoprotein, and (ii) a decrease in drug uptake, which is easily achieved by the inactivation of any one of the two proteins known to be responsible for the MIL uptake, the MIL transporter LdMT and its beta subunit LdRos3. Policies concerning a proper use of this drug should be followed and supervised by health authorities of endemic areas to minimalize the risk for the appearance of drug failures and to ensure a long life span for this effective oral drug.

Phospholipid translocation and miltefosine potency require both L. donovani miltefosine transporter and the new protein LdRos3 in Leishmania parasites.

The antitumor drug miltefosine has been recently approved as the first oral drug active against visceral leishmaniasis. We have previously identified the L. donovani miltefosine transporter (LdMT) as a P-type ATPase involved in phospholipid translocation at the plasma membrane of Leishmania parasites. Here we show that this protein is essential but not sufficient for the phospholipid translocation activity and, thus, for the potency of the drug. Based on recent findings in yeast, we have identified the putative beta subunit of LdMT, named LdRos3, as another protein factor required for the translocation activity. LdRos3 belongs to the CDC50/Lem3 family, proposed as likely beta subunits for P4-ATPases. The phenotype of LdRos3-defective parasites was identical to that of the LdMT-/-, including a defect in the uptake of 7-nitrobenz-2-oxa-1,3-diazol-4-yl-amino)-phosphatidylserine, generally considered as not affected in Lem3p-deficient yeast. Both LdMT and LdRos3 normally localized to the plasma membrane but were retained inside the endoplasmic reticulum in the absence of the other protein or when inactivating point mutations were introduced in LdMT. Modulating the expression levels of either protein independently, we show that any one of them could behave as the protein limiting the level of flippase activity. Thus, LdMT and LdRos3 seem to form part of the same translocation machinery that determines flippase activity and miltefosine sensitivity in Leishmania, further supporting the consideration of CDC50/Lem3 proteins as beta subunits required for the normal functioning of P4-ATPases.