Saturable and non-saturable components of choline transport in Plasmodium-infected mammalian erythrocytes: possible role of experimental conditions

Antiplasmodial Cyclodecapeptides from Tyrothricin Share a Target with Chloroquine

Previous research found that the six major cyclodecapeptides from the tyrothricin complex, produced by Brevibacillus parabrevis, showed potent activity against chloroquine sensitive (CQS) Plasmodium falciparum. The identity of the aromatic residues in the aromatic dipeptide unit in cyclo-(D-Phe¹-Pro²-(Phe³/Trp³)-D-Phe⁴/D-Trp⁴)-Asn⁵-Gln⁶-(Tyr⁷/Phe⁷/Trp⁷)-Val⁸-(Orn⁹/Lys⁹)-Leu¹⁰ was proposed to have an important role in activity. CQS and resistant (CQR) P. falciparum strains were challenged with three representative cyclodecapeptides. Our results confirmed that cyclodecapeptides from tyrothricin had significantly higher antiplasmodial activity than the analogous gramicidin S, rivaling that of CQ. However, the previously hypothesized size and hydrophobicity dependent activity for these peptides did not hold true for P. falciparum strains, other than for the CQS 3D7 strain. The Tyr⁷ in tyrocidine A (TrcA) with Phe³-D-Phe⁴ seem to be related with loss in activity correlating with CQ antagonism and resistance, indicating a shared target and/or resistance mechanism in which the phenolic groups play a role. Phe⁷ in phenycidine A, the second peptide containing Phe³-D-Phe⁴, also showed CQ antagonism. Conversely, Trp⁷ in tryptocidine C (TpcC) with Trp³-D-Trp⁴ showed improved peptide selectivity and activity towards the more resistant strains, without overt antagonism towards CQ. However, TpcC lead to similar parasite stage inhibition and parasite morphology changes than previously observed for TrcA. The disorganization of chromatin packing and neutral lipid structures, combined with amorphous hemozoin crystals, could account for halted growth in late trophozoite/early schizont stage and the nanomolar non-lytic activity of these peptides. These targets related to CQ antagonism, changes in neural lipid distribution, leading to hemozoin malformation, indicate that the tyrothricin cyclodecapeptides and CQ share a target in the malaria parasite. The differing activities of these cyclic peptides towards CQS and CQR P. falciparum strains could be due to variable target interaction in multiple modes of activity. This indicated that the cyclodecapeptide activity and parasite resistance response depended on the aromatic residues in positions 3, 4 and 7. This new insight on these natural cyclic decapeptides could also benefit the design of unique small peptidomimetics in which activity and resistance can be modulated.

Transport and pharmacodynamics of albitiazolium, an antimalarial drug candidate

BACKGROUND AND PURPOSE

Choline analogues, a new type of antimalarials, exert potent in vitro and in vivo antimalarial activity. This has given rise to albitiazolium, which is currently in phase II clinical trials to cure severe malaria. Here we dissected its mechanism of action step by step from choline entry into the infected erythrocyte to its effect on phosphatidylcholine (PC) biosynthesis.

EXPERIMENTAL APPROACH

We biochemically unravelled the transport and enzymatic steps that mediate de novo synthesis of PC and elucidated how albitiazolium enters the intracellular parasites and affects the PC biosynthesis.

KEY RESULTS

Choline entry into Plasmodium falciparum-infected erythrocytes is achieved both by the remnant erythrocyte choline carrier and by parasite-induced new permeability pathways (NPP), while parasite entry involves a poly-specific cation transporter. Albitiazolium specifically prevented choline incorporation into its end-product PC, and its antimalarial activity was strongly antagonized by choline. Albitiazolium entered the infected erythrocyte mainly via a furosemide-sensitive NPP and was transported into the parasite by a poly-specific cation carrier. Albitiazolium competitively inhibited choline entry via the parasite-derived cation transporter and also, at a much higher concentration, affected each of the three enzymes conducting de novo synthesis of PC.

CONCLUSIONS AND IMPLICATIONS

Inhibition of choline entry into the parasite appears to be the primary mechanism by which albitiazolium exerts its potent antimalarial effect. However, the pharmacological response to albitiazolium involves molecular interactions with different steps of the de novo PC biosynthesis pathway, which would help to delay the development of resistance to this drug.

The neutral lipid composition present in the digestive vacuole of Plasmodium falciparum concentrates heme and mediates β-hematin formation with an unusually low activation energy

In eukaryotic cells, neutral lipids serve as major energy storage molecules; however, in Plasmodium falciparum, a parasite responsible for causing malaria in humans, neutral lipids may have other functions during the intraerythrocytic stage of the parasite life cycle. Specifically, experimental data suggest that neutral lipid structures behave as a catalyst for the crystallization of hemozoin, a detoxification byproduct of several blood-feeding organisms, including malaria parasites. Synthetic neutral lipid droplets (SNLDs) were produced by depositing a lipid blend solution comprised of mono- and diglycerides onto an aqueous surface. These lipid droplets are able to mediate the production of brown pigments that are morphologically and chemically identical to hemozoin. The partitioning of heme into these SNLDs was examined by employing Nile Red, a lipid specific dye. Soluble ferriprotoporphyrin IX was observed to spontaneously localize to the lipid droplets, partitioning in a pH-dependent manner with an estimated log P of 2.6. Interestingly, the pH profile of heme partitioning closely resembles that of β-hematin formation. Differential scanning calorimetry and kinetic studies demonstrated that the SNLDs provide a unique environment that promotes hemozoin formation. SNLD-mediated formation of the malaria pigment displayed an activation energy barrier lower than those of individual lipid components. In particular, lipid droplets composed of diglycerides displayed activation barriers lower than those composed of monoglycerides. This difference was attributed to the greater fluidity of these lipids. In conjunction with the known pattern of lipid body proliferation, it is suggested that neutral lipid structures within the digestive vacuole not only are the location of in vivo hemozoin formation but are also essential for the survival of the parasite by functioning as a kinetically competent and site specific mediator for heme detoxification.

Transport of phospholipid synthesis precursors and lipid trafficking into malaria-infected erythrocytes

Phospholipid biosynthesis in Plasmodium is of crucial importance considering the high degree of membrane biogenesis. In the de novo phosphatidylcholine pathway, the major plasmodial phospholipid, choline, first enters infected erythrocytes by a transport-mediated process, whose main kinetic characteristics are the same as in normal cells except for a considerable increase in Vm. The kinetic and functional characterizations of the choline carrier (affinity, specificity, stereoselectivity, asymmetric cyclic model, ionic dependence, limiting step in carrier translocation) have now been done, although there is no information concerning its nature and structure, despite the fact that it is likely an outstanding pharmacological target. Other unanswered questions concern the mechanisms for choline entry into the parasite. The intense lipid trafficking between the intracellular parasite and the host cell membrane also indicates that Plasmodium controls its own lipid composition as well as that of its host cell. Organelles that house the machinery for lipid synthesis, and mechanisms for trafficking and sorting, have not yet been described because of the lack of appropriate tools, but they could address fundamental questions in the contemporary cell biology of this parasite.

Increased choline transport in erythrocytes from mice infected with the malaria parasite Plasmodium vinckei vinckei

Parasitized erythrocytes from mice infected with the murine malaria parasite Plasmodium vinckei vinckei showed a marked increase in the rate of influx of choline compared with erythrocytes from uninfected mice. In contrast, uninfected erythrocytes from P. vinckei-infected animals transported choline at the same rate as those from uninfected mice. The increased influx of choline into parasitized cells was via two discrete routes. One was a saturable pathway with a Km similar to that of the choline carrier of normal erythrocytes but a Vmax approx. 20-fold higher than that observed in uninfected cells. The other was a non-saturable pathway inhibited by furosemide. At choline concentrations within the normal physiological plasma concentration range, the former pathway contributed approx. two-thirds and the latter approx. one-third of the influx of choline into parasitized cells. The characteristics of the furosemide-sensitive pathway were similar to those of a broad-specificity pathway that is induced in human erythrocytes infected in vitro with Plasmodium falciparum. The results of this study rule out the possibility that the induced transport pathway of P. falciparum-infected erythrocytes is an artifact arising in vitro from the long-term culture of parasitized cells and provide evidence that this pathway makes a significant contribution to the uptake of choline into the parasitized cells of malaria-infected animals.

Characterization of the enhanced transport of L- and D-lactate into human red blood cells infected with Plasmodium falciparum suggests the presence of a novel saturable lactate proton cotransporter

Human erythrocytes parasitized with the malarial protozoan Plasmodium falciparum showed rates of L-lactate, D-lactate, and pyruvate uptake many fold greater than control cells. Thus it was necessary to work at 0 degrees C to resolve true initial rates of transport. Studies on the dependence of the rate of transport on substrate concentration implied the presence in parasitized cells of both a saturable mechanism blocked by alpha-cyano-4-hydroxycinnamate (CHC) and a nonsaturable mechanism insensitive to CHC. The former was dominant at physiological substrate concentrations with Km values for pyruvate and D-lactate of 2.3 and 5.2 mM, respectively, with no stereoselectivity for L- over D-lactate. CHC was significantly less effective as an inhibitor of lactate transport in parasitized erythrocytes than in uninfected cells, whereas p-chloromercuribenzenesulfonate, a potent inhibitor in control cells, gave little or no inhibition of lactate transport into parasitized erythrocytes. Inhibition of transport into infected cells was also observed with phloretin, furosemide, niflumic acid, stilbenedisulfonate derivatives, and 5-nitro-2-(3-phenylpropylamino)benzoic acid at concentrations similar to those that inhibit the lactate carrier of control erythrocytes. These compounds were more effective inhibitors of the rapid transport of chloride into infected cells than of lactate transport, whereas CHC was more effective against lactate transport. This implies that different pathways are involved in the parasite-induced transport pathways for lactate and chloride. The transport of L-lactate into infected erythrocytes was also inhibited by D-lactate, pyruvate, 2-oxobutyrate, and 2-hydroxybutyrate. The intracellular accumulation of L-lactate at equilibrium was dependent on the transmembrane pH gradient, suggesting a protogenic transport mechanism. Our data are consistent with lactate and pyruvate having direct access to the malarial parasite, perhaps via the proposed parasitophorous duct or some close contact between the host cell and parasite plasma membranes, with transport across the latter by both a proton-linked carrier (CHC-sensitive, saturable, and the major route) and free diffusion of the undissociated acid (CHC-insensitive, unsaturable, and a minor route).

Parasite-regulated membrane transport processes and metabolic control in malaria-infected erythrocytes

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