Selective elimination of malaria infected erythrocytes by a modified phospholipase A2 in vitro

Unlocking the potential of snake venom-based molecules against the malaria, Chagas disease, and leishmaniasis triad

Malaria, leishmaniasis and Chagas disease are vector-borne protozoal infections with a disproportionately high impact on the most fragile societies in the world, and despite malaria-focused research gained momentum in the past two decades, both trypanosomiases and leishmaniases remain neglected tropical diseases. Affordable effective drugs remain the mainstay of tackling this burden, but toxicicty, inneficiency against later stage disease, and drug resistance issues are serious shortcomings. One strategy to overcome these hurdles is to get new therapeutics or inspiration in nature. Indeed, snake venoms have been recognized as valuable sources of biomacromolecules, like peptides and proteins, with antiprotozoal activity. This review highlights major snake venom components active against at least one of the three aforementioned diseases, which include phospholipases A2, metalloproteases, L-amino acid oxidases, lectins, and oligopeptides. The relevance of this repertoire of biomacromolecules and the bottlenecks in their clinical translation are discussed considering approaches that should increase the success rate in this arduous task. Overall, this review underlines how venom-derived biomacromolecules could lead to pioneering antiprotozoal treatments and how the drug landscape for neglected diseases may be revolutionized by a closer look at venoms. Further investigations on poorly studied venoms is needed and could add new therapeutics to the pipeline.

In vitro anti-Plasmodium falciparum properties of the full set of human secreted phospholipases A2

We have previously shown that secreted phospholipases A2 (sPLA2s) from animal venoms inhibit the in vitro development of Plasmodium falciparum, the agent of malaria. In addition, the inflammatory-type human group IIA (hGIIA) sPLA2 circulates at high levels in the serum of malaria patients. However, the role of the different human sPLA2s in host defense against P. falciparum has not been investigated. We show here that 4 out of 10 human sPLA2s, namely, hGX, hGIIF, hGIII, and hGV, exhibit potent in vitro anti-Plasmodium properties with half-maximal inhibitory concentrations (IC50s) of 2.9 ± 2.4, 10.7 ± 2.1, 16.5 ± 9.7, and 94.2 ± 41.9 nM, respectively. Other human sPLA2s, including hGIIA, are inactive. The inhibition is dependent on sPLA2 catalytic activity and primarily due to hydrolysis of plasma lipoproteins from the parasite culture. Accordingly, purified lipoproteins that have been prehydrolyzed by hGX, hGIIF, hGIII, and hGV are more toxic to P. falciparum than native lipoproteins. However, the total enzymatic activities of human sPLA2s on purified lipoproteins or plasma did not reflect their inhibitory activities on P. falciparum. For instance, hGIIF is 9-fold more toxic than hGV but releases a lower quantity of nonesterified fatty acids (NEFAs). Lipidomic analyses of released NEFAs from lipoproteins demonstrate that sPLA2s with anti-Plasmodium properties are those that release polyunsaturated fatty acids (PUFAs), with hGIIF being the most selective enzyme. NEFAs purified from lipoproteins hydrolyzed by hGIIF were more potent at inhibiting P. falciparum than those from hGV, and PUFA-enriched liposomes hydrolyzed by sPLA2s were highly toxic, demonstrating the critical role of PUFAs. The selectivity of sPLA2s toward low- and high-density (LDL and HDL, respectively) lipoproteins and their ability to directly attack parasitized erythrocytes further explain their anti-Plasmodium activity. Together, our findings indicate that 4 human sPLA2s are active against P. falciparum in vitro and pave the way to future investigations on their in vivo contribution in malaria pathophysiology.

In vitro antiplasmodial activity of phospholipases A2 and a phospholipase homologue isolated from the venom of the snake Bothrops asper

The antimicrobial and antiparasite activity of phospholipase A₂ (PLA₂) from snakes and bees has been extensively explored. We studied the antiplasmodial effect of the whole venom of the snake Bothrops asper and of two fractions purified by ion-exchange chromatography: one containing catalytically-active phospholipases A₂ (PLA₂) (fraction V) and another containing a PLA₂ homologue devoid of enzymatic activity (fraction VI). The antiplasmodial effect was assessed on in vitro cultures of Plasmodium falciparum. The whole venom of B. asper, as well as its fractions V and VI, were active against the parasite at 0.13 ± 0.01 µg/mL, 1.42 ± 0.56 µg/mL and 22.89 ± 1.22 µg/mL, respectively. Differences in the cytotoxic activity on peripheral blood mononuclear cells between the whole venom and fractions V and VI were observed, fraction V showing higher toxicity than total venom and fraction VI. Regarding toxicity in mice, the whole venom showed the highest lethal effect in comparison to fractions V and VI. These results suggest that B. asper PLA₂ and its homologue have antiplasmodial potential.

Antiplasmodial effect of the venom of Crotalus durissus cumanensis, crotoxin complex and Crotoxin B

The antiplasmodial activity of phospholipases A(2) (PLA(2)) isolated from different animals has been studied. We explored the in vitro anti Plasmodium falciparum effect of a fraction containing crotoxin, Crotoxin B and whole venom of the rattlesnake Crotalus durissus cumanensis. Fraction II (crotoxin complex) was obtained by size exclusion chromatography, whereas Crotoxin B was purified by RP-HPLC. The whole venom is active against the parasite at concentrations of 0.17±0.03 μg/ml, fraction II at 0.76±0.17 μg/ml and Crotoxin B at 0.6±0.04 μg/ml. Differences were observed in the cytotoxic activity against peripheral mononuclear cells, with Crotoxin B exhibiting the highest cytotoxicity. The concentration of Crotoxin B required to exert cytotoxic activity was higher than that required to exert antiplasmodial activity. Lethality in mice confirmed the higher toxicity and neurotoxicity of whole venom and fraction II, whereas Crotoxin B was not lethal at the doses tested. These results suggest the potential of Crotoxin B as a lead compound for antimalarial activity.

Lipid compartmentalization in erythrocytes parasitized by Plasmodium spp

Although reasonably well protected from the host immune system by the erythrocyte membrane, the intraerythrocytic malaria parasite has to make that membrane compatible with its own requirements for development and multiplication. The development of Plasmodium spp brings about major changes in the lipid composition of the host cell membrane, as well as in its physical properties. The parasite itself has a lipid composition that differs from that of the host cell and an intense lipid trafficking seems to occur between intracellular parasite and host cell membrane. Here, Ana Paula Simões, Ben Roelofsen and Jos Op den Kamp discuss how, despite serious methodological limitations and the existence of some conflicting results, an overall picture of lipid compartmentalization within the parasitized erythrocyte is perceived.

Anti-Plasmodium properties of group IA, IB, IIA and III secreted phospholipases A2 are serum-dependent

Antibacterial, antiparasitidal and antiviral properties have recently been attributed to members of the secreted phospholipases A(2) (sPLA(2)s) superfamily. Seven sPLA(2)s from groups IA, IB, IIA and III, were tested here in different culture conditions for inhibition of the in vitro intraerythrocytic development of Plasmodium falciparum, the causative agent of the most severe form of human malaria. In the presence of human serum, all sPLA(2)s were inhibitory, with three out of seven exhibiting IC(50)<0.1 nM. In all cases, inhibition could be induced by enzymatic pre-treatment of the serum. By contrast, no effect was observed when parasites were grown in a semi-defined medium (AlbuMAX II) devoid of lipoproteins and containing 10 times less phospholipids than the medium with human serum, strongly suggesting that hydrolysis of serum generating toxic lipid by-products, rather than a direct interaction of the sPLA(2) with the infected erythrocyte, is a general feature of the anti-Plasmodium properties of sPLA(2)s. Furthermore, in serum, six out of the seven sPLA(2)s were toxic against both trophozoite and schizont stages of the parasite development, contrasting with the trophozoite-selective bee venom enzyme's toxicity. Deciphering the molecular mechanisms at play in the phenotypic singularity of the bee venom enzyme toxicity might offer new prospects in antimalarial fight.

Bee venom phospholipase A2 induces stage-specific growth arrest of the intraerythrocytic Plasmodium falciparum via modifications of human serum components

Secreted phospholipases A(2) (sPLA(2)s) from snake and insect venoms and from mammalian pancreas are structurally related enzymes that have been associated with several toxic, pathological, or physiological processes. We addressed the issue of whether toxic sPLA(2)s might exert specific effects on the Plasmodium falciparum intraerythrocytic development. We showed that both toxic and non-toxic sPLA(2)s are lethal to P. falciparum grown in vitro, with large discrepancies between respective IC(50) values; IC(50) values from toxic PLA(2)s ranged from 1.1 to 200 pm, and IC(50) values from non-toxic PLA(2)s ranged from 0.14 to 1 microm. Analysis of the molecular mechanisms responsible for cytotoxicity of bee venom PLA(2) (toxic) and hog pancreas PLA(2) (non-toxic) demonstrated that, in both cases, enzymatic hydrolysis of serum phospholipids present in the culture medium was responsible for parasite growth arrest. However, bee PLA(2)-lipolyzed serum induced stage-specific inhibition of P. falciparum development, whereas hog PLA(2)-lipolyzed serum killed parasites at either stage. Sensitivity to bee PLA(2)-treated serum appeared restricted to the 19-26-h period of the 48 h parasite cycle. Analysis of the respective role of the different lipoprotein classes as substrates of bee PLA(2) showed that enzyme treatment of high density lipoproteins, low density lipoproteins, and very low density lipoproteins/chylomicrons fractions induces cytotoxicity of either fraction. In conclusion, our results demonstrate that toxic and non-toxic PLA(2)s 1) are cytotoxic to P. falciparum via hydrolysis of lipoprotein phospholipids and 2) display different killing processes presumably involving lipoprotein by-products recognizing different targets on the infected red blood cell.

Ionophore-Phospholipid Interactions in Langmuir Films in Relation to Ionophore Selectivity toward Plasmodium-Infected Erythrocytes

Carboxylic true ionophores were previously demonstrated to have efficient antimalarial activity against the human parasite Plasmodium falciparum, with a 50% inhibitory concentration around nM and generally high selectivity as compared to their toxic effects against mammalian cell lines. The decreased molecular packing of the erythrocyte membrane outer leaflet after malarial infection could explain the preferential ionophore interaction with infected erythrocytes. Monolayer penetration experiments using different phospholipid films showed strong incorporation of true carboxylic ionophores, from classes 1 (nigericin) and 2 (lasalocid), up to a surface pressure close to film collapse. The interaction was slightly higher with PC (phosphatidylcholine) monolayers than with monolayers composed of cholesterol-containing total lipid extracts from either malaria-infected or normal erythrocytes, and the two latter induced identical interactions with 5-bromo lasalocid. Surface pressure-area isotherms for pure ionophores on water and surface tension of ionophore aqueous solutions clearly highlighted the surface-active characteristics of these ionophores and allowed determination of their molecular area in compact monolayers. The estimated ionophore concentration in the mixed interfacial layers indicates that higher amounts (threefold more) of ionophores might be integrated in infected erythrocyte membrane due to their impaired molecular packing as compared to normal erythrocytes. This infection-enhanced penetration efficiency does not appear directly related to the change in erythrocyte membrane lipid composition, but it could be the basis of ionophore selectivity for infected erythrocytes. Copyright 1999 Academic Press.

Effect of tryptophan-N-formylated gramicidin on growth of Plasmodium berghei in mice

The effect of tryptophan-N-formylated gramicidin (NFG) on the growth of Plasmodium berghei in mice was tested in three different experiments. NFG was shown to be capable of inhibiting the growth of the parasite in a dose-dependent way, although its action did not result in elimination of the parasite and was only temporary, preventing mice from early death, presumably due to cerebral malaria, but not from fatal generalized malaria. Intriguingly, a similar observation was made with two other drugs, (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine, an inhibitor of viral and eukaryotic DNA polymerases, and the presumed topoisomerase II inhibitor, a bisquaternary quinolinium salt. A rise in the level of parasitemia after 8 days, despite continued treatment, was not due to parasite-induced reticulocytosis, as demonstrated in experiments in which this condition was induced artificially. NFG was added in the form of lipid vesicles in which the peptide had been incorporated. The inhibitory action of NFG was not modulated by the lipid composition of the vesicles. Control experiments did not demonstrate any toxicity of NFG when it was administered in lipid vesicles. The main observation is that NFG is able to inhibit the growth of a malaria parasite in vivo at concentrations that are well tolerated by the host.

Characterization of the potent in vitro and in vivo antimalarial activities of ionophore compounds

Large-scale in vitro screening of different types of ionophores previously pinpointed nine compounds that were very active and selective in vitro against Plasmodium falciparum; their in vitro and in vivo antimalarial effects were further studied. Addition of the ionophores to synchronized P. falciparum suspensions revealed that all P. falciparum stages were sensitive to the drugs. However, the schizont stages were three- to ninefold more sensitive, and 12 h was required for complete parasite clearance. Pretreatment of healthy erythrocytes with toxic doses of ionophores for 24 to 48 h showed that the activity was not due to an irreversible effect on the host erythrocyte. No preferential ionophore adsorption in infected or uninfected erythrocytes occurred. On the other hand, ionophore molecules strongly bound to serum proteins since increasing the serum concentration from 2 to 50% led to almost a 25-fold parallel increase in the ionophore 50% inhibitory concentration. Mice infected with the malaria parasites Plasmodium vinckei petteri or Plasmodium chabaudi were successfully treated with eight ionophores in a 4-day suppressive test. The 50% effective dose after intraperitoneal administration ranged from 0.4 to 4.1 mg/kg of body weight, and the therapeutic indices were about 5 for all ionophores except monensin A methyl ether, 5-bromo lasalocid A, and gramicidin D, whose therapeutic indices were 12, 18, and 344, respectively. These three compounds were found to be curative, with no recrudescence. Gramicidin D, which presented impressive antimalarial activity, requires parenteral administration, while 5-bromo lasalocid A has the major advantage of being active after oral administration. Overall, the acceptable levels of toxicity and the good in vivo therapeutic indices in the rodent model highlight the interesting potential of these ionophores for the treatment of malaria in higher animals.

Targeting of porcine pancreatic phospholipase A2 to human platelets. Introduction of an RGD sequence and acyl-group by chemical modification

In the present study we prepared by chemical modification a series of porcine pancreatic phospholipase A2 (PLA) derivatives, that bind to the activated glycoprotein (GP) IIb/IIIa complex and hydrolyse phospholipids in the outer leaflet of the platelet membrane. To the native enzyme, an RGD-containing peptide was coupled to introduce affinity for GPIIb/IIIa in combination with lauric acid to improve binding to the membrane. As controls, derivatives containing only one of these modifications were prepared. Acylation of the enzyme improved the affinity for densely packed phospholipids, as deduced by kinetic analyses. After stimulation of platelets, the RGD-containing PLAs bound to GPIIb/IIIa since GRGDS peptide and a monoclonal antibody against the complex interfered with binding. No binding was found with native PLA. The binding seen with lauric acid PLA was not mediated by GPIIb/IIIa. All modified PLAs induced 1-3% hydrolysis of [3H]arachidonic-acid-labelled phospholipids in resting platelets. After activation with alpha-thrombin, hydrolysis increased to 17%, corresponding to about 90% of [3H]arachidonate-labelled phospholipids in the outer leaflet of the plasma membrane. RGD-containing PLAs were more active than lauroyl PLA, and their activity was mediated via GPIIb/IIIa since GRGDS inhibited release of [3H]arachidonic acid. Acylation of the RGD-containing PLAs did not further improve the hydrolytic properties. We conclude that chemical modification of PLA leads to a targetted hydrolytic action and could be a basis for the design of enzymes that specifically destroy activated platelets.

Differential in vitro activities of ionophore compounds against Plasmodium falciparum and mammalian cells

Twenty-two ionophore compounds were screened for their antimalarial activities. They consisted of true ionophores (mobile carriers) and channel-forming quasi-ionophores with different ionic specificities. Eleven of the compounds were found to be extremely efficient inhibitors of Plasmodium falciparum growth in vitro, with 50% inhibitory concentrations of less than 10 ng/ml. Gramicidin D was the most active compound tested, with 50% inhibitory concentration of 0.035 ng/ml. Compounds with identical ionic specificities generally had similar levels of antimalarial activity, and ionophores specific to monovalent cations were the most active. Compounds were further tested to determine their in vitro toxicities against mammalian lymphoblast and macrophage cell lines. Nine of the 22 compounds, i.e., alborixin, lonomycin, nigericin, narasin, monensin and its methylated derivative, lasalocid and its bromo derivative, and gramicidin D, most specific to monovalent cations, were at least 35-fold more active in vitro against P. falciparum than against the two other mammalian cell lines. The enhanced ability to penetrate the erythrocyte membrane after infection could be a factor that determines ionophore selectivity for infected erythrocytes.

Stage-dependent effects of analogs of gramicidin A on the growth of Plasmodium falciparum in vitro

Tryptophan-N-formylated gramicidin A, a nonhemolytic derivative of the toxic peptide antibiotic gramicidin A, has previously been shown to induce potassium leakage from Plasmodium falciparum-infected erythrocytes in vitro and to inhibit the growth of the parasite. In the present study the antimalarial activities of two other nonhemolytic derivatives of gramicidin A, viz., acylated gramicidin A and desformylated gramicidin A, were tested and compared with those of gramicidin A and tryptophan-N-formylated gramicidin A. The 50% growth-inhibitory concentrations (IC50 values) of the four compounds varied from 0.3 to 18.3 nM, and complete growth inhibition was detected within one parasitic growth cycle. Using highly synchronized cultures of P. falciparum, it was furthermore shown that the gramicidin analogs are inhibitory to all developmental stages of the parasite, although their efficiency in accomplishing growth inhibition was found, as expected, to be clearly stage-dependent and to increase with the age of the parasite.

Induction of circulating group II phospholipase A2 expression in adults with malaria

High levels of interleukin 1 and tumor necrosis factor are found in both cases of malaria and cases of septic shock. Since both interleukin 1 and tumor necrosis factor induce expression of the proinflammatory enzyme phospholipase A2 (PLA2), we examined serum PLA2 levels in 14 adults with malaria. Mean serum PLA2 activity was elevated 40-fold above normal (P less than 0.001). Serum PLA2 activity correlated with PLA2 immunoreactivity (r = 0.987; P less than 0.001) by an enzyme-linked immunosorbent assay specific for human group II PLA2, showing that serum PLA2 in cases of malaria is host derived. This article describes the novel finding of elevated PLA2 levels in cases of malaria, further strengthening the notion that mediators of the host response in cases of malaria are similar to those in cases of septic shock.

Replacement of vertebrate serum with lipids and other factors in the culture of invertebrate cells, tissues, parasites, and pathogens

Culture medium supplementation with vertebrate serum results in the selection of fibroblastoid insect cell lines and a general decline during continuous subculturing of both morphologic and functional differentiation of the surviving cells. Essential lipid mixtures can substitute for vertebrate serum in the culture of insect and some vertebrate cells, tissues, parasites, and pathogens. The provision of sterols and essential (with nonessential) polyunsaturated fatty acids as phospholipids in oxidation-protected peptoliposomes or proteoliposomes allows cells in culture to duplicate in vivo specific membranes more accurately. Such lipid-corrected membranes allow cultured cells to communicate with neighboring cells through the extracellular matrix, effectively transmit hormonal signals directly and via receptor control, and respond with various tissue-specific functions and differentiation states as directed.

Selective internalization of choline-phospholipids in Plasmodium falciparum parasitized human erythrocytes

We have incubated control and Plasmodium falciparum parasitized human erythrocytes with lipid vesicles containing radiolabeled long-chain phosphatidylcholine and sphingomyelin, in the presence of a nonspecific lipid transfer protein. Most of the radiolabeled phospholipids were, immediately thereafter, available for extracellular phospholipases, suggesting that uptake of vesicles as such did not occur. In time, the amount of phosphatidylcholine inserted in the outer leaflet of the host cell membrane of parasitized erythrocytes decreased, indicating that phosphatidylcholine was being internalized in parasitized erythrocytes. The exclusion of sphingomyelin from the internalization process suggests that the removal of phosphatidylcholine from the outer leaflet of the erythrocyte membrane is caused by transbilayer migration, rather than by endocytosis. The extent of phosphatidylcholine internalization indicates that part of it does not remain in the inner leaflet of the host cell membrane, but is taken up by the intraerythrocytic parasite. Individual phosphatidylcholine species, containing 16:0/18:1-, 16:0/18:2- and 16:0/20:4-fatty acids, showed similar extents of internalization, after being incorporated in parasitized erythrocytes by a phosphatidylcholine specific transfer protein.

Growth inhibition of Plasmodium falciparum in in vitro cultures by selective action of tryptophan-N-formylated gramicidin incorporated in lipid vesicles

We studied the differential effect of tryptophan-N-formylated gramicidin on uninfected and Plasmodium falciparum-infected erythrocytes. Trp-N-formylated gramicidin induces a much faster leakage of K+ from infected cells than from uninfected cell whereas, and at an even lower concentration, gramicidin A' causes a rapid K+ leakage from both uninfected and infected cells. We also studied the effect of Trp-N-formylated gramicidin and gramicidin A' incorporated in liposomes on the growth of Plasmodium falciparum in an in vitro culture. Incorporation of Trp-N-formylated gramicidin in the membranes of so-called 'stealth' vesicles strongly decreases the concentration needed to induce 50% inhibition of parasite growth. Moreover, no decrease in the K+ content of uninfected cells was observed when cells were exposed to liposome-incorporated Trp-N-formylated gramicidin at a concentration which causes full inhibition of parasite growth. These observations strongly suggest that Trp-N-formylated gramicidin incorporated in 'stealth' vesicles ends up specifically in the infected cell, thereby inhibiting the growth of the growth of the malaria parasite.