Novel anion dependence of induced cation transport in malaria-infected erythrocytes

Characterisation of complexes formed by parasite proteins exported into the host cell compartment of Plasmodium falciparum infected red blood cells

During its intraerythrocytic life cycle, the human malaria parasite Plasmodium falciparum supplements its nutritional requirements by scavenging substrates from the plasma through the new permeability pathways (NPPs) installed in the red blood cell (RBC) membrane. Parasite proteins of the RhopH complex: CLAG3, RhopH2, RhopH3, have been implicated in NPP activity. Here, we studied 13 exported proteins previously hypothesised to interact with RhopH2, to study their potential contribution to the function of NPPs. NPP activity assays revealed that the 13 proteins do not appear to be individually important for NPP function, as conditional knockdown of these proteins had no effect on sorbitol uptake. Intriguingly, reciprocal immunoprecipitation assays showed that five of the 13 proteins interact with all members of the RhopH complex, with PF3D7_1401200 showing the strongest association. Mass spectrometry‐based proteomics further identified new protein complexes; a cytoskeletal complex and a Maurer's clefts/J‐dot complex, which overall helps clarify protein–protein interactions within the infected RBC (iRBC) and is suggestive of the potential trafficking route of the RhopH complex itself to the RBC membrane.

The Functioning of Na+-ATPases from Protozoan Parasites: Are These Pumps Targets for Antiparasitic Drugs?

The ENA ATPases (from exitus natru: the exit of sodium) belonging to the P-type ATPases are structurally very similar to the sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA); they exchange Na⁺ for H⁺ and, therefore, are also known as Na⁺-ATPases. ENA ATPases are required in alkaline milieu, as in the case for Aspergillus, where other transporters cannot mediate an uphill Na⁺ efflux. They are also important for salt tolerance, as described for Arabidopsis. During their life cycles, protozoan parasites might encounter a high pH environment, thus allowing consideration of ENA ATPases as possible targets for controlling certain severe parasitic diseases, such as Chagas' Disease. Phylogenetic analysis has now shown that, besides the types IIA, IIB, IIC, and IID P-type ATPases, there exists a 5th subgroup of ATPases classified as ATP4-type ATPases, found in Plasmodium falciparum and Toxoplasma gondii. In malaria, for example, some drugs targeting PfATP4 destroy Na⁺ homeostasis; these drugs, which include spiroindolones, are now in clinical trials. The ENA P-type (IID P-type ATPase) and ATP4-type ATPases have no structural homologue in mammalian cells, appearing only in fungi, plants, and protozoan parasites, e.g., Trypanosoma cruzi, Leishmania sp., Toxoplasma gondii, and Plasmodium falciparum. This exclusivity makes Na⁺-ATPase a potential candidate for the biologically-based design of new therapeutic interventions; for this reason, Na⁺-ATPases deserves more attention.

Mir-155 is overexpressed in systemic sclerosis fibroblasts and is required for NLRP3 inflammasome-mediated collagen synthesis during fibrosis

BACKGROUND

Despite the important role that microRNAs (miRNAs) play in immunity and inflammation, their involvement in systemic sclerosis (SSc) remains poorly characterized. miRNA-155 (miR-155) plays a role in pulmonary fibrosis and its expression can be induced with interleukin (IL)-1β. SSc fibroblasts have activated inflammasomes that are integrally involved in mediating the myofibroblast phenotype. In light of this, we investigated whether miR-155 played a role in SSc and if its expression was dependent on inflammasome activation.

METHODS

miR-155 expression was confirmed in SSc dermal and lung fibroblasts by quantitative polymerase chain reaction (PCR). Wild-type and NLRP3-deficient murine fibroblasts were utilized to explore the regulation of miR-155 during inflammasome activation. miR-155-deficient fibroblasts and retroviral transductions with a miR-155 expression or control vectors were used to understand the contribution of miR-155 in fibrosis.

RESULTS

miR-155 was significantly increased and the highest expressing miRNA in SSc lung fibroblasts. Its expression was dependent on inflammasome activation as miR-155 expression could be blocked when inflammasome signaling was inhibited. In the absence of miR-155, inflammasome-mediated collagen synthesis could not be induced but was restored when miR-155 was expressed in miR-155-deficient fibroblasts.

CONCLUSIONS

miR-155 is upregulated in SSc. These results suggest that the inflammasome promotes the expression of miR-155 and that miR-155 is a critical miRNA that drives fibrosis.

Decreased redox-sensitive erythrocyte cation channel activity in aquaporin 9-deficient mice

Survival of the malaria pathogen Plasmodium falciparum in host erythrocytes requires the opening of new permeability pathways (NPPs) in the host cell membrane, accomplishing entry of nutrients, exit of metabolic waste products such as lactate and movement of inorganic ions such as Cl⁻, Na⁺ and Ca²⁺. The molecular identity of NPPs has remained largely elusive but presumably involves several channels, which partially can be activated by oxidative stress in uninfected erythrocytes. One NPP candidate is aquaporin 9 (AQP9), a glycerol-permeable water channel expressed in erythrocytes. Gene-targeted mice lacking functional AQP9 (aqp⁻/⁻) survive infection with the malaria pathogen Plasmodium berghei better than their wild-type littermates (aqp9⁺/⁺). In the present study whole-cell patch-clamp recordings were performed to explore whether ion channel activity is different in erythrocytes from aqp⁻/⁻ and aqp9⁺/⁺ mice. As a result, the cation conductance (K⁺ > Na⁺ > Ca²⁺ ≫ NMDG⁺) was significantly lower in erythrocytes from aqp⁻/⁻ than in erythrocytes from aqp9⁺/⁺ mice. Oxidative stress by exposure for 15-30 min to 1 mM H₂O₂ or 1 mM tert-butyl-hydroperoxide enhanced the cation conductance and increased cytosolic Ca²⁺ concentration, effects significantly less pronounced in erythrocytes from aqp⁻/⁻ than in erythrocytes from aqp9⁺/⁺ mice. In conclusion, lack of AQP9 decreases the cation conductance of erythrocytes, an effect that possibly participates in the altered susceptibility of AQP9-deficient mice to infection with P. berghei.

Anion channels in Plasmodium-falciparum-infected erythrocytes and protein kinase A

By replicating within red blood cells, malaria parasites are largely hidden from immune recognition; however, in the cells, nutrients are limiting and hazardous metabolic end products can rapidly accumulate. Therefore, to survive within erythrocytes, parasites alter the permeability of the host plasma membrane, either by upregulating existing transporters or by creating new permeation pathways. Recent electrophysiological studies of Plasmodium-infected erythrocytes have demonstrated that membrane permeability is mediated by transmembrane transport through ion channels in the infected erythrocyte. This article discusses the evidence and controversies concerning the nature of these channels and surveys the potential role of phosphorylation in activating anion channels that could be important in developing novel strategies for future malarial chemotherapies.

Artemisinin inhibits cation currents in malaria-infected human erythrocytes

BACKGROUND

Previous patch-clamp studies have demonstrated inwardly and outwardly rectifying anion currents, ClC-2 Cl- currents, and nonselective Ca(++)-permeable cation currents in Plasmodium falciparum-infected human erythrocytes.

METHODS

The current work studied the effect of the potent antimalarial drug artemisinin on the P falciparum infection-induced whole cell currents in human erythrocyte.

RESULTS

Artemisinin had no significant effect on the outwardly rectifying anion currents but inhibited the cation-selective currents with an apparent half-maximal inhibitory concentration of < or =10 micromol/L.

CONCLUSION

Because artemisinin reportedly inhibits the asexual parasite amplification with much higher potency, the antimalarial action of the drug cannot be attributed to the artemisinin effect on the cation currents. However, artemisinin may be used as a pharmacologic tool to dissect different current fractions in P falciparum-infected erythrocytes.

Solute transport via the new permeability pathways in Plasmodium falciparum-infected human red blood cells is not consistent with a simple single-channel model

After infection of a red blood cell (RBC), the malaria parasite, Plasmodium falciparum, increases the permeability of the host's plasma membrane by inducing new permeability pathways (NPPs). Single-channel patch-clamp experiments have shown the presence in infected RBCs of novel anion-selective channel types with low open-state probabilities at positive membrane potentials. These channels have been postulated to form the NPPs. Here, we have used a range of transport techniques to study whether electroneutral solutes use these channels or altered/separate pathways. Transport of the electroneutral solute sorbitol via the NPPs was found to increase by a small but significant amount after gross membrane depolarization. This is inconsistent with transport via a channel with a reduced open-state probability at positive membrane potentials. As has been demonstrated previously for parasite-induced anion currents, sorbitol transport in infected RBCs was found to be sensitive to the presence of bovine serum albumin (BSA). However, it remains to be shown whether the effect is due to serum/BSA altering a single channel type or activating a new pathway. In addition, the study highlights problems that can occur when using different transport techniques to study the NPPs.

Purinoceptors are involved in the induction of an osmolyte permeability in malaria-infected and oxidized human erythrocytes

In human erythrocytes, infection by the malaria parasite Plasmodium falciparum or oxidative stress induces a new organic osmolyte and anion permeability. To examine a role for autocrine purinoceptor signaling during this induction process, erythrocytic purinoceptor expression, and ATP release were determined. Furthermore, using pharmacological and genetic approaches the dependence on purinoceptor signaling of osmolyte permeability and Plasmodium development, both in vitro and in vivo, were assessed. Extracellular ATP did not induce an osmolyte permeability in non-infected or non-oxidized erythrocytes. ATP and other purinoceptor agonists increased the induction of osmolyte permeability during infection or oxidation as measured by isosmotic hemolysis and patch-clamp recording. Purinoceptor antagonists and apyrase decreased the induced permeability. The observed pharmacology suggested the involvement of P2Y purinoceptors. Accordingly, human erythrocytes expressed P2Y1 protein. Moreover, P2Y1-deficient mouse erythrocytes exhibited a delayed appearance of the osmolyte permeability during P. berghei infection- or oxidation compared with wild-type erythrocytes. Furthermore, the nonspecific purinoceptor antagonist suramin decreased in vitro growth and DNA/RNA amplification of P. falciparum in human erythrocytes and decreased in vivo growth of P. berghei. P. berghei developed slower in P2Y1-deficient mice in vivo compared with wild-type animals. In conclusion, induction of the osmolyte permeability in Plasmodium-infected erythrocytes involves autocrine purinoceptor signaling.

Permselectivity and pH-dependence of Plasmodium falciparum-induced anion currents in human erythrocytes

Intraerythrocytic survival of the malaria pathogen Plasmodium falciparum requires delivery of nutrients and disposal of waste products across the host erythrocyte membrane. Recent patch-clamp experiments have demonstrated inwardly and outwardly rectifying anion conductances in infected but not in control erythrocytes. A ClC-2-generated fraction of the inwardly rectifying current is activated by cell swelling and presumably subserves host cell volume regulation. In contrast, the outwardly rectifying current is insensitive to cell volume but allows the passage of lactate and is involved in the transport of nutrients. The present study was performed to characterize the permselectivity and pH sensitivity of the anion conductances using whole-cell recording. The outwardly rectifying and the inwardly rectifying currents exhibited permselectivities of Cl- > or = Br- approximately I- > SCN- and SCN- > I- > Br- > Cl-, respectively, as evident from the reversal potentials recorded under biionic conditions. While the inwardly rectifying current was not affected significantly by alterations of pH between 6.0 and 8.4, the outward rectifier was inhibited strongly by alkalinization to pH > or = 7.8. Fluxes of 14C-lactate and parasite growth were decreased markedly by the increase of bath pH, an effect that may at least in part be due to inhibition of the outward rectifier and subsequently impaired transport across the erythrocyte membrane.

Patch-clamp analysis of the "new permeability pathways" in malaria-infected erythrocytes

The intraerythrocytic amplification of the malaria parasite Plasmodium falciparum induces new pathways of solute permeability in the host cell's membrane. These pathways play a pivotal role in parasite development by supplying the parasite with nutrients, disposing of the parasite's metabolic waste and organic osmolytes, and adapting the host's electrolyte composition to the parasite's needs. The "new permeability pathways" allow the fast electrogenic diffusion of ions and thus can be analyzed by patch-clamp single-channel or whole-cell recording. By employing these techniques, several ion-channel types with different electrophysiological profiles have been identified in P. falciparum-infected erythrocytes; they have also been identified in noninfected cells. This review discusses a possible contribution of these channels to the new permeability pathways on the one hand and their supposed functions in noninfected erythrocytes on the other.

Plasmodium Induces Swelling-activated ClC-2 Anion Channels in the Host Erythrocyte

Intraerythrocytic growth of the human malaria parasite Plasmodium falciparum depends on delivery of nutrients. Moreover, infection challenges cell volume constancy of the host erythrocyte requiring enhanced activity of cell volume regulatory mechanisms. Patch clamp recording demonstrated inwardly and outwardly rectifying anion channels in infected but not in control erythrocytes. The molecular identity of those channels remained elusive. We show here for one channel type that voltage dependence, cell volume sensitivity, and activation by oxidation are identical to ClC-2. Moreover, Western blots and FACS analysis showed protein and functional ClC-2 expression in human erythrocytes and erythrocytes from wild type (Clcn2(+/+)) but not from Clcn2(-/-) mice. Finally, patch clamp recording revealed activation of volume-sensitive inwardly rectifying channels in Plasmodium berghei-infected Clcn2(+/+) but not Clcn2(-/-) erythrocytes. Erythrocytes from infected mice of both genotypes differed in cell volume and inhibition of ClC-2 by ZnCl(2) (1 mm) induced an increase of cell volume only in parasitized Clcn2(+/+) erythrocytes. Lack of ClC-2 did not inhibit P. berghei development in vivo nor substantially affect the mortality of infected mice. In conclusion, activation of host ClC-2 channels participates in the altered permeability of Plasmodium-infected erythrocytes but is not required for intraerythrocytic parasite survival.

Organic osmolyte permeabilities of the malaria-induced anion conductances in human erythrocytes

Infection of human erythrocytes with the malaria parasite Plasmodium falciparum induces new permeability pathways (NPPs) in the host cell membrane. Isotopic flux measurements demonstrated that the NPP are permeable to a wide variety of molecules, thus allowing uptake of nutrients and release of waste products. Recent patch-clamp recordings demonstrated the infection-induced up-regulation of an inwardly and an outwardly rectifying Cl(-) conductance. The present experiments have been performed to explore the sensitivity to cell volume and the organic osmolyte permeability of the two conductances. It is shown that the outward rectifier has a high relative lactate permeability (P(lactate)/P(Cl) = 0.4). Sucrose inhibited the outward-rectifier and abolished the infection-induced hemolysis in isosmotic sorbitol solution but had no or little effect on the inward-rectifier. Furosemide and NPPB blocked the outward-rectifying lactate current and the sorbitol hemolysis with IC(50)s in the range of 0.1 and 1 microM, respectively. In contrast, the IC(50)s of NPPB and furosemide for the inward-rectifying current were >10 microM. Osmotic cell-shrinkage inhibited the inwardly but not the outwardly rectifying conductance. In conclusion, the parasite-induced outwardly-rectifying anion conductance allows permeation of lactate and neutral carbohydrates, whereas the inward rectifier seems largely impermeable to organic solutes. All together, these data should help to resolve ongoing controversy regarding the number of unique channels that exist in P. falciparum-infected erythrocytes.

Channel-induced apoptosis of infected host cells-the case of malaria

Infection of erythrocytes by the malaria pathogen Plasmodium falciparum leads to activation of several distinct anion channels and a non-selective, Ca2+-permeable cation channel. All channel types are presumably activated by the oxidative stress generated by the pathogen. Similar or identical channels are activated by oxidation of non-infected erythrocytes. Activation of the non-selective cation channel allows entry of Ca2+ and Na+, both of which are required for intracellular growth of the pathogen. The entry of Ca2+ stimulates an intraerythrocytic scramblase that facilitates bi-directional phospholipid migration across the bilayer, resulting in breakdown of the phosphatidylserine asymmetry of the cell membrane. The exposure of phosphatidylserine at the outer surface of the cell membrane is presumably followed by binding to phosphatidylserine receptors on macrophages and subsequent phagocytosis of the affected erythrocyte. The lysosomal degradation may eventually eliminate the pathogen. The channel may thus play a dual role in pathogen survival. Absence of the channels is not compatible with pathogen growth, enhanced channel activity accelerates erythrocyte "apoptosis" that may represent a host defence mechanism serving to eliminate infected erythrocytes.

Channels and transporters as drug targets in the Plasmodium-infected erythrocyte

Throughout the intraerythrocytic phase of its lifecycle the malaria parasite is separated from the extracellular medium by the plasma membrane of its host erythrocyte and by the parasitophorous vacuole in which the parasite is enclosed. The intracellular parasite itself has, at its surface, a plasma membrane, and has a variety of membrane-bound organelles which carry out a range of biochemical functions. Each of the various membranes of the infected cell have in them proteins that facilitate the movement of molecules and ions from one side of the membrane to the other. These 'channels' and 'transporters' play a central role in the physiology of the parasitised cell. From a clinical viewpoint they are of interest both as potential targets in their own right, and as potential drug targeting routes capable of mediating the entry of cytotoxic drugs into the appropriate compartment of the infected cell. In this review both of these aspects are considered.

Extracellular lysines on the plasmodial surface anion channel involved in Na+ exclusion

The human malaria parasite, Plasmodium falciparum, induces an unusual ion channel, the plasmodial surface anion channel (PSAC), on its host red blood cell (RBC) membrane. PSAC has a broad selectivity with permeability to anions, sugars, amino acids, purines, and certain vitamins, suggesting a role in nutrient acquisition by the intracellular parasite. Permeating solutes cover a range of molecular sizes and may be either neutral or carry a net negative or positive charge. Despite this broad selectivity, PSAC must efficiently exclude Na+ to avoid osmotic lysis of infected RBCs in the bloodstream. Here, we used amine-reactive N-hydroxysulfosuccinimide esters to probe PSAC's unusual selectivity. PSAC permeation rates, measured with both a kinetic osmotic lysis assay and single-channel patch-clamp, irreversibly decrease after treatment with these reagents. Sequential labelings with different esters and the effects of their chain length suggest that PSAC has multiple lysine residues near its extracellular pore mouth and that inhibition occurs via steric hindrance of its pore by the amide-linked side chain. When combined with the effects of pH on permeation, these findings implicate a combination of cation repulsion by pore mouth charges and a weak binding site for permeant solutes in PSAC's broad selectivity yet effective exclusion of Na+.

Heme binding contributes to antimalarial activity of bis-quaternary ammoniums

Quaternary ammonium compounds have received recent attention due to their potent in vivo antimalarial activity based on their ability to inhibit de novo phosphatidylcholine synthesis. Here we show that in addition to this, heme binding significantly contributes to the antimalarial activity of these compounds. For the study, we used a recently synthesized bis-quaternary ammonium compound, T16 (1,12-dodecanemethylene bis[4-methyl-5-ethylthiazolium] diodide), which exhibits potent antimalarial activity (50% inhibitory concentration, approximately 25 nM). Accumulation assays reveal that this compound is readily concentrated several hundredfold (cellular accumulation ratio, approximately 500) into parasitized erythrocytes. Approximately 80% of the drug was shown to be distributed within the parasite, approximately 50% of which was located in the parasite food vacuoles. T16 uptake was affected by anion substitution (permeation increasing in the order Cl(-) < Br(-) = NO(3)(-) < I(-) < SCN(-)) and was sensitive to furosemide-properties similar to substrates of the induced new permeability pathway in infected erythrocytes. Scatchard plot analysis of in situ T16 binding revealed high-affinity and low-affinity binding sites. The high-affinity binding site K(d) was similar to that measured in vitro for T16 and ferriprotoporphyrin IX (FPIX) binding. Significantly, the capacity but not the K(d) of the high-affinity binding site was decreased by reducing the concentration of parasite FPIX. Decreasing the parasite FPIX pool also caused a marked antagonism of T16 antimalarial activity. In addition, T16 was also observed to associate with parasite hemozoin. Binding of T16 to FPIX in the digestive food vacuole is shown to be critical for drug accumulation and antimalarial activity. These data provide additional new mechanisms of antimalarial activity for this promising new class of antimalarial compounds.

Novel antiplasmodial agents

The morbidity and mortality associated with malaria have spurred efforts to find novel antimalarial agents with improved potency and selectivity. Leads for agents continue to be obtained from natural sources (plants and microorganisms) and chemical syntheses. Screening of commercial or specialized databases have also yielded promising leads. The structural diversity of compounds with good (micromolar and lower) activity point to the considerable tolerance for different structural elements in the "antimalarial pharmacophore." It may also be a reflection of the varied targets present in the plasmodia. The challenge in malaria chemotherapy is to find safe and selective agents whose potencies will not be compromised by plasmodial resistance. Modification of potential leads should also aim at improving "drug-like" character, viz. to ensure acceptable oral bioavailability. A review of the literature shows that there is a growing trend towards the development of target-specific antimalarial agents (for example, agents inhibiting plasmodial farnesyl transferase, cyclin dependent kinases, proteases, choline transport). An increasing number of reports focus on the development of chemosensitizers, agents that are capable of reversing plasmodial resistance.

Amazing chloride channels: an overview

AIM

This review describes molecular and functional properties of the following Cl- channels: the ClC family of voltage-dependent Cl- channels, the cAMP-activated transmembrane conductance regulator (CFTR), Ca2+ activated Cl- channels (CaCC) and volume-regulated anion channels (VRAC). If structural data are available, their relationship with the function of Cl- channels will be discussed. We also describe shortly some recently discovered channels, including high conductance Cl- channels and the family of bestrophins. We illustrate the growing physiological importance of these channels in the plasma membrane and in intracellular membranes, including their involvement in transepithelial transport, pH regulation of intracellular organelles, regulation of excitability and volume regulation. Finally, we discuss the role of Cl- channels in various diseases and describe the pathological phenotypes observed in knockout mice models.

A two-compartment model of osmotic lysis in Plasmodium falciparum-infected erythrocytes

We recently identified a voltage-dependent anion channel on the surface of human red blood cells (RBCs) infected with the malaria parasite, Plasmodium falciparum. This channel, the plasmodial erythrocyte surface anion channel (PESAC), likely accounts for the increased permeability of infected RBCs to various small solutes, as assessed quantitatively with radioisotope flux and patch-clamp studies. Whereas this increased permeability has also been studied by following osmotic lysis of infected cells in permeant solutes, these experiments have been limited to qualitative comparisons of lysis rates. To permit more quantitative examination of lysis rates, we have developed a mathematical model for osmotic fragility of infected cells based on diffusional uptake via PESAC and the two-compartment geometry of infected RBCs. This model, combined with a simple light scattering assay designed to track osmotic lysis precisely, produced permeability coefficients that match both previous isotope flux and patch-clamp estimates. Our model and light scattering assay also revealed Michaelian kinetics for inhibition of PESAC by furosemide, suggesting a 1:1 stoichiometry for their interaction.

Modulation of cathepsin G expression in severe atopic dermatitis following medium-dose UVA1 phototherapy

BACKGROUND

During the last decade, medium-dose UVA1 phototherapy (50 J/cm2) has achieved great value within the treatment of severe atopic dermatitis (AD). The purpose of our study was to investigate to what extent UVA1 irradiation is able to modulate the status of protease activity by the use of a monoclonal antibody labeling cathepsin G.

METHODS

In order to further elucidate the mechanisms by which medium-dose UVA1 irradiation leads to an improvement of skin status in patients with AD, biopsy specimens from 15 patients before and after treatment were analyzed immunohistochemically for proteolytic activation.

RESULTS

Compared to lesional skin of patients with AD before UVA1 irradiation, the number of cells positive for cathepsin G within the dermal infiltrate decreased significantly after treatment. The decrease of cathepsin G+ cells was closely linked to a substantial clinical improvement in skin condition.

CONCLUSIONS

In summary, our findings demonstrated that medium-dose UVA1 irradiation leads to a modulation of the expression of cathepsin G in the dermal inflammatory infiltrate in patients with severe AD. Cathepsin G may attack laminin, proteoglycans, collagen I and insoluble fibronectin, to provoke proinflammatory events, to degrade the basement membrane, to destroy the tissue inhibitor of metalloproteinases and to increase the endothelial permeability. Therefore, its down-regulation by UVA1 phototherapy may induce the reduction of skin inflammation as well as improvement of the skin condition.

Two functionally distinct organic osmolyte pathways in Plasmodium gallinaceum-infected chicken red blood cells

Red cells infected with the human malaria parasite Plasmodium falciparum have an increased permeability to a range of small, structurally unrelated solutes via a malaria-induced pathway. We report here a similar pathway present in parasitised red cells from chickens infected with the avian malaria parasite, Plasmodium gallinaceum. Parasitised cells showed a marked increase in the rate of influx of sorbitol (76-fold) and, to a lesser degree, taurine (3-fold) when compared with red cells from uninfected chickens. Pharmacological data suggest that both sorbitol and taurine are transported via a single malaria-induced pathway, which is sensitive to inhibition by 5-nitro-2-(3-phenylpropylamino)benzoic acid (IC(50) approximately 7 microM). The malaria-induced pathway differed in its inhibition by a range of anion channel inhibitors when compared to the endogenous, volume-activated osmolyte pathway of chicken red cells. There were also differences in the selectivity of sorbitol and taurine by the two permeation routes. The data presented here are consistent with the presence of two distinct organic solute pathways in infected chicken red cells. The first is an endogenous volume-activated pathway, which is not activated by the parasite and the second is a malaria-induced pathway, similar to those that are induced by other types of malaria in other host species.

Oxidation induces a Cl(-)-dependent cation conductance in human red blood cells

Oxidative stress induces complex alterations of membrane proteins in red blood cells (RBCs) eventually leading to haemolysis. To study changes of membrane ion permeability induced by oxidative stress, whole-cell patch-clamp recordings and haemolysis experiments were performed in control and oxidised human RBCs. Control RBCs exhibited a small cation-selective whole-cell conductance (236 +/- 38 pS; n = 8) which was highly sensitive to the external Cl(-) concentration: replacement of NaCl in the bath by sodium gluconate induced an increase of this cation conductance by about 85 %. Exposing RBCs to t-butylhydroxyperoxide (1 mM for 10 min) induced a twofold increase in this cation conductance which was further stimulated after replacement of extracellular Cl(-) by gluconate, Br(-), I(-) or SCN(-). In addition, lowering the ionic strength of the bath solution by isosmotic substitution of NaCl by sorbitol activated the cation conductance. The Cl(-)-sensitive and oxidation-induced cation conductance was Ca(2+) permeable, exhibited a permselectivity of Cs(+) > K(+) > Na(+) = Li(+) >> NMDG(+), and was partially inhibited by amiloride (1 mM) and almost completely inhibited by GdCl(3) (150 microM), but was insensitive to TEA, BaCl(2), NPPB, flufenamic acid or quinidine. DIDS (100 microM) reversibly inhibited the activation of the cation conductance by removal of external Cl(-). Oxidation induced haemolysis in NaCl-bathed human RBCs. This haemolysis was attenuated by amiloride (1 mM) and inhibited by replacement of bath Na(+) by the impermeant cation NMDG(+). The Na(+)- and Ca(2+)-permeable conductance might be involved in haemolytic diseases induced by elevated oxidative stress, such as glucose-6-phosphate dehydrogenase deficiency.

Nutrient acquisition by intracellular apicomplexan parasites: staying in for dinner

The intracellular forms of the apicomplexan parasites Plasmodium, Toxoplasma and Eimeria reside within a parasitophorous vacuole. The nutrients required by these intracellular parasites to support their high rate of growth and replication originate from the host cell which, in turn, takes up such compounds from the extracellular milieu. Solutes moving from the external medium to the interior of the parasite, are confronted by a series of three membranes --the host cell membrane, the parasitophorous vacuole membrane and the parasite plasma membrane. Each constitutes a potential permeability barrier which must be either crossed or bypassed. It is the mechanisms by which this occurs that are the subject of this review.

Characterization of P-type ATPase 3 in Plasmodium falciparum

We report the nucleotide sequence, derived amino acid sequence and expression profile of P-type ATPase 3 (PfATPase3) from Plasmodium falciparum. An open reading frame of 7362 nucleotides, interrupted by a single intron of 168 nt, encoded a protein product of 2394 amino acids with a predicted MW of 282791 Da. Hydropathy analysis of PfATPase3 revealed six amino-terminal and six carboxyl-terminal membrane spanning regions (M1-12) flanking a large hydrophilic domain with a smaller hydrophilic loop between M4 and M5. Based on a phylogenetic comparison of conserved domains present in P-type ATPases from other organisms, PfATPase3 resembled a Type-V ATPase for which the transport affinity is unknown. The PfATPase3 topology was interrupted by four regions, termed 'inserts', unique to malarial P-type ATPases, which were high in asparagine residues and charged amino acids (inserts I1-I4). Inserts I1 and I3 also contained repeated amino acid motifs. The number and composition of repeated amino acid motifs in insert I3 were variable in seven P. falciparum strains tested. PfATPase3 was 80.2% similar to the non-insert portions of P. yoelii ATPase3, although their inserts differed in length and composition. PfATPase3 mRNA was most abundant relative to beta-tubulin during the latter half of the erythrocytic cycle and was also present in gametocytes. Using affinity-purified antibody to a 14 amino acid PfATPase3 epitope, a 260 kDa protein was detected by Western analysis. Based on immunofluorescence, the PfATPase3 protein was located intracellularly in gametocytes and, to a lesser extent, in late erythrocytic stages.

Perturbation of the pump-leak balance for Na+ and K+ in malaria-infected erythrocytes

In human erythrocytes infected with the mature form of the malaria parasite Plasmodium falciparum, the cytosolic concentration of Na+ is increased and that of K+ is decreased. In this study, the membrane transport changes underlying this perturbation were investigated using a combination of86Rb+, 43K+, and22Na+ flux measurements and a semiquantitative hemolysis technique. From >15 h postinvasion, there appeared in the infected erythrocyte membrane new permeation pathways (NPP) that caused a significant increase in the basal ion permeability of the erythrocyte membrane and that were inhibited by furosemide (0.1 mM). The NPP showed the selectivity sequence Cs+ > Rb+ > K+ > Na+, with the K+-to-Na+permeability ratio estimated as 2.3. From 18 to 36 h postinvasion, the activity of the erythrocyte Na+/K+ pump increased in response to increased cytosolic Na+ (a consequence of the increased leakage of Na+ via the NPP) but underwent a progressive decrease in the latter 12 h of the parasite's occupancy of the erythrocyte (36–48 h postinvasion). Incorporation of the measured ion transport rates into a mathematical model of the human erythrocyte indicates that the induction of the NPP, together with the impairment of the Na+/K+pump, accounts for the altered Na+ and K+levels in the host cell cytosol, as well as predicting an initial decrease, followed by a lytic increase in the volume of the host erythrocyte.

Diamidine compounds: selective uptake and targeting in Plasmodium falciparum

Extensive drug resistance in Plasmodium falciparum emphasizes the urgent requirement for novel antimalarial agents. Here we report potent antimalarial activity of a number of diamidine compounds. The lead compound pentamidine is concentrated 500-fold by erythrocytes infected with P. falciparum. Pentamidine accumulation can be blocked by inhibitors of hemoglobin digestion, suggesting that the drug binds to ferriprotoporphyrin IX (FPIX). All of the compounds bound to FPIX in vitro and inhibited the formation of hemozoin. Furthermore, inhibitors of hemoglobin digestion markedly antagonized the antimalarial activity of the diamidines, indicating that binding to FPIX is crucial for the activity of diamidine drugs. Pentamidine was not accumulated into uninfected erythrocytes. Pentamidine transport into infected cells exhibits an initial rapid phase, nonsaturable in the micromolar range and sensitive to inhibition by furosemide and glibenclamide. Changing the counter-ion in the order Cl(-) < Br(-) < NO(2)(-) < I(-) <SCN(-) markedly stimulated pentamidine transport. These data suggest that pentamidine is transported although a pore or ion channel with properties similar to those of the recently characterized 'induced permeability pathway' on the infected red cell membrane. In summary, the diamidines exhibit two levels of selectivity against P. falciparum. The route of entry and molecular target are both specific to malaria-infected cells and are distinct from targets in other protozoa. Drugs that target the hemoglobin degradation pathway of malaria parasites have a proven record of accomplishment. The employment of induced permeability pathways to access this target represents a novel approach to antiparasite chemotherapy and offers an additional level of selectivity.

Membrane transport in the malaria-infected erythrocyte

The malaria parasite is a unicellular eukaryotic organism which, during the course of its complex life cycle, invades the red blood cells of its vertebrate host. As it grows and multiplies within its host blood cell, the parasite modifies the membrane permeability and cytosolic composition of the host cell. The intracellular parasite is enclosed within a so-called parasitophorous vacuolar membrane, tubular extensions of which radiate out into the host cell compartment. Like all eukaryote cells, the parasite has at its surface a plasma membrane, as well as having a variety of internal membrane-bound organelles that perform a range of functions. This review focuses on the transport properties of the different membranes of the malaria-infected erythrocyte, as well as on the role played by the various membrane transport systems in the uptake of solutes from the extracellular medium, the disposal of metabolic wastes, and the origin and maintenance of electrochemical ion gradients. Such systems are of considerable interest from the point of view of antimalarial chemotherapy, both as drug targets in their own right and as routes for targeting cytotoxic agents into the intracellular parasite.

A voltage-dependent channel involved in nutrient uptake by red blood cells infected with the malaria parasite

Growth of the malaria parasite in human red blood cells (RBCs) is accompanied by an increased uptake of many solutes including anions, sugars, purines, amino acids and organic cations. Although the pharmacological properties and selectivity of this uptake suggest that a chloride channel is involved, the precise mechanism has not been identified. Moreover, the location of this uptake in the infected RBC is unknown because tracer studies are complicated by possible uptake through fluid-phase pinocytosis or membranous ducts. Here we have studied the permeability of infected RBCs using the whole-cell voltage-clamp method. With this method, uninfected RBCs had ohmic whole-cell conductances of less than 100 pS, consistent with their low tracer permeabilities. In contrast, trophozoite-infected RBCs exhibited voltage-dependent, non-saturating currents that were 150-fold larger, predominantly carried by anions and abruptly abolished by channel blockers. Patch-clamp measurements and spectral analysis confirmed that a small (< 10 pS) ion channel on the infected RBC surface, present at about 1,000 copies per cell, is responsible for these currents. Because its pharmacological properties and substrate selectivities match those seen with tracer studies, this channel accounts for the increased uptake of small solutes in infected RBCs. The surface location of this new channel and its permeability to organic solutes needed for parasite growth indicate that it may have a primary role in a sequential diffusive pathway for parasite nutrient acquisition.

Isolation and functional characterization of the PfNT1 nucleoside transporter gene from Plasmodium falciparum

Plasmodium falciparum, the causative agent of the most lethal form of human malaria, is incapable of de novo purine synthesis, and thus, purine acquisition from the host is an indispensable nutritional requirement. This purine salvage process is initiated by the transport of preformed purines into the parasite. We have identified a gene encoding a nucleoside transporter from P. falciparum, PfNT1, and analyzed its function and expression during intraerythrocytic parasite development. PfNT1 predicts a polypeptide of 422 amino acids with 11 transmembrane domains that is homologous to other members of the equilibrative nucleoside transporter family. Southern analysis and BLAST searching of The Institute for Genomic Research (TIGR) malaria data base indicate that PfNT1 is a single copy gene located on chromosome 14. Northern analysis of RNA from intraerythrocytic stages of the parasite demonstrates that PfNT1 is expressed throughout the asexual life cycle but is significantly elevated during the early trophozoite stage. Functional expression of PfNT1 in Xenopus laevis oocytes significantly increases their ability to take up naturally occurring D-adenosine (K(m) = 13.2 microM) and D-inosine (K(m) = 253 microM). Significantly, PfNT1, unlike the mammalian nucleoside transporters, also has the capacity to transport the stereoisomer L-adenosine (K(m) > 500 microM). Inhibition studies with a battery of purine and pyrimidine nucleosides and bases as well as their analogs indicate that PfNT1 exhibits a broad substrate specificity for purine and pyrimidine nucleosides. These data provide compelling evidence that PfNT1 encodes a functional purine/pyrimidine nucleoside transporter whose expression is strongly developmentally regulated in the asexual stages of the P. falciparum life cycle. Moreover, the unusual ability to transport L-adenosine and the vital contribution of purine transport to parasite survival makes PfNT1 an attractive target for therapeutic evaluation.

Transport properties of the host cell membrane

The malaria-infected erythrocyte shows an increased permeability to a wide range of solutes. The increase is mediated in part by parasite-induced new permeation pathways (NPP) and in part (for some solutes, under some conditions) by increased activity of endogenous transporters. The NPP provide the major route for the influx into the infected cell of a number of essential nutrients, but although the functional characteristics of these pathways are understood in some detail, they are yet to be identified at a molecular level. Lucifer yellow, a fluorescent anion, is taken up by malaria-infected erythrocytes to a much greater extent than uninfected erythrocytes via a pathway that differs in its pharmacological characteristics from the NPP. The nature, origin and location of this pathway remain to be established.

The effects of transport perturbations on the homeostasis of erythrocytes

The control of erythrocyte volume, pH, membrane potential and ion content results from the interaction of many passive and active transport systems, cytoplasmic buffers, and from the charge and osmotic properties of haemoglobin and other impermeant solutes. The complexity of the system is such that the understanding of cell responses to experimental, physiological and pathophysiological challenges is beyond intuitive grasp. Mathematical models of erythrocyte and reticulocyte homeostasis have delivered a wealth of novel and unexpected predictions that have been confirmed experimentally. Those concerning effects of Ca(2)+ and K+ permeabilization on cell volume, pH and osmolality have helped solve long-standing issues on the pathophysiology of sickle-cell dehydration and will be briefly reviewed here. To study the effects of parasite growth and of new permeation pathways (NPP) on host cell homeostasis, we have developed a model of a Plasmodium falciparum- infected erythrocyte. Modelling NPP to fit reported changes in both Na+/K+ fluxes and gradients predicted large variations in host cell haemoglobin concentration, [Hb]. However, preliminary estimates seem to indicate that host cell [Hb] is conserved throughout the parasite's asexual cycle, suggesting that the properties of the NPP vary in subtle, stage-dependent ways.

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 permeability of the malaria-infected erythrocyte to organic cations

The human malaria parasite, Plasmodium falciparum, induces in the plasma membrane of its host red blood cell new permeation pathways (NPP) that allow the influx of a variety of low molecular weight solutes. In this study we have demonstrated that the NPP confer upon the parasitised erythrocyte a substantial permeability to a range of monovalent organic (quaternary ammonium) cations, the largest having an estimated minimum cross-sectional diameter of 11-12 A. The rate of permeation of these cations showed a marked dependence on the nature of the anion present, increasing with the lyotropicity of the anion. There was no clear relationship between the permeation rate and either the size or the hydrophobicity of these solutes. However, the data were consistent with the rate of permeation being influenced by a combination of these two factors, with the pathways showing a marked preference for the relatively small and hydrophobic phenyltrimethylammonium ion over larger or less hydrophobic solutes. Large quaternary ammonium cations inhibited flux via the NPP, as did long-chain n-alkanols. For both classes of compound the inhibitory potency increased with the size and hydrophobicity of the solute. This study extends the range of solutes known to permeate the NPP of malaria-infected erythrocytes as well as providing some insight into the factors governing the rate of permeation.

IL-1 rescues scleroderma myofibroblasts from serum-starvation-induced cell death

Normal senescent adult human fibroblasts are relatively insensitive to the effects of serum deprivation in culture. In contrast, the myofibroblast phenotype, which is predominant in scleroderma-affected skin and cultured fibroblasts from scleroderma-affected skin, shows signs of deterioration within 3 days of serum starvation. The addition of IL-1 (5 ng/ml) prevents this deterioration. We propose that IL-1 is a factor promoting myofibroblast longevity with consequent fibrosis in scleroderma skin.

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.

Swelling detection for volume regulation in the primitive eukaryote Giardia intestinalis: a common feature of volume detection in present-day eukaryotes

It is increasingly evident that cell swelling is associated with the triggering of many biological processes, including progression of the cell cycle, hormonal response, and gene expression. However, the mechanism by which cell swelling is initially sensed and converted into intracellular signals is still ill-defined. We report here an early event in the detection of cell swelling and initiation of the volume regulatory response in Giardia intestinalis, an ancient representative of the eukaryotic kingdom. Giardial cell swelling, irrespective of the extent, was sensed at a cell volume of 1.06 x isosmotic volume (the threshold volume), at which the transition of the volume regulatory transport system from the 'resting' to the 'open' state occurred. Irreversible modification by p-chloromercuribenzoate (pCMB) and N-ethylmaleimide (NEM) of reduced thiols affected the threshold volume, but in opposing manners: pCMB increased the threshold volume to 1.14 x and NEM decreased to 0.85 x isosmotic volume. The simple modification of the threshold volume by NEM caused a drastic reduction of giardial cell volume under isosmotic conditions, with a process strikingly similar to the opening of mitochondrial permeability transition pore, a causative event in stress-induced programmed cell death. Substantial evidence supports the hypothesis that modulation of the membrane thiol moieties at the threshold volume, causing the 'all-or-nothing' type of swelling detection, represents the event linking cell swelling to the second messenger systems for volume regulation in present eukaryotes. Pathophysiological implications of alteration of the threshold volume are discussed.

Antimalarial Activity of 77 Phospholipid Polar Head Analogs: Close Correlation Between Inhibition of Phospholipid Metabolism and In Vitro Plasmodium Falciparum Growth

Seventy-seven potential analogs of phospholipid polar heads, choline and ethanolamine, were evaluated in vitro as inhibitors ofPlasmodium falciparum growth. Their IC50 ranged from 10−3 to 10−7 mol/L. Ten compounds showed similar antimalarial activity when tested against three different parasite strains (2 chloroquine-sensitive strains and 1 chloroquine-resistant strain). Compounds showing marked antimalarial activity were assayed for their effects on phospholipid metabolism. The most active compounds (IC50 of 1 to 0.03 μmol/L) were inhibitors of de novo phosphatidylcholine (PC) biosynthesis from choline. For a series of 50 compounds, there was a close correlation between impairment of phospholipid biosynthesis and inhibition of in vitro malaria parasite growth. High choline concentrations caused a marked specific shift in the curves for PC biosynthesis inhibition. Concentrations inhibiting 50% PC metabolism from choline were in close agreement with the Ki of these compounds for the choline transporter inPlasmodium knowlesi-infected erythrocytes. By contrast, measurement of the effects of 12 of these compounds on rapidly dividing lymphoblastoid cells showed a total absence of correlation between parasite growth inhibition and human lymphoblastoid cell growth inhibition. Specific antimalarial effects of choline or ethanolamine analogs are thus likely mediated by their alteration of phospholipid metabolism. This indicates that de novo PC biosynthesis from choline is a very realistic target for new malaria chemotherapy, even against pharmacoresistant strains.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Transforming growth factor-beta 1 modulates myofibroblastic phenotype of rat palatal fibroblasts in vitro

The effects of transforming growth factor-beta 1 (TGF-beta 1) on normal rat palatal fibroblasts in vitro were investigated in the present study in order to unravel the precise mechanisms by which the phenotypic modulation of fibroblasts occurs during the scar formation process. TGF-beta 1 dramatically changed the morphology of normal palatal fibroblasts from polygonal into an elongated shape, which was very similar to that of fibroblasts derived from experimental immature scar tissue in rat palatal mucosa. This morphological transition was concomitant with an increase in the expression of alpha-smooth muscle (alpha-SM) actin protein, a marker for myofibroblasts, when determined by immunocytochemistry. An immunoblot study also revealed that alpha-SM actin expression in palatal fibroblasts became evident after 24 h of TGF-beta 1 treatment and increased time-dependently up to 72 h. Northern blot analysis showed that TGF-beta 1 stimulated endogenous TGF-beta 1 mRNA expression in palatal fibroblasts within 24 h. Neither epidermal growth factor nor basic fibroblast growth factor had any effect on either alpha-SM actin expression or TGF-beta 1 mRNA expression. Pretreatment of palatal fibroblasts with TGF-beta 1 significantly increased the contractile capacity in a three-dimensional collagen gel culture, even when the culture medium was deprived of TGF-beta 1 for 72 h of the experimental period. Moreover, the contractility of scar fibroblasts, which highly expressed alpha-SM actin protein and TGF-beta 1 mRNA, was significantly lowered by a neutralizing antibody to TGF-beta 1. These data strongly suggest that TGF-beta 1 is a potential inducer of phenotypic expression of myofibroblasts in palatal fibroblasts and that auto-induction of TGF-beta 1 mRNA expression may play an important role in the scar formation process in palatal mucosa.

Angiotensin II induces TIMP-1 production in rat heart endothelial cells

Angiotensin II (AII) was found to upregulate tissue inhibitor of metalloproteineses-1 (TIMP-1) gene expression in rat heart endothelial cells in a dose and time-dependent manner. The maximal stimulation of TIMP-1 mRNA was achieved by 2 h after the addition of AII. This effect was blocked by losartan, an AT1 receptor antagonist and by calphostin C, a protein kinase C inhibitor. Addition of cycloheximide superinduced and actinomycin D abolished the induction. These results suggest that AII stimulates TIMP-1 production by a protein kinase C dependent pathway which is dependent upon de novo RNA synthesis. Immunoprecipitation experiment showed an enhanced band of 28 kDa from the conditioned medium of AII-treated cultures. Immunoblot analysis revealed that TIMP-1 was detectable in the conditioned medium 4 h after AII stimulation. Since endothelial cells line the blood vessels and sense the rise in AII associated with hypertension, the TIMP-1 released by these cells may provide an initial trigger leading to cardiac fibrosis in angiotensin-renin dependent hypertension.