Stage-specific expression of the proline-alanine transporter in the human pathogen Leishmania

Leishmania are obligatory intracellular parasites that cycle between the sand fly midgut (extracellular promastigotes) and mammalian macrophage phagolysosomes (intracellular amastigotes). They have developed mechanisms of adaptation to the distinct environments of host and vector that favor utilization of both proline and alanine. LdAAP24 is the L. donovani proline-alanine transporter. It is a member of Leishmania system A that translocates neutral amino acids. Since system A is promastigote-specific, we aimed to assess whether LdAAP24 is also expressed exclusively in promastigotes. Herein, we established that upon exposing L. donovani promastigotes to amastigote differentiation signal (pH 5.5 and 37 °C), parasites rapidly and completely degrade LdAAP24 protein in both axenic and in spleen-derived amastigotes. In contrast, LdAAP24 mRNA remained unchanged throughout differentiation. Addition of either MG132 or Bafilomycin A1 partially inhibited LdAAP24 protein degradation, indicating a role for both lysosome- and proteasome-mediated degradation. This work provides the first evidence for post-translational regulation of stage-specific expression of LdAAP24.

Multiple levels of gene regulation mediate differentiation of the intracellular pathogen Leishmania

For many years, mRNA abundance has been used as the surrogate measure of gene expression in biological systems. However, recent genome-scale analyses in both bacteria and eukaryotes have revealed that mRNA levels correlate with steady-state protein abundance for only 50-70% of genes, indicating that translation and post-translation processes also play important roles in determining gene expression. What is not yet clear is whether dynamic processes such as cell cycle progression, differentiation, or response to environmental changes change the relationship between mRNA and protein abundance. Here, we describe a systems approach to interrogate promastigote-to-amastigote differentiation in the obligatory intracellular parasitic protozoan Leishmania donovani. Our results indicate that regulation of mRNA levels plays a major role early in the differentiation process, while translation and post-translational regulation are more important in the latter part. In addition, it appears that the differentiation signal causes a transient global increase in the rate of protein synthesis, which is subsequently down-regulated by phosphorylation of α-subunit of translation initiation factor 2. Thus, Leishmania dynamically changes the relationship between mRNA and protein abundance as it adapts to new environmental circumstances. It is likely that similar mechanisms play a more important role than previously recognized in regulation of gene expression in other organisms.

Analysis of the Leishmania donovani transcriptome reveals an ordered progression of transient and permanent changes in gene expression during differentiation

Leishmania donovani is an intracellular protozoan parasite that causes kala-azar in humans. During infection the extracellular insect forms (promastigotes) undergo rapid differentiation to intracellular amastigotes that proliferates in phagolysosomes of mammalian macrophages. We used microarray-based expression profiling to investigate the time-course of changes in RNA abundance during promastigote-to-amastigote differentiation in a host-free system that mimics this process. These studies revealed that several hundred genes underwent an ordered progression of transient or permanent up- and down-regulation during differentiation. Genes that were permanently up-regulated in amastigotes were enriched for transporters and surface proteins, but under-represented in genes involved in protein and other metabolism. Most of these changes occurred late in the differentiation process, when morphological differentiation was essentially complete. Down-regulated genes were over-represented in those involved in cell motility, growth and/or maintenance, and these changes generally occurred earlier in the process. Genes that were transiently up- or down-regulated during differentiation included those encoding heat shock proteins, ubiquitin hydrolases, RNA binding proteins, protein kinases, a protein phosphatase, and a histone deacetylase. These results suggest that changes in mRNA abundance may be important in signal transduction, as well as protein and mRNA turnover, during differentiation. In addition to these mRNA changes, other transcripts including one or more rRNAs and snoRNAs, and non-coding RNAs from several telomeres, also showed substantial changes in abundance during the differentiation process. This paper provides the first genome-scale quantitative analysis of gene expression during the transition from promastigotes to amastigotes and demonstrates the utility of the host-free differentiation system.

Novel Intracellular SbV reducing activity correlates with antimony susceptibility in Leishmania donovani

The standard treatment of human visceral leishmaniasis involves the use of pentavalent antimony (Sb(V)). Its mechanism of action is unknown because of the limited information available about intracellular antimony metabolism and about the genes that regulate these processes. Herein, flow injection-inductively coupled plasma mass spectrometry (ICP-MS), flow injection hydride generation ICP-MS, and ion chromatography ICP-MS were used to measure antimony accumulation and intracellular metabolism in the human protozoan parasite Leishmania donovani. Amastigotes (the intracellular form) and promastigotes (the extracellular form) accumulate Sb(V) and Sb(III) via separate transport systems. Stage-specific intracellular Sb(V) reducing activity was apparent in amastigotes, which reduced the negligibly toxic Sb(V) to highly toxic Sb(III). This amastigote-specific reducing activity was deficient in the Pentostam-resistant mutant L. donovani Ld1S.20. These data indicate that parasite susceptibility to Sb(V) correlates with its level of Sb(V) reducing activity. Also, in promastigotes of both wild-type L. donovani and the Pentostam-resistant mutant L. donovani Ld1S.20, Sb(V) inhibited the toxicity of Sb(III) but not of As(III). Both Sb(V) and Sb(III) were toxic to wild-type amastigotes. However, as observed in promastigotes, in mutant amastigotes Sb(V) inhibits Sb(III) but not As(III) activity. Anion exchange chromatography showed that intracellular antimony metabolism occurred in both promastigotes and amastigotes. These data demonstrate that the interaction between the two antimony oxidation states occurs intracellularly, within the parasite. The results also indicate that Sb(V) anti-leishmanial activity is dependent on its reduction to Sb(III). The mechanism of this novel intracellular Sb(V) reduction has yet to be identified, and it may or may not be enzymatic. This is the first description of intracellular Sb(V) reducing activity in Leishmania as well as in any prokaryotic or eukaryotic cell.

Speciation of antimony(III) and antimony(V) in cell extracts by anion chromatography/inductively coupled plasma mass spectrometry

An analytical method for the separation and quantification of Sb(III) and Sb(V) using anion chromatography with ICP-MS is presented. The optimum conditions for the separation of the antimony species were established with 15 mmol/L nitric acid at pH 6 as eluent system on a PRP-X100 column. The retention times for antimony(V) and antimony(III) were 85 s and 300 s with detection limits of 0.06 microg/L and 0.29 microg/L, respectively. The proposed method was applied to cell extracts of Leishmania donovani, which were incubated with antimony(III) and antimony(V). Some metabolism seemed to occur within the cells.

Developmental regulation of proline transport in Leishmania donovani

Leishmania donovani are the causative agents of kala azar in humans. These organisms cycle between the proline-rich environment of the sand fly vector (extracellular promastigotes) and the sugar-rich condition in the mammalian host (intracellular amastigotes). Parasites have adapted to these extreme changes in proline concentrations: promastigotes utilize proline as a carbon source, whereas amastigotes utilize sugars and fatty acids. Previous studies have suggested that promastigotes and amastigotes express distinct proline transporters. However, the information available on these transporters is limited. In this work, proline transport was investigated in axenic L. donovani cultures. Three transport systems were identified: cation-dependent and -independent proline transporters in promastigotes (systems A and B, respectively) and a single cation-independent transporter in amastigotes (system C). Systems A and C have broad specificity to almost all amino acids and obtain optimum activity at acidic pH ranges (pH 6 and 5, respectively). System B is more specific to proline, as it is inhibited by only five amino acids. Temperature response analyses indicated that the transporters of both promastigotes and amastigotes perform best at 37 degrees C. The activity of system A during parasite differentiation was assessed. The transport activity of system A disappeared 3 days after promastigotes were induced to differentiate into amastigotes. In these cells, elevated temperature and acidic pH each suppressed the activity of system A. When amastigotes were induced to differentiate back into promastigotes, system A resumed its activity 24 h after differentiation was initiated. In conclusion, L. donovani obtain proline transport systems that are stage specific, regulated by both pH and temperature. This paper constitutes the first investigation of amino acid transport in axenic L. donovani.

Stage-specific activity of pentavalent antimony against Leishmania donovani axenic amastigotes

The standard treatment of human visceral leishmaniasis involves the use of pentavalent antimony (SbV) compounds. In recent years increasing numbers of clinical failures of treatment with SbV have been reported, probably due to the development of parasite resistance to this compound. The mode of action and mechanisms of resistance to SbV have not been fully elucidated. In the present study an axenic amastigote culture was used to study the in vitro responses of Leishmania donovani to SbV. Susceptibility to both sodium stibogluconate and meglumine antimoniate was found to be stage specific. Amastigotes were 73 to 271 times more susceptible to SbV than were promastigotes. As opposed to SbV, trivalent antimony (SbIII) was similarly toxic to both developmental stages. When promastigotes were transformed to amastigotes, susceptibility to meglumine antimoniate developed after 4 to 5 days, upon the completion of differentiation. In contrast, with transformation from amastigotes to promastigotes, resistance to meglumine antimoniate was acquired rapidly, within 24 h, before the completion of differentiation. The culture of promastigotes at an acidic pH (5.5) or at an elevated temperature (37 degrees C) alone did not lead to the appearance of SbV susceptibility, emphasizing the requirement of both these environmental factors for the development of SbV susceptibility. A previously isolated sodium stibogluconate (Pentostam)-resistant L. donovani mutant (Ld1S.20) is also resistant to meglumine antimoniate, indicating cross-resistance to SbV-containing compounds. In contrast, no cross-resistance was found with SbIII, suggesting a mechanism of SbV resistance different from that described in Leishmania tarentolae. These data show that L. donovani susceptibility to SbV is parasite intrinsic, stage specific, and macrophage independent.

Characterization of developmentally-regulated activities in axenic amastigotes of Leishmania donovani

Leishmania donovani is an obligatory intracellular parasite which cycles between the midgut of sand flies (extracellular promastigote) and the phagolysosomes of mammalian macrophages (intracellular amastigote). Promastigotes have been readily cultured, whereas axenic cultures of amastigotes have only recently been developed. A new method for in vitro differentiation of L. donovani promastigotes into amastigotes is presented, in which promastigotes are exposed to environmental changes that mimic the in vivo process. First, promastigotes are subjected to 37 degrees C + 5% CO2 for 24 h, and then are shifted to pH 5.5. Under these conditions, differentiation is completed within 120 h. In the reverse process, amastigotes are induced to differentiate back to promastigotes by transferring them to promastigote growth conditions (medium 199 at pH 7.4 and 26 degrees C). Axenic amastigotes closely resemble animal-derived amastigotes. They manifest all seven proteins of the amastigote-specific A2 gene family. They down-regulate lipophosphoglycan (LPG) synthesis and do not express it on their surface. LPG is up-regulated 2 h after inducing amastigotes to differentiate to promastigotes. Within 6 h, parasites resume the promastigote level of this molecule, although differentiation is completed only after 48 h. Axenic amastigotes also express amastigote-like metabolic activities of proline uptake, as well as thymidine and proline incorporation. In conclusion, the results indicate that the method developed for in vitro differentiation of L. donovani promastigotes to amastigotes is efficient and yields organisms resembling animal-derived amastigotes. Being able to induce in vitro differentiation of L. donovani provides us with an excellent tool to study Leishmania development and differentiation.

Biosynthesis of Leishmania lipophosphoglycan: solubilization and partial characterization of the initiating mannosylphosphoryltransferase

Lipophosphoglycan (LPG) is the predominant surface glycoconjugate of Leishmania promastigotes and consists of a capped polymer of Gal(beta1,4)Man(alpha1)-PO4 repeating units attached through a glycan core to a phosphatidylinositol anchor. We have solubilized the mannosylphosphoryltransferase from L. donovani promastigotes that initiates repeating unit synthesis using beta-dodecylmaltoside and other nonionic detergents with long alkyl chains. The detergent-solubilized enzyme, in the presence of GDP-Man and Mn2+, transferred Man(alpha1)-PO4 to two exogenous acceptor substrates: the glycan core from LPG and stachyose, a tetrasaccharide terminating in the same Gal(alpha1,6)Gal(alpha) disaccharide as glycan core. The activity is saturable with respect to GDP-Man, but not with respect to stachyose, suggesting that more than Gal(alpha1,6)Gal(alpha) is required in the acceptor substrate for optimal activity. In contrast to promastigotes, Leishmania amastigotes express lower levels of LPG by downregulating the addition of the repeating units. We compared the relative activity of the initiating mannosylphosphoryltransferase in microsomal fractions from axenic amastigotes and its promastigote counterpart, using stachyose as the acceptor substrate. The promastigote membranes were 3-fold more active relative to the amastigote membranes. These results provide evidence that the initiating mannosylphosphoryltransferase is developmentally regulated during the life-cycle of the Leishmania parasite.

Pentostam induces resistance to antimony and the preservative chlorocresol in Leishmania donovani promastigotes and axenically grown amastigotes

An axenic amastigote culture system was utilized to directly assess the stage-specific antileishmanial effects of antimony on amastigotes of Leishmania donovani devoid of the macrophage host cell. Pentostam, which contains antimony in the form of sodium stibogluconate and the preservative chlorocresol, was used. Cell density was quantified by measuring the activity of the stable enzyme ornithine decarboxylase. Dose-response curve analyses show that Leishmania promastigotes are susceptible to Pentostam, with the 50% inhibitory concentration (IC50) being 104 microg/ml, while amastigotes are more susceptible, with the IC50 being 24 microg/ml. Promastigotes and amastigotes are also susceptible to chlorocresol, with IC50s being 1.27 and 1.82 microg/ml, respectively. Given that promastigotes are insensitive to antimony, these results suggest that the increased susceptibility of amastigotes to Pentostam is due to the stage-specific activity of sodium stibogluconate. To further study this phenomenon, spontaneous resistance to Pentostam was induced in L. donovani promastigotes by increasing the concentration of Pentostam in the growth medium in a stepwise fashion. Two mutants, Ld1S.04 and Ld1S.20, grew at 0.4 and 2.0 mg of Pentostam per ml, respectively. Promastigotes of these mutants were 11 and 21 times, respectively, more resistant to Pentostam than the wild type. Amastigotes were 40 and 148 times, respectively, more resistant than the wild type. The mutants were also chlorocresol resistant; promastigotes were 6 and 9 times, respectively, more resistant than the wild type, and amastigotes were 14 and 35 times, respectively, more resistant than the wild type. These data show that resistance to Pentostam induced in antimony-insensitive promastigotes is manifested in amastigotes as resistance both to pentavalent antimony and to chlorocresol. The axenic amastigote system is a unique tool which enables direct evaluation of the activity of antileishmanial compounds on the amastigote devoid of its host cell.

The accumulation and compartmentalization of isometamidium chloride in Trypanosoma congolense, monitored by its intrinsic fluorescence

Interaction of the trypanocide isometamidium chloride with components of Trypanosoma congolense results in characteristic shifts in the intrinsic fluorescence of the drug. The specificity of this interaction was investigated by analysing the effects of various physicochemical manipulations on its fluorescence properties. The characteristic shifts involved a preferential increase in the intensity of one emission peak over the other, resulting in a systematic increase in the ratio of fluorescence intensities. These effects were apparently due to constraints on fluorophore free rotation in the solution (that is, viscosity). Purified DNA produced similar effects in a saturable manner displaying high affinity for the drug, indicating that the constraint involves binding of the drug to high-affinity binding sites within the DNA. Such binding sites were demonstrated in lysates derived from trypanosomal cells. The binding sites were associated with macromolecular species (M(r) > 12000), and were partly disrupted by thermal denaturation and proteolysis. Treatment with DNase 1 produced high levels of disruption of the binding sites (> 85%), indicating an involvement of DNA in the binding. BSA demonstrated weak non-specific binding of the drug. Entry of drug into live trypanosomal cells (monitored by 14C-labelled drug uptake) was paralleled by fluorescence shifts observed under comparable conditions of drug concentration and buffer conditions. Both systems (fluorescence shifts and accumulation of labelled drug) indicated the presence of a saturable membrane transporter with high affinity for the drug. We conclude that monitoring the fluorescence shifts of isometamidium constitutes a sensitive and highly specific probe for entry of the drug into trypanosomal cells, thereby enabling resolution of the transport events involved.

The role of pH and temperature in the development of Leishmania parasites

Protozoans of the genus Leishmania are obligate intracellular parasites that cycle between the midgut of sandflies and the phagolysosomes of mammalian macrophages and therefore are exposed to extreme environmental changes. Recent evidence obtained from in vitro experiments indicate that such environmental changes trigger a developmental program in the parasites. Thus, following heat shock, promastigotes from certain Leishmania species differentiate to amastigotes. Promastigotes also respond to acidification of their environment by changing the expression of a number of genes. However, the combination of both low pH and high temperature induces the transformation of the promastigote to the amastigote in all Leishmania species examined to date. This review discusses the role of pH and heat shock in gene regulation and its contribution to the differentiation processes in Leishmania spp. Cycling between cold-blooded insect vectors and the warm-blooded mammalian host is not unique to Leishmania spp., but typical to most parasitic protozoa. It is therefore likely that the mechanism of stress-induced differentiation is shared by other mammalian parasites.

Regulation of L-proline transport in Leishmania donovani by extracellular pH

We have previously shown that Leishmania donovani promastigotes adapted to long-term culture at acidic pH can serve as a model to study parasite development in a lysosomal-like environment. In this study we investigated the effect of growth pH on L. donovani L-proline transport systems. Reducing the pH of the growth medium causes an up to 7-fold decrease in the extent of L-proline transport. Transport resumes after switching the culture from pH 4.5 to pH 7 for 48 h by a protein synthesis-dependent process. The pH optimum for transport changes from 7.5 in promastigotes grown at pH 7 to 5.5 in cells grown at pH 4.5. In addition, kinetic analysis of L-proline transport showed that Vmax in pH 4.5-grown L. donovani promastigotes is one-tenth that of cells grown at pH 7 (4.5 and 44.7 nmol min-1 (10(8) cells)-1, respectively). The apparent Km for L-proline in pH 4.5 promastigotes is one-half of the Km in pH 7 cells (0.30 and 0.65 mM, respectively). In contrast to L-proline transport, D-glucose transport demonstrates a growth pH-independent activity: Km and Vmax as well as optimum pH of transport are similar in promastigotes grown at either pH 7 or pH 4.5. Taken together, the results indicate that in L. donovani, expression and activity of L-proline transport is regulated by culture pH. The pH-dependent expression of L-proline transporters may be of physiological significance during the promastigote-amastigote transition.

Fluorescence analysis of the interaction of isometamidium with Trypanosoma congolense

Isometamidium chloride (Samorin) is the only compound recommended for prophylaxis against bovine trypanosomiasis in sub-Saharan Africa. The fluorescence property of this compound was used to investigate the interaction of the molecule with in vitro-derived bloodstream forms of Trypanosoma congolense IL 1180. Incubation of isometamidium with trypanosomes at 37 degrees C for 180 min resulted in a gradual alteration of the lambda max. with time (from 600 to 584 nm) and an increase in the intensity of trypanosome-associated fluorescence of approx. 2-fold. The alteration in fluorescence was temperature-dependent and inhibited by the addition of N-ethylmaleimide. In contrast, with intact cells addition of digitonin caused a rapid increase in fluorescence intensity to approximately four times that observed with intact cells. Uptake of isometamidium was also determined using radiolabelled drug; the results indicated that the time course of the uptake process resembled the fluorescence profile and was temperature-dependent. The results therefore indicate that the alteration of fluorescence is due to interaction of isometamidium with an intracellular component(s) and that isometamidium is transported across the plasma membrane via a protein carrier. The data also indicate that the described fluorescence technique can be used to investigate the role of membrane transport in resistance to isometamidium.

Leishmania donovani: characterization of a 38-kDa membrane protein that cross-reacts with the mammalian G-protein transducin

We investigated the presence in Leishmania donovani promastigotes of proteins with homology to the G-proteins known to mediate signal transduction in other organisms. [alpha 32P]GTP binding experiments revealed the presence in the promastigote membrane of GTP-binding sites with high affinity and specificity. Experiments with antisera directed against mammalian G-proteins showed that the promastigotes possess a 38-kDa protein (p38) which strongly reacts with an antiserum directed against a decapeptide containing the C-terminal sequence of transducin, the G-protein that mediates visual signal transduction. The interaction of p38 with the antiserum is specifically blocked by the decapeptide antigen. p38 is enriched in plasma membranes and is absent in cytosol and in a mitochondria-enriched fraction. p38 was also detected in two other Leishmania species, L. mexicana and L. major. The migration of p38 upon sucrose gradient centrifugation of detergent extract of L. donovani membranes corresponded to Mr of approximately 70,000, indicating that p38 is part of an oligomeric structure. The findings suggest that p38 may be a component of a transmembrane signal transduction system in Leishmania.

Localization of the plasma membrane and mitochondrial H(+)-ATPases in Leishmania donovani promastigotes

Immunochemical methods were used to characterize the proton-translocating ATPases (H(+)-ATPases) of the plasma membrane and mitochrondrion of Leishmania donovani promastigotes. Antisera directed against the plasma membrane H(+)-ATPase of Saccharomyces cerevisiae reacted with a 66 kDa membrane protein of L. donovani promastigotes. By immunocytochemistry, the antiserum was shown to label the cell and flagellar surface of promastigotes as well as the Golgi apparatus and the membrane of intracellular organelles. The target antigen was shown to possess ATPase activity resembling the leishmanial H(+)-ATPase activity. Antisera raised against the beta-subunit of the F0F1-ATPase of Escherichia coli reacted with a 56 kDa protein in L. donovani promastigotes. Ultrastructurally, the anti-beta-subunit antibody was exclusively associated with the mitochondrion in these cells. This antiserum immunoprecipitates ATP hydrolytic activity typical of the F1 beta-subunit activity of the mitochondria of higher eukaryotes.

Tricyclic drugs reduce proton motive force in Leishmania donovani promastigotes

Tricyclic compounds have been suggested as potential anti-leishmanial drugs. We have studied the effect of tricyclic drugs on several cellular functions in L. donovani promastigotes. Imipramine inhibits proline transport and reduces delta pH and cellular ATP at relatively high concentrations (IC50 = 50-80 microM). High concentrations of imipramine are also required to kill L. donovani promastigotes (LD50 greater than 50 microM). The presence of a chlorine atom in the side ring of either imipramine or promazine results in a three-fold increase in both IC50 and LD50 values. Tricyclic compounds in which the nitrogen in the middle ring was substituted with a carbon atom (amitryptyline and chlorprothixene) are most effective in causing cell death and in decreasing proline transport and delta pH (IC50 congruent to 5 microM), whereas depletion of cellular ATP requires a higher drug concentration (IC50 = 12 microM). Transchlorprothixene has IC50 values for proline transport, delta pH and cellular ATP that are similar to those of amitriptyline, whereas the cis isomer is less active. Imipramine, chlomipramine and chlorpromazine decrease the membrane potential in promastigotes. There is a direct correlation between inhibition of membrane transport of proline and the size of the membrane potential at various concentrations of the drugs. Taken together, the multiple effects of the tricyclic drugs on cellular functions in Leishmania suggest that the drugs cause cellular death by non-specific mechanisms, probably involving a general increase in membrane permeability.

Maintenance of cytoplasmic pH and proton motive force in promastigotes of Leishmania donovani

Three methods were used to measure intracellular pH (pHi) of Leishmania donovani promastigotes: (a) measurement of the fluorescence of the pH indicator 2',7'-bis-(carboxyethyl)-5,6-carboxyfluorescein; (b) pH null point assays; and (c) determination of the distribution across the promastigote plasma membrane of the fluorescent amine acridine orange and of the weak acid 5,5-dimethyl-2,4-oxazolidinedione. The three methods gave similar results and showed that promastigotes of L. donovani maintain pHi at a narrow range of 6.4-6.7, throughout an extracellular pH (pHo) range of 5.5-7.4. L-Proline transport in L. donovani promastigotes, which is known to be coupled to proton translocation, was used to estimate the proton electrochemical gradient across parasite plasma membrane. While proline uptake is optimal at pHo 7.5, an outward-directed concentration gradient is obtained at steady state throughout a pHo range of 5-8. The calculated electrochemical gradient of proline across the parasite plasma membrane at steady state is 90-100 mV within a pHo range of 5-8, suggesting an almost constant proton electrochemical gradient at this pHo range. Taken together, the results show that the parasites regulate both pHi and the size of the chemiosmotic energy required to drive active transport of nutrients.

Regulation of intracellular calcium in promastigotes of the human protozoan parasite Leishmania donovani

By using the fluorescent Ca2+ indicator fura 2, we show that the concentration of free calcium in the cytoplasm of Leishmania donovani promastigotes is maintained at very low levels (73.5 +/- 10-94 +/- 8 nM at a [Ca2+]i range of 0-1 mM). The maintenance of low [Ca2+]i is energy-dependent as it is disrupted by KCN, H+-ATPase inhibitors, and ionophores. KCN, nigericin, and N,N'-dicyclohexylcarbodiimide increase cytosolic free calcium by mobilizing calcium from intracellular pools. Monensin and oligomycin increase [Ca2+]i by allowing influx of calcium from the external medium through the plasma membrane, but they have no effect on intracellular pools. Intracellular traffic of calcium was examined by measuring the transport of 45Ca2+ in digitonin-permeabilized promastigotes. Two transport systems for calcium were identified in these cells. One is respiration-dependent, suggesting a mitochondrial localization. A second system is respiration-independent but requires either endogenous or externally added ATP. The ATP-dependent Ca2+ transport is optimal at pH 7.1, has high affinity for calcium (Km = 92 nM, Vmax = 1 nmol/min/mg of protein), and is sensitive to orthovanadate. These properties suggest the presence of a Ca2+-ATPase similar to that of mammalian endoplasmic reticulum. Taken together, the results indicate that [Ca2+]i in L. donovani promastigotes is regulated at low concentration by mechanisms similar to those found in higher eukaryotic cells.

Binding of thyrotropin to selected Mycoplasma species: detection of serum antibodies against a specific Mycoplasma membrane antigen in patients with autoimmune thyroid disease

Radiolabeled human (hTSH) and bovine (bTSH) thyroid stimulating hormone was shown to bind to five species of Mycoplasma, the wall-less prokaryotes. The maximum binding capacity of 125I-bTSH to these five species was about 7.9 x 10(-13) moles-1.4 x 10(-12) moles for 50-100 micrograms protein with dissociation constants of approximately 1.7 to 2.2 x 10(-7)M. Approximately 50% of the 125I-bTSH binding was displaced by excess, unlabeled bTSH or hTSH, but labeled bTSH was not effectively displaced by growth hormone, LH, FSH, prolactin, or the beta subunit of hTSH, FSH and LH. Antisera prepared against Mycoplasma gallisepticum and Mycoplasma pneumoniae bound to human thyroid membranes and guinea pig fat cells, suggesting that receptors on human thyroid tissues and on Mycoplasma cells may have similarities in antigenicity. These findings were substantiated by the occurrence of TSH binding to Mycoplasma antisera. Further, sera from three of six patients with Graves' disease containing antibodies to thyroid tissues also reacted to a 108 Kd polypeptide of Mycoplasma gallisepticum.

Identification of a surface membrane proton-translocating ATPase in promastigotes of the parasitic protozoan Leishmania donovani

ATPase activities were measured in surface membranes and mitochondria isolated from promastigotes of the parasitic protozoan Leishmania donovani. The two enzymes were differentiated on the basis of pH optima, inhibitor sensitivity and by immunochemical methods. The surface-membrane (SM-) ATPase had an activity of 100 nmol/min per mg of protein, which was optimal at pH 6.5. The enzyme was Mg2+-dependent, partially inhibited by Ca2+, and unaffected by Na+ or K+. The SM-ATPase was inhibited by orthovanadate, NN'-dicyclohexylcarbodi-imide, and N-ethylmaleimide [IC50 (concentration causing half-maximal inhibition) 7.5, 25 and 520 microM respectively]; however, it was unaffected by ouabain, azide or oligomycin. The SM-ATPase demonstrated a Km of 1.05 mM and a Vmax. of 225 nmol/min per mg of protein. Moreover, fine-structure cytochemical results demonstrated that the SM-ATPase was localized to the cytoplasmic lamina of the parasite SM. A method was devised for the isolation of SM-derived vesicles. These were used to demonstrate the proton-pumping capacity of the SM-ATPase. Cumulatively, these results constitute the first demonstration of a surface-membrane proton-translocating ATPase in a parasitic protozoan.

Immunochemical evidence for an active (F1-F0)-ATPase in mycoplasmas

In all Acholeplasma, Mycoplasma and Spiroplasma species tested, a protein capable of reacting with antibodies prepared against the beta subunit of the proton-ATPase complex from yeast, chloroplasts and Escherichia coli was detected. The reactive protein of M. gallisepticum was found to be catalytically active, suggesting that mycoplasmas, as other bacteria, possess a proton-translocating ATPase. Characterization of the ATPase activity of M. gallisepticum indicates that this organism also possesses a Na+-stimulated ATPase activity that differs from the proton-ATPase in its pH profile and its resistance to dicyclohexylcarbodiimide (DCCD).

Kinetic correlation of the acquisition of resistance to immune attack in schistosomula of Schistosoma mansoni with a developmental change in membrane potential

When Schistosoma mansoni cercariae penetrate the skin of the mammalian host they rapidly pass from fresh water to a high salt physiologic environment and transform into schistosomula. Following this transition, the parasites migrate from the skin to the lungs during which time they change from being highly susceptible to immune attack to being refractory, as measured by in vitro cytotoxicity assays. In this study, in vivo or in vitro schistosomula of different ages were examined for developmentally linked changes in membrane function which might correlate with the attainment of the resistant state. In particular, alterations in the distribution of tetraphenylphosphonium (TPP+), a synthetic lipophilic cation which shows a potential dependent partition across membranes, were followed. Three-hour-old schistosomula, which are greater than 75% susceptible to antibody-dependent complement-mediated attack or lymphokine-activated macrophage-mediated cytotoxicity, acquired TPP+ at a similar rate and steady state level to 5-day-old lung worms, which were completely resistant to both these effector mechanisms. The addition of ouabain, a Na+/K+-ATPase inhibitor, caused a 50% decrease in both the rate and steady state of TPP+ uptake by 3 h parasites but had little effect on these parameters in lung worms. Valinomycin, a K+-ionophore, completely inhibited TPP+ influx in both stages. The characteristics of TPP+ efflux from 3-h and 5-day-old parasites preloaded with the cation were found to be dissimilar. Whereas 30% of acquired TPP+ was lost from lung worms within 2 h, only 10% of acquired cation was released from 3-h schistosomula during the same period.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and biochemical characterization of the plasma membrane glucose transporter of Leishmania donovani

The plasma membrane glucose transporter of Leishmania donovani, an obligate intracellular protozoan parasite of humans, was specifically labeled, identified, and biochemically characterized. Cytochalasin B, a known inhibitor of D-glucose transport in mammalian cells, but not cytochalasin E inhibited the transport of 2-deoxy-D-glucose in the extracellular promastigote form of this organism. Hydroxysuccinimidyl-4-azido-benzoate was used to photochemically cross-link [3H] cytochalasin B to the glucose transporter in isolated surface membranes and plasma membrane vesicles of L. donovani promastigotes. In sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), the covalently labeled glucose transporter migrated as a 20-30-kDa protein band. This band was eluted from SDS-PAGE gels and subsequently analyzed by isoelectric focusing. The latter revealed two major peaks focusing at pH 6.8 and 6.6 [3H]Cytochalasin B-labeled membrane activity was detergent-solubilized, bound to concanavalin A-agarose beads, and specifically eluted with alpha-methyl mannoside. Analysis of the eluted material by SDS-PAGE revealed a D-glucose-inhibitable cytochalasin B peak with an apparent Mr approximately 20,000. The cumulative results indicate that the glucose transporter of L. donovani promastigotes is a glycoprotein which contains mannose as its major carbohydrate constituent.

The beta-subunit of the F1F0-ATPase is conserved in mycoplasmas

Monospecific polyclonal antibodies that were generated against the beta-subunit of Escherichia coli ATPase (F1Fo) cross-reacted with a protein present in the cells of several Mycoplasma and Acholeplasma species. In Mycoplasma gallisepticum, the reactive protein was found almost exclusively in the cell membrane. This protein had an apparent molecular mass of approximately 52 kDa and could not be released from the membranes by repeated washings with either low or high salt solutions in the presence or absence of EDTA. The reactive protein was found to be catalytically active, exhibiting up to 44% of the total membrane-bound ATPase activity. We suggest that mycoplasmas possess a F1Fo-ATPase which undergoes structural modification(s) allowing its integration into the membrane.

Antidepressants cause lethal disruption of membrane function in the human protozoan parasite Leishmania

The antidepressant compounds clomipramine and nitroimipramine were cidal to extracellular promastigotes of both human protozoan parasites Leishmania donovani and Leishmania major. Clomipramine also killed amastigotes of both species within murine macrophages with no apparent toxicity to the host cells. Further, amastigotes were more sensitive than promastigotes to clomipramine. Clomipramine (100 micromoles per liter or 0.2 nanomole per 1 X 10(6) cells) inhibited L-proline transport in promastigotes. Synergistic inhibition of L-proline transport was observed with clomipramine after addition of either of the ionophores valinomycin or nigericin. These observations suggest that the cytotoxic effects of clomipramine result from its disruption of the proton electrochemical gradient of the parasite surface membrane.

Glucose transport in Leishmania donovani promastigotes

Mid-log phase Leishmania donovani promastigotes accumulated 2-deoxy-D-glucose (2-DOG) via a carrier mediated transport system, maintaining an apparent Km of 24.4 microM and a Vmax of 3.12 nmol mg-1 protein min-1. D-Glucose but not L-glucose competitively inhibited the 2-DOG transport with an apparent Ki of 18.7 microM. Transport of 2-DOG was inhibited by 2,4-dinitrophenol and NaN3. The parasites maintained a 2-DOG gradient of at least 79 fold across the surface membrane, demonstrating the active nature of the transport system.

Escherichia coli intracellular pH, membrane potential, and cell growth

We studied the changes in various cell functions during the shift to alkaline extracellular pH in wild-type Escherichia coli and in strain DZ3, a mutant defective in pH homeostasis. A rapid increase in membrane potential (delta psi) was detected in both the wild type and the mutant immediately upon the shift, when both cell types failed to control intracellular pH. Upon reestablishment of intracellular pH - extracellular pH and growth in the wild type, delta psi decreased to a new steady-state value. The electrochemical proton gradient (delta muH+) was similar in magnitude to that observed before the pH shift. In the mutant DZ3, delta psi remained elevated, and even though delta muH+ was higher than in the wild type, growth was impaired. Cessation of growth in the mutant is not a result of cell death. Hence, the mutant affords an interesting system to explore the intracellular-pH-sensitive steps that arrest growth without affecting viability. In addition to delta muH+, we measured respiration rates, protein synthesis, cell viability, induction of beta-galactosidase, DNA synthesis, and cell elongation upon failure of pH homeostasis. Cell division was the only function arrested after the shift in extracellular pH. The cells formed long chains with no increase in colony-forming capacity.

A monoclonal antibody to rhesus erythrocyte band 3 inhibits invasion by malaria (Plasmodium knowlesi) merozoites

Receptors on erythrocytes and malaria parasites mediate specific attachment and junction formation between these cells that lead to invasion of the erythrocytes. We identified monoclonal antibody A9 and its subclone A9D3 that bound to rhesus erythrocytes and blocked invasion of the erythrocytes by Plasmodium knowlesi merozoites. The monoclonal antibodies did not block attachment, the initial step in invasion, although swelling and crenation of the erythrocyte, which normally occur after attachment, were rarely observed in the presence of antibody. The monoclonal antibody immunoprecipitated rhesus erythrocyte band 3. It bound to erythrocytes of another Old World monkey, the kra monkey, but not to erythrocytes of New World monkeys, chimpanzees, or man. Since the antibody did not bind to human erythrocytes, we could test for nonspecific toxicity to the parasite by studying the effect of the ascites and purified antibody on invasion of human erythrocytes. The antibody caused a minimal reduction in invasion of human erythrocytes, a reduction no greater than that seen with an unrelated monoclonal antibody. Further evidence that the inhibition was specific came from study of Fab fragments of A9D3. Column-purified Fab fragments reduced invasion of rhesus erythrocytes without affecting invasion of human erythrocytes. Fab fragments preabsorbed with rhesus erythrocytes did not inhibit invasion. From the above data, we conclude that band 3 is involved in a stage in the invasion process after initial recognition.

Effects of ionophores and dicyclohexylcarbodiimide on Mycoplasma gallisepticum adherence to erythrocytes

To test the influence of the electrochemical ion gradient across mycoplasma membranes on the capacity of organisms to adhere to host cells, Mycoplasma gallisepticum cells were treated with valinomycin, carbonylcyanide m-chlorophenylhydrazone, and N,N'-dicyclohexylcarbodiimide (DCCD) singly or in combination. Uptake of [3H]tetraphenylphosphonium by the treated cells was employed as a measure of the effects of the ionophores on membrane potential. In the absence of K+, valinomycin increased, whereas carbonylcyanide m-chlorophenylhydrazone, and DCCD decreased [3H]tetraphenylphosphonium uptake. However, with a high level of K+ or with DCCD, uptake of [3H]tetraphenylphosphonium in the presence of valinomycin decreased below control levels, indicating that, generally, the ionophores affected membrane potential in the expected manner. The treated organisms were tested for their capacity to attach to glutaraldehyde-fixed human erythrocytes. DCCD was the best inhibitor of mycoplasma attachment, and in combination with valinomycin attachment, capacity decreased by about 40%. The combination of valinomycin plus carbonylcyanide m-chlorophenylhydrazone was less effective; it decreased attachment by about 15 to 25%. It was concluded that the dissipation of ion gradients across cell membranes decreases only partially mycoplasma adherence, in line with previous findings that isolated mycoplasma membranes retain the major part of the attachment capacity of intact cells.

The proton electrochemical gradient in Escherichia coli cells

The internal pH of Escherichia coli cells was estimated from the distribution of either 5,5-[14C]dimethyl-2,4-oxazolidinedione or [14C]methylamine. EDTA/valinomycin treatment of cells was employed to estimate delta psi from 86Rb+ distribution concomitant with the delta pH for calculation of delta muH. Respiring intact cells maintained an internal pH more alkaline by 0.63-0.75 unit than that of the milieu at extracellular pH 7, both in growth medium and KCl solutions. The delta pH decreased when respiration was inhibited by anaerobiosis or in the presence of KCN. The delta muH, established by EDTA/valinomycin-treated cells, was constant (122-129 mV) over extracellular potassium concentration of 0.01 mM-1 mM. At the lower potassium concentration delta psi (110-120 mV) was the predominant component, and at the higher concentration delta pH increased to 0.7 units (42 mV). At 150 mM potassium delta muH was reduced to 70 mV mostly due to a delta pH component of 0.89 (53 mV). The interchangeability of the delta muH components is consistent with an electronic proton pump and with potassium serving as a counter ion in the presence of valinomycin. Indeed both parameters of delta muH decreased in the presence of carbonylcyanide p-trifluoromethoxyphenylhydrazone. The highest delta pH of 2 units was observed in the intact cells at pH 6; increasing the extracellular pH decreased the delta pH to 0 at pH 7.65 and to -0.51 at pH 9. A similar pattern of dependence of delta pH on extracellular pH was observed in EDTA/valinomycin-treated cells but the delta psi was almost constant over the whole range of extracellular pH values (6-8) implying electroneutral proton movement. Potassium is specifically required for respiration of EDTA-treated E. coli K12 cells since other monovalent or divalent cations could not replace potassium and valinomycin was not required.