Goutam Mandal

Increased levels of thiols protect antimony unresponsive Leishmania donovani field isolates against reactive oxygen species generated by trivalent antimony

The current trend of antimony (Sb) unresponsiveness in the Indian subcontinent is a major impediment to effective chemotherapy of visceral leishmaniasis (VL). Although contributory mechanisms studied in laboratory-raised Sb-R parasites include an up-regulation of drug efflux pumps and increased thiols, their role in clinical isolates is not yet substantiated. Accordingly, our objectives were to study the contributory role of thiols in the generation of Sb unresponsiveness in clinical isolates. Promastigotes were isolated from VL patients who were either Sb responsive (n=2) or unresponsive (n=3). Levels of thiols as measured by HPLC and flow cytometry showed higher basal levels of thiols and a faster rate of thiol regeneration in Sb unresponsive strains as compared with sensitive strains. The effects of antimony on generation of reactive oxygen species (ROS) in normal and thiol-depleted conditions as also their H2O2 scavenging activity indicated that in unresponsive parasites, Sb-mediated ROS generation was curtailed, which could be reversed by depletion of thiols and was accompanied by a higher H2O2 scavenging activity. Higher levels of thiols in Sb-unresponsive field isolates from patients with VL protect parasites from Sb-mediated oxidative stress, thereby contributing to the antimony resistance phenotype.

Anti-inflammatory effect of allylpyrocatechol in LPS-induced macrophages is mediated by suppression of iNOS and COX-2 via the NF-κB pathway

The crude ethanol extract of Piper betle leaf is reported to possess anti-inflammatory activity which has been suggested to be mediated by allylpyrocatechol (APC). In the present study, we have demonstrated the anti-inflammatory effects of APC (10 mg/kg, p.o.) in an animal model of inflammation. To investigate the mechanism(s) of this anti-inflammatory activity, we examined its effects on the lipopolysaccaride (LPS)-induced production of NO and PGE(2) in a murine macrophage cell line, RAW 264.7. APC inhibited production of NO and PGE(2) in a dose dependent manner as also decreased mRNA expression of iNOS, COX-2, IL-12p40 and TNF-alpha. Since nuclear factor-kappaB (NF-kappaB) appears to play a central role in transcriptional regulation of these proteins, we investigated the effects of APC on this transcription factor. APC inhibited LPS induced nuclear factor-kappaB (NF-kappaB) activation, by preventing degradation of the inhibitor kappaB (IkappaB). Taken together, our data indicates that APC targets the inflammatory response of macrophages via inhibition of iNOS, COX-2 and IL-12 p40 through down regulation of the NF-kappaB pathway, indicating that APC may have therapeutic potential in inflammation associated disorders.

Elevated levels of tryparedoxin peroxidase in antimony unresponsive Leishmania donovani field isolates

Enhancement of the anti-oxidant metabolism of Leishmania parasites, dependent upon the unique dithiol trypanothione, has been implicated in laboratory-generated antimony resistance. Here, the role of the trypanothione-dependent anti-oxidant pathway is studied in antimony-resistant clinical isolates. Elevated levels of tryparedoxin and tryparedoxin peroxidase, key enzymes in hydroperoxide detoxification, were observed in antimonial resistant parasites resulting in an increased metabolism of peroxides. These data suggest that enhanced anti-oxidant defences may play a significant role in clinical resistance to antimonials.

Flow cytometric determination of intracellular non-protein thiols in Leishmania promastigotes using 5-chloromethyl fluorescein diacetate.

Leishmania parasites lack catalase and therefore, their anti-oxidant system hinges primarily upon non-protein thiols; accordingly, depletion of thiols could potentially serve as an effective drug target. We have developed a flow cytometry based assay using 5-chloromethyl fluorescein diacetate based upon its selective staining of non-protein thiols. Its specificity was confirmed using buthionine sulphoximine (a gamma-glutamyl cysteine synthetase inhibitor), diamide (an oxidizing agent of intracellular thiols) and N-ethylmaleimide (a covalent modifier of cysteine residues) as evidenced by reduction in fluorescence; furthermore, restoration of fluorescence by N-acetyl cysteine corroborated specificity of 5-chloromethyl fluorescein diacetate to measure non-protein thiols. Differences in basal level of thiols in antimony sensitive and antimony resistant Leishmania field isolates were detected. The depletion of non-protein thiols by conventional anti-leishmanial drugs e.g. antimony and miltefosine was demonstrated. Furthermore, fluorescence was unaffected by depletion of ATP in majority of the strains studied, indicating that 5-chloromethyl fluorescein diacetate is not a substrate for the pump operative in most Leishmania donovani strains. Taken together, measurement of 5-chloromethyl fluorescein diacetate fluorescence is an effective method for monitoring non-protein thiols in Leishmania promastigotes.

Monitoring of intracellular nitric oxide in leishmaniasis: Its applicability in patients with visceral leishmaniasis

Nitric oxide (NO) has been demonstrated to be a principal effector molecule responsible for mediating intracellular killing of Leishmania parasites, the causative organism of leishmaniasis. As measurement of intracellular NO remains a challenge to biologists, we have developed a flow cytometric approach to perform real time biological detection of NO within Leishmania parasites and parasitized macrophages using a membrane permeable derivative of diaminofluorescein [4,5‐diaminofluorescein diacetate (DAF‐2DA)]. Initially, assay optimization was performed in Leishmania donovani promastigotes, assay specificity being confirmed using both a NO donor [S‐nitroso‐N‐acetyl‐penicillamine (SNAP)] and a NO scavenger [2‐(4‐carboxyphenyl)‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide, C‐PTIO]. Using 40 μM DAF‐2DA, basal levels of intracellular NO were measured which varied in different Leishmania species; addition of conventional anti‐leishmanial drugs, antimony and miltefosine translated into a dramatic increase in DAF‐2T fluorescence. Furthermore, the assay also measured levels of NO in macrophages, but needed a 20 fold lower concentration of DAF‐2DA, being 2 μM. Following parasitization, levels of NO decreased which was normalized following treatment with anti‐leishmanial drugs. Similarly monocytes of patients with visceral leishmaniasis at disease presentation showed decreased levels of NO which too reverted on completion of treatment. Taken together, this study opens new perspectives of research regarding monocyte function and provides a real time approach for monitoring the effect of anti‐leishmanial compounds.

Generation of an aquaglyceroporin AQP1 null mutant in Leishmania major.

The Leishmania aquaglyceroporin AQP1 plays an important physiological role in water and uncharged polar solutes transport, volume regulation, osmotaxis, and is a key determinant of antimony resistance. By targeted gene disruption, we generated a Leishmania major promastigote AQP1 null mutant. This required several attempts but a chromosomal null AQP1 mutant was obtained by loss of heterozygosity in the presence of a rescue plasmid encoding AQP1. Growth in the absence of selection led to the loss of the rescuing plasmid, indicating that AQP1 is not essential for Leishmania viability. The AQP1-null mutant was resistant to antimonyl tartrate (SbIII) and arsenite (AsIII) due to a decrease import of these metalloids. It also exhibited alterations in its osmoregulation abilities compared with wild-type cells. This is the first report of the generation of a genetic AQP1 null mutant in Leishmania parasite, confirming its physiological function and role in resistance to antimonials, the therapeutic mainstay against Leishmania.