Identification of GR and TrxR systems in Setaria cervi: Purification and characterization of glutathione reductase

In silico exploration and in vitro validation of the filarial thioredoxin reductase inhibitory activity of Scytonemin and its derivatives

Lymphatic filariasis (LF) caused by the vector borne parasitic nematode Wuchereria bancrofti is of major concern of the World Health Organization (WHO). Lack of potential drug candidates worsens the situation. Presently available drugs are promising in killing the microfilaria (mf) but are not effective as adulticidal therapeutics. Previous studies have revealed that routine administration of the available drugs (albendazole, ivermectin and albendazole) sometime is associated with severe adverse effects (SAEs) in co-infection state. Therefore, potential and safe therapeutics are still required. Earlier studies on filarial thioredoxin reductase (TrxR) have shown that successful inhibition of it can lead to apoptotic death of the parasites. TrxR in filarial parasites plays a significant role in disease progression and pathogenesis, hence efficient non-reversible inhibition of TrxR can be a good strategy to treat LF. In this research, inhibitory potential of Scytonemin, a cyanobacterial metabolite on filarial TrxR was evaluated via different in silico methods and validated through in vitro experiments. Parasite death upon exposure to Scytonemin can be correlated with the TrxR inhibiting capacity of the compound. Therefore, this cyanobacterial-derived compound may possibly be used further as novel and safe therapeutic candidate against filarial infection.Communicated by Ramaswamy H. Sarma.

Antifilarial efficacy of andrographolide: Ex vivo studies on bovine filarial parasite Setaria cervi

Lymphatic filariasis caused by filarial nematode is an important disease leading to considerable morbidity throughout tropical countries. Even after specific elimination programs, the disease continue to spread in endemic countries. Thus newer therapeutic interventions are urgently needed to control the spread. In the present study, we have seen the effect of andrographolide (andro), a diterpenoid lactone from the leaves of Andrographis paniculata on filarial parasite Setaria cervi. There was time and concentration dependent decrease in motility and viability leading to death of parasite after 6 h of the exposure of andro. Andro showed potential antifilarial activity with an IC₅₀ value of 24.80 μM assessed through MTT assay. There was concentration dependent decrease in the antioxidant enzymes activity and increase in proapoptotic markers after 5 h exposure of andro. Further, molecular docking analysis revealed that andro binds with filarial glutathione-S-transferase at glutathione (GSH) binding site and inhibiting enzyme activity competitively. Andro induced oxidative stress mediated apoptosis in parasites as evidenced by increase in the intracellular reactive oxygen species (ROS) and apoptotic markers.Therefore this study suggested that andro could be further explored as a new antifilarial drug.

Inhibition of thioredoxin reductase (TrxR) triggers oxidative stress-induced apoptosis in filarial nematode Setaria cervi channelized through ASK-1-p38 mediated caspase activation

Inhibition of an imperative antioxidant enzyme with subsequent death is a victorious and widely accepted strategy to combat various infectious diseases. Among different antioxidant enzymes, thioredoxin reductase (TrxR) is an exclusive one. Studies have revealed that direct inhibition of TrxR by different classes of chemical moieties promptly results in the death of an organism. Especially the structural as well as biochemical modifications of the enzyme upon inhibition project serious threat towards the subject organism. Herein, an attempt was made to inhibit TrxR of filarial species by administering Auranofin, 1 chloro 2,4 dinitrobenzene (CDNB), Curcumin, and a novel carbamo dithioperoxo(thioate) derivative (4a). Our study has revealed that inhibition of TrxR resulted in the induction of the classical CED pathway of apoptosis along with the intrinsic and extrinsic pathways of apoptosis (Caspase mediated) routed through the ASK-1/p38 axis. Druggability analysis of filarial TrxR for the selected compounds was performed in silico through molecular docking studies. Therefore, this study attempts to decipher the mechanism of apoptosis induction following TrxR inhibition. The safety of those four compounds in terms of dose and toxicity was taken under consideration. Thitherto, the mechanism of TrxR mediated initiation of cell death in filarial parasite has remained undercover, and therefore, it is a maiden report on the characterization of apoptosis induction upon TrxR inhibition which will eventually help in generating effective antifilarial drugs in the future.

Role of anti-filarial drugs in inducing ER stress mediated signaling in bovine filarial parasitosis Setaria cervi

In this ex vivo study, S. cervi parasitoses were treated with Ivermectin (50 μM), Albendazole (200 μM) alone and Ivermectin + Albendazole (50 + 200 μM) at 37°C for 8 h and the motility and viability of the parasitoses were evaluated. Individually both drugs Ivermectin (Iver) and Albendazole (Alb) are reported to affect the function and integrity of ER, however till date, no reports are available on the functional changes in ER due to a combined Iver and Alb treatment of bovine helminth parasitosis. Here, we report the lethal effect of a combination treatment of Iver and Alb against adult bovine filarial parasitosis Setaria cervi. The underlying mechanism of drug action was elucidated by performing a systematic biochemical, molecular and proteomics based study. Altered calcium homeostasis in drug treated parasitoses lead to reduction in levels of total Endoplasmic Reticulum (ER) calcium by 50 % and 61 % and elevation by 50 % and 63 % in cytosol in Iver alone and Iver + Alb treated parasitoses respectively. Further, it was found that upregulated expression of ER localized GRP94, galactosyltransferase and glycosyltransferase activity in addition to reduction in activity of PDI indicated ER stress mechanisms being operative under combined drug treatment. Marked rise of 79 % reactive oxygen species and reduced antioxidant levels induced oxidative stress in drug treated parasitosis. The collective effect of both ER and oxidative stress might have triggered apoptosis, as evidenced by the elevated calpain activity, reduction of 67 % in cytochrome c oxidase and 83 % rise in caspase-3 activity in the Iver + Alb treated parasitoses respectively. The ER proteome analysis by 2D gel electrophoresis revealed 76 spots in the control and 56 spots in the treated proteome. A MALDI-MS/MS analysis of some of the differentially expressed spots of the combination drug treated parasitoses identified glucuronosyltransferase as a major upregulated protein with a fold change of 1.81. Trafficking protein, acyl transferase, MATH involved in protein folding were also found to be downregulated. Thus, this study based on biochemical and proteomic approaches indicates that a combination of anti-filarial drugs Iver and Alb can alter calcium homeostasis in bovine filarial parasitosis leading to induction of ER stress culminating into apoptosis.

Differential expression of disulfide reductase enzymes in a free-living platyhelminth (Dugesia dorotocephala)

A search of the disulfide reductase activities expressed in the adult stage of the free-living platyhelminth Dugesia dorotocephala was carried out. Using GSSG or DTNB as substrates, it was possible to obtain a purified fraction containing both GSSG and DTNB reductase activities. Through the purification procedure, both disulfide reductase activities were obtained in the same chromatographic peak. By mass spectrometry analysis of peptide fragments obtained after tryptic digestion of the purified fraction, the presence of glutathione reductase (GR), thioredoxin-glutathione reductase (TGR), and a putative thioredoxin reductase (TrxR) was detected. Using the gold compound auranofin to selectively inhibit the GSSG reductase activity of TGR, it was found that barely 5% of the total GR activity in the D. dorotocephala extract can be assigned to GR. Such strategy did allow us to determine the kinetic parameters for both GR and TGR. Although It was not possible to discriminate DTNB reductase activity due to TrxR from that of TGR, a chromatofocusing experiment with a D. dorotocephala extract resulted in the obtention of a minor protein fraction enriched in TrxR, strongly suggesting its presence as a functional protein. Thus, unlike its parasitic counterparts, in the free-living platyhelminth lineage the three disulfide reductases are present as functional proteins, albeit TGR is still the major disulfide reductase involved in the reduction of both Trx and GSSG. This fact suggests the development of TGR in parasitic flatworms was not linked to a parasitic mode of life.

A new thioredoxin reductase with additional glutathione reductase activity in Haemonchus contortus

We report, herein, the purification to homogeneity and the biochemical and kinetic characterization of HcTrxR3, a new isoform of thioredoxin reductase (TrxR) from Haemonchus contortus. HcTrxR3 was found to have a relative molecular weight of 134,000, while the corresponding value per subunit obtained under denaturing conditions, was of 67,000. By peptide mass spectrophotometric analysis, HcTrxR3 was determined to have 99% identity with the H. contortus HcTrxR1 although, and most importantly, they are different in their amino acid sequence in two amino acid positions: 48 (isoleucine instead of leucine) and 460 (leucine instead of proline). The enzyme catalyzes NADPH-dependent reduction of DTNB and, unexpectedly, it follows the pattern of glutathione reductases (GR) performing the reduction of oxidized glutathione (GSSG) to reduced glutathione using NADPH as the reducing cofactor. Hence, it is important to highlight this enzyme's new and unexpected condition that makes so special and one our main finding. Enzyme K_cat values for DTNB, GSSG and NADPH were 12, 3 and 8 s^-1, respectively. HcTrxR3 developed, into specific TrxR substrates: ebselen and sodium selenite, with activity at 0.5 and 0.068 (U/mg), respectively; and 0.044 (U/mg) for S-nitrosoglutathione through its GR activity. The enzyme was inhibited by gold compound auranofin (AU), a selective inhibitor of thiol-dependent flavoreductases. Although HcTrxR3 has both TrxR and GR activity as thioredoxin glutathione reductase (TGR) does, it is a TrxR because it has no glutaredoxin domain and it does not develop any hysteretic behavior as does TGR. The importance of this new enzyme is potential to further clarify the detoxification and haemostasis redox mechanism in H. contortus. Likewise, this enzyme could also be a protein model to recognize more differences between TrxR and GR.

Inhibition of Setaria cervi protein tyrosine phosphatases by Phenylarsine oxide: A proteomic and biochemical study

Phenylarsine oxide (PAO), a specific protein tyrosine phosphatase (PTP) inhibitor significantly decreased the motility and viability of Setaria cervi ultimately leading to its death. The PTP activity present in the cytosolic and detergent soluble fractions as well as on surface of these parasites was significantly inhibited by PAO. A marked alteration in protein spots abundance after proteomic analysis showed 14 down-regulated and 9 upregulated spots in the treated parasites as compared to the control. The PTP inhibition led to increase in the cytosolic and mitochondrial calpain activity in these parasites. PAO also blocked the ATP generation in the parasite depicted by reduced activity of phosphoglycerate kinase and expression of enolase. An increased ROS level, induced lipid peroxidation/protein carbonyl formation and decreased activity of different antioxidant enzymes like thioredoxin reductase, glutathione reductase and glutathione transferases was also observed in the PAO treated parasites. PAO, thus disturbs the overall homeostasis of the filarial parasite by inhibiting PTPs. Thereby suggesting that these molecules could be used as a good chemotherapeutic target for lymphatic filariasis.

Oxidative stress plays major role in mediating apoptosis in filarial nematode Setaria cervi in the presence of trans-stilbene derivatives

Lymphatic filariasis, affecting around 120 million people in 80 countries worldwide, is an extremely painful disease and caused permanent and long term disability. Owing to its alarming prevalence there is immediate need for development of new therapeutics. A series of trans-stilbene derivatives were synthesized using aqueous reaction condition showing potential as antifilarial agents demonstrated in vitro. MTT reduction assay and dye exclusion test were performed to evaluate the micro and macrofilaricidal potential of these compounds. Amid 20 trans-stilbene derivatives together with Resveratrol (RSV), a multifunctional natural product was screened; nine compounds (28, 29, 33, 35, 36, 38, 39, 41 and 42) have showed promising micro and macrofilaricidal activities and four of them (28, 39, 41 and 42) showed better effectiveness than RSV. In the treated parasites apoptosis was established by DNA laddering, in situ DNA fragmentation and FACS analysis. The generation of ROS in the treated parasites was indicated by the depletion in the level of GSH, GR and GST activity and elevation of SOD, catalase, GPx activity and superoxide anion and H2O2 level. Along with the ROS generation and oxidative stress, the decreased expression of anti-apoptotic ced-9 gene and increased expression of nematode specific pro-apoptotic genes, egl-1, ced-4 and ced-3 at the level of transcription and translation level; the up-regulation of caspase-3 activity and involvement of caspase-8,9,3, cytochrome-c and PARP were also observed and which denotes the probable existence of both extrinsic and intrinsic pathways apoptosis in parasitic nematodes. This observation is reported first time and thus it confirmed the mode of action and effectiveness of the compounds. Further, the comparative bioavailability-pharmacokinetics studies showed that compound 28 possesses comparable properties with Ivermectin. This study will certainly intensify our understanding of the pharmacological importance of trans-stilbenes as an anti-filarial agent.

Reduction of mitochondrial protein mitoNEET [2Fe-2S] clusters by human glutathione reductase

The human mitochondrial outer membrane protein mitoNEET is a newly discovered target of the type 2 diabetes drug pioglitazone. Structurally, mitoNEET is a homodimer with each monomer containing an N-terminal transmembrane α helix tethered to the mitochondrial outer membrane and a C-terminal cytosolic domain hosting a redox-active [2Fe-2S] cluster. Genetic studies have shown that mitoNEET has a central role in regulating energy metabolism in mitochondria. However, the specific function of mitoNEET remains largely elusive. Here we find that the mitoNEET [2Fe-2S] clusters can be efficiently reduced by Escherichia coli thioredoxin reductase and glutathione reductase in an NADPH-dependent reaction. Purified human glutathione reductase has the same activity as E. coli thioredoxin reductase and glutathione reductase to reduce the mitoNEET [2Fe-2S] clusters. However, rat thioredoxin reductase, a human thioredoxin reductase homolog that contains selenocysteine in the catalytic center, has very little or no activity to reduce the mitoNEET [2Fe-2S] clusters. N-ethylmaleimide, a potent thiol modifier, completely inhibits human glutathione reductase from reducing the mitoNEET [2Fe-2S] clusters, indicating that the redox-active disulfide in the catalytic center of human glutathione reductase may be directly involved in reducing the mitoNEET [2Fe-2S] clusters. Additional studies reveal that the reduced mitoNEET [2Fe-2S] clusters in mouse heart cell extracts can be reversibly oxidized by hydrogen peroxide without disruption of the clusters, suggesting that the mitoNEET [2Fe-2S] clusters may undergo redox transition to regulate energy metabolism in mitochondria in response to oxidative signals.

Inhibition of cathepsin B by E-64 induces oxidative stress and apoptosis in filarial parasite

BACKGROUND

Current available antifilarial drug strategies only eliminate the larval stages of filarial parasites. Therefore, there is an urgent need of drugs which are macrofilaricidals. Identification of molecular targets crucial for survival of parasite is a prerequisite for drug designing. Cathepsin B, a cysteine protease family member is known to play crucial role in the normal growth, digestion of nutrients, exsheathment of the helminth parasites. Therefore, we targeted this enzyme in the filarial parasite using its specific inhibitor, E-64.

METHODS AND FINDINGS

We have exposed the parasites to E-64 and observed their motility and viability at various time intervals. It caused marked decrease in the motility and viability of the parasites ultimately leading to their death after 8 hours. It is well known that E-64 protects the cell from apoptosis, however, it causes apoptotic effect in carcinoma cell lines. To understand the mechanism of action of E-64 on parasite survival, we have measured levels of different apoptotic markers in the treated parasites. E-64 significantly reduced the level of ced-9 and activity of tyrosine phosphatases, cytochrome c oxidase. It also activated ced-3, homolog of mammalian caspase 3 suggesting initiation of an apoptotic like event in the filarial parasites. Different antioxidant enzymes were also evaluated to further explore the mechanism behind the death of the parasites. There was marked decrease in the level of GSH and activity of Glutathione reductase and glutathione-s-transferase leading to increased generation of reactive oxygen species. This led to the induced oxidation of fatty acids and protein which might alter the mitochondrial membrane permeability.

CONCLUSION

This study suggests that inhibition of cathepsin B by E-64 generates oxidative stress followed by mitochondrial mediated apoptotic like event in filarial parasites leading to their death. Hence, suggesting filarial cathepsin B as a potential chemotherapeutic target for lymphatic filariasis.