Giardial lipid rafts share virulence factors with secreted vesicles and participate in parasitic infection in mice.

Giardia lamblia, a protozoan parasite, is a major cause of waterborne infection, worldwide. While the trophozoite form of this parasite induces pathological symptoms in the gut, the cyst form transmits the infection. Since Giardia is a noninvasive parasite, the actual mechanism by which it causes disease remains elusive. We have previously reported that Giardia assembles cholesterol and GM1 glycosphingolipid-enriched lipid rafts (LRs) that participate in encystation and cyst production. To further delineate the role of LRs in pathogenesis, we isolated LRs from Giardia and subjected them to proteomic analysis. Various cellular proteins including potential virulence factors-e.g., giardins, variant surface proteins, arginine deaminases, elongation factors, ornithine carbomyltransferases, and high cysteine-rich membrane proteins-were found to be present in LRs. Since Giardia secretes virulence factors encapsulated in extracellular vesicles (EVs) that induce proinflammatory responses in hosts, EVs released by the parasite were isolated and subjected to nanoparticle tracking and proteomic analysis. Two types of EV-i.e., small vesicles (SVs; <100 nm, exosome-like particles) and large vesicles (LVs; 100-400 nm, microvesicle-like particles)-were identified and found to contain a diverse group of proteins including above potential virulence factors. Although pretreatment of the parasite with two giardial lipid raft (gLR) disruptors, nystatin (27 µM) and oseltamivir (20 µM), altered the expression profiles of virulence factors in LVs and SVs, the effects were more robust in the case of SVs. To examine the potential role of rafts and vesicles in pathogenicity, Giardia-infected mice were treated with oseltamivir (1.5 and 3.0 mg/kg), and the shedding of cysts were monitored. We observed that this drug significantly reduced the parasite load in mice. Taken together, our results suggest that virulence factors partitioning in gLRs, released into the extracellular milieu via SVs and LVs, participate in spread of giardiasis and could be targeted for future drug development.

Triazoxins: Novel nucleosides with anti-Giardia activity.

Novel nucleoside analogues named "triazoxins" were synthesized. Of these, two analogues were found to be highly effective against Giardia lamblia, an intestinal parasite and a major cause of waterborne infection, worldwide. While compound 7 reduced the growth of trophozoites in culture (IC50, ~5 µM), compound 21 blocked the in vitro cyst production (IC50 ~5 µM). Compound 21 was also effective against trophozoites (IC50, ~36 µM). A third analogue (compound 8) was effective against both trophozoites (IC50, ~36 µM) and cysts (IC50, ~20 µM) although at higher concentration. Thus triazoxin analogues are unique and exhibit morphology (i.e., trohozoites or cysts) -specific effects against Giardia.

A Targeted Mass Spectrometric Analysis Reveals the Presence of a Reduced but Dynamic Sphingolipid Metabolic Pathway in an Ancient Protozoan, Giardia lamblia.

Giardia lamblia, a single-celled eukaryote, colonizes and thrives in the small intestine of humans. Because of its compact and reduced genome, Giardia has adapted a "minimalistic" life style, as it becomes dependent on available resources of the small intestine. Because Giardia expresses fewer sphingolipid (SL) genes-and glycosphingolipids are critical for encystation-we investigated the SL metabolic cycle in this parasite. A tandem mass spectrometry (MS/MS) analysis reveals that major SLs in Giardia include sphingomyelins, sphingoid bases, ceramides, and glycosylceramides. Many of these lipids are obtained by Giardia from the growth medium, remodeled at their fatty acyl chains and end up in the spent medium. For instance, ceramide-1-phosphate, a proinflammatory molecule that is not present in the culture medium, is generated from sphingosine (abundant in the culture medium) possibly by remodeling reactions. It is then subsequently released into the spent medium. Thus, the secretion of ceramide-1-phospate and other SL derivatives by Giardia could be associated with inflammatory bowel disease observed in acute giardiasis. Additionally, we found that the levels of SLs increase in encysting Giardia and are differentially regulated throughout the encystation cycle. We propose that SL metabolism is important for this parasite and, could serve as potential targets for developing novel anti-giardial agents.

Sphingolipids, Lipid Rafts, and Giardial Encystation: The Show Must Go On.

Sphingolipids are sphingosine-based phospholipids, which are present in the plasma and endomembranes of many eukaryotic cells. These lipids are involved in various cellular functions, including cell growth, differentiation, and apoptosis. In addition, sphingolipid and cholesterol-enriched membrane microdomains (also called "lipid rafts") contain a set of proteins and lipids, which take part in the signaling process in response to intra- or extracellular stimuli. Recent findings suggest that sphingolipids, especially glucosylceramide, play a critical role in inducing encystation and maintaining the cyst viability in Giardia. Similarly, the assembly/disassembly of lipid rafts modulates the encystation and cyst production of this ubiquitous enteric parasite. In this review article, we discuss the overall progress in the field and examine whether sphingolipids and lipid rafts can be used as novel targets for designing therapies to control infection by Giardia, which is rampant in developing countries, where children are especially vulnerable.

The assembly of GM1 glycolipid- and cholesterol-enriched raft-like membrane microdomains is important for giardial encystation.

Although encystation (or cyst formation) is an important step of the life cycle of Giardia, the cellular events that trigger encystation are poorly understood. Because membrane microdomains are involved in inducing growth and differentiation in many eukaryotes, we wondered if these raft-like domains are assembled by this parasite and participate in the encystation process. Since the GM1 ganglioside is a major constituent of mammalian lipid rafts (LRs) and known to react with cholera toxin B (CTXB), we used Alexa Fluor-conjugated CTXB and GM1 antibodies to detect giardial LRs. Raft-like structures in trophozoites are located in the plasma membranes and on the periphery of ventral discs. In cysts, however, they are localized in the membranes beneath the cyst wall. Nystatin and filipin III, two cholesterol-binding agents, and oseltamivir (Tamiflu), a viral neuraminidase inhibitor, disassembled the microdomains, as evidenced by reduced staining of trophozoites with CTXB and GM1 antibodies. GM1- and cholesterol-enriched LRs were isolated from Giardia by density gradient centrifugation and found to be sensitive to nystatin and oseltamivir. The involvement of LRs in encystation could be supported by the observation that raft inhibitors interrupted the biogenesis of encystation-specific vesicles and cyst production. Furthermore, culturing of trophozoites in dialyzed medium containing fetal bovine serum (which is low in cholesterol) reduced raft assembly and encystation, which could be rescued by adding cholesterol from the outside. Our results suggest that Giardia is able to form GM1- and cholesterol-enriched lipid rafts and these raft domains are important for encystation.

Synthesis, characterization, and evaluation of cis-diphenyl pyridineamine platinum(II) complexes as potential anti-breast cancer agents.

Although cisplatin is considered as an effective anti-cancer agent, it has shown limitations and may produce toxicity in patients. Therefore, we synthesized two cis-dichlorideplatinum(II) compounds (13 and 14) composed of meta- and para-N,N-diphenyl pyridineamine ligands through a reaction of the amine precursors and PtCl2 with respective yields of 16 and 47 %. We hypothesized that compounds 13 and 14, with lipophilic ligands, should transport efficiently in cancer cells and demonstrate more effectiveness than cisplatin. When tested for biological activity, compounds 13 and 14 were found to inhibit the growth of MCF 7 and MDA-MB-231 cells (IC50s 1 ± 0.4 µM and 1 ± 0.2 µM for 13 and 14, respectively, and IC50 7.5 ± 1.3 µM for compound 13 and 1 ± 0.3 µM for compound 14). Incidentally, these doses were found to be lower than cisplatin doses (IC50 5 ± 0.7 µM for MCF 7 and 10 ± 1.1 µM for MDA-MB-231). Similar to cisplatin, 13 and 14 interacted with DNA and induced apoptosis. However, unlike cisplatin, they blocked the migration of MDA-MB-231 cells suggesting that in addition to apoptotic and DNA-binding capabilities, these compounds are useful in blocking the metastatic migration of breast cancer cells. To delineate the mechanism of action, computer-aided analyses (DFT calculations) were conducted for compound 13. Results indicate that in vivo, the pyridineamine ligands are likely to dissociate from the complex, forming a platinum DNA adduct with anti-proliferative activity. These results suggest that complexes 13 and 14 hold promise as potential anti-cancer agents.

Glucosylceramide transferase activity is critical for encystation and viable cyst production by an intestinal protozoan, Giardia lamblia.

The production of viable cysts by Giardia is essential for its survival in the environment and for spreading the infection via contaminated food and water. The hallmark of cyst production (also known as encystation) is the biogenesis of encystation-specific vesicles (ESVs) that transport cyst wall proteins to the plasma membrane of the trophozoite before laying down the protective cyst wall. However, the molecules that regulate ESV biogenesis and maintain cyst viability have never before been identified. Here, we report that giardial glucosylceramide transferase-1 (gGlcT1), an enzyme of sphingolipid biosynthesis, plays a key role in ESV biogenesis and maintaining cyst viability. We find that overexpression of this enzyme induced the formation of aggregated/enlarged ESVs and generated clustered cysts with reduced viability. The silencing of gGlcT1 synthesis by antisense morpholino oligonucleotide abolished ESV production and generated mostly nonviable cysts. Interestingly, when gGlcT1-overexpressed Giardia was transfected with anti-gGlcT1 morpholino, the enzyme activity, vesicle biogenesis, and cyst viability returned to normal, suggesting that the regulated expression of gGlcT1 is important for encystation and viable cyst production. Furthermore, the overexpression of gGlcT1 increased the influx of membrane lipids and fatty acids without altering the fluidity of plasma membranes, indicating that the expression of gGlcT1 activity is linked to lipid internalization and maintaining the overall lipid balance in this parasite. Taken together, our results suggest that gGlcT1 is a key player of ESV biogenesis and cyst viability and therefore could be targeted for developing new anti-giardial therapies.

Phosphonoxins III: synthesis of α-aminophosphonate analogs of antifungal polyoxins with anti-Giardia activity.

A synthesis of α-aminophosphonate analogs of polyoxins, termed phosphonoxin C1, C2, and C3, has been achieved. The key step was the addition of lithium dimethyl phosphite to the aldehyde of a protected threose derivative. α-Hydroxyphosphonate analogs C4 and C5 were also obtained by taking advantage of an unprecedented conversion of an azide to hydroxyl during treatment with hydrogen on palladium on carbon. The resulting phosphonoxin C5 inhibited the growth of an intestinal protozoan, Giardia lamblia, at low micromolar concentration.