Siddhartha Das

University of Texas at El Paso | UTEP · Department of Biological Sciences

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...

Giardia lamblia is an intestinal parasite responsible for transmitting giardiasis, worldwide. This parasite exists in two morphologic forms: an infective cyst and a replicating trophozoite. The symptoms include diarrhea, vomiting, dehydration and stunted growth in children. Metronidazole (Mz) is a commonly prescribed medicine to treat giardiasis, h...

The βγ subunit of heterotrimeric G proteins, a key molecule in the G protein-coupled receptors (GPCRs) signaling pathway, has been shown to be an important factor in the modulation of the microtubule cytoskeleton. Gβγ has been shown to bind to tubulin, stimulate microtubule assembly, and promote neurite outgrowth of PC12 cells. In this study, we de...

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 crit...

Microtubules (MTs) constitute a crucial part of the cytoskeleton and are essential for cell division and differentiation, cell motility, intracellular transport, and cell morphology. Precise regulation of MT assembly and dynamics is essential for the performance of these functions. Although much progress has been made in identifying and characteriz...

Background: The migration of tumor cells is critical in spreading cancers through the lymphatic nodes and circulatory systems. Although arachidonic acid (AA) and its soluble metabolites have been shown to induce the migration of breast and colon cancer cells, the mechanism by which it induces such migration has not been fully understood. Objectiv...

The stage differentiation from trophozoite to cyst (i.e., encystation) is an essential step for Giardia to survive outside its human host and spread the infection via the fecal-oral route. We have previously shown that Giardia expresses glucosylceramide transferase 1 (GlcT1) enzyme, the activity of which is elevated during encystation. We have also...

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...

Although encystation (or cyst formation) is an important step of the life cycle of Giardia, the cellular events that trigger encystation is 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...

Phospholipid remodeling and eicosanoid synthesis are central to lipid-based inflammatory reactions. Studies have revealed that membrane phospholipid remodeling by fatty acids through deacylation/reacylation reactions increases the risk of colorectal cancers (CRC) by allowing the cells to produce excess inflammatory eicosanoids, such as prostaglandi...

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...

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...

Assembly and disassembly of microtubules (MTs) is critical for axon and dendrites formation, and for neurite outgrowth. During neuronal degeneration, the architecture of MTs and proteins associated with them are severely altered. We have shown earlier that the βγ (beta-gamma) subunit of G proteins binds to tubulin stimulating MT assembly. More rece...

s: Thirty-Fourth Annual CTRC‐AACR San Antonio Breast Cancer Symposium‐‐ Dec 6‐10, 2011; San Antonio, TX Background: In invasive breast cancers, cancerous cells spread outside the ducts of the breast and metastasize to lung and other tissues. Although the rapid arachidonic acid (AA) metabolism and concomitant increase of eicosanoid molecules are in...

In addition to plasma membrane, Giardia lamblia contains numerous membrane-enveloped, primitive organelles, which house a variety of metabolic processes. It has been proposed earlier that this intestinal pathogen lacks the ability to synthesize the majority of its own lipids de novo and depends on supplies from outside sources. Therefore, the quest...

Neurite outgrowth and differentiation is a complex and finely regulated process that requires tight coordination between assembly/disassembly of microtubules and actin filaments. However, the precise molecular mechanisms that drive these cytoskeletal changes during neurite outgrowth are poorly understood. Heterotrimeric G proteins are important for...

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 hydr...

Giardia lamblia, a protozoan parasite, infects a wide variety of vertebrates, including humans. Studies indicate that this anaerobic protist possesses a limited ability to synthesize lipid molecules de novo and depends on supplies from its environment for growth and differentiation. It has been suggested that most lipids and fatty acids are taken u...

PURPOSE: Giardiasis, a clinical syndrome caused by the intestinal protozoan Giardia lamblia, is a re-emerging waterborne illness worldwide. Although giardiasis can be effectively treated with the metronidazole group of compounds, these drugs can be problematic, resulting in unpleasant side effects and ultimately leading to drug-resistant parasites....

Sphingolipids regulate various cellular functions such as differentiation, apoptosis, and signaling in mammalian cells. Recent results suggest that ceramide and other sphingolipids are also important for encystation and cyst production by intestinal protozoa, Giardia and Entamoeba. The genomic analyses indicated that while Giardia contains fewer sp...

The pathogenic protozoan Giardia lamblia is known to not synthesize membrane lipids de novo. Therefore, it is possible that lipids in the small intestine, where trophozoites colonize, play key roles in regulating the growth and differentiation of this important pathogen. The focus of the current study is to conduct a complete lipidomic analysis and...

The early-divergent protozoan Giardia lamblia, which is a major cause of waterborne enteric disease worldwide, was shown to possess limited lipid synthesis ability and to depend upon preformed lipid molecules for energy production and membrane biosynthesis. Therefore, questions regarding how Giardia imports and utilizes exogenous lipids are importa...

The early-divergent protozoan Giardia lamblia, which is a major cause of waterborne enteric disease worldwide, was shown to possess limited lipid synthesis ability and to depend upon preformed lipid molecules for energy production and membrane biosynthesis. Therefore, questions regarding how Giardia imports and utilizes exogenous lipids are importa...

Giardia and Cryptosporidium are both parasites of considerable global interest due to the gastrointestinal problems the organisms can cause in humans as well as domestic and wild animals. This book presents an overview of recent research. The chapters discuss topics from taxonomy; nomenclature and evolution to molecular epidemiology; advances in di...

Neurite outgrowth is a complex and finely regulated process that requires tight coordination between assembly/disassembly of microtubules and actin filaments and is regulated by G-protein coupled receptors in neurons. We have shown earlier that βγ subunits of G proteins are involved in promoting microtubule assembly in cultured NIH3T3 and PC12 cell...

Although encystation (cyst formation) is important for the survival of Giardia lamblia outside its human host, the molecular events that prompt encystation have not been fully elucidated. Here, we demonstrate that sphingolipids (SLs), which are important for the growth and differentiation of many eukaryotes, play key roles in giardial encystation....

The βγ subunit of G proteins (Gβγ) is known to transfer signals from cell surface receptors to intracellular effector molecules. Recent results suggest that Gβγ also interacts with microtubules and is involved in the regulation of the mitotic spindle. In the current study, the anti-microtubular drug nocodazole was employed to investigate the mechan...

Naegleria fowleri cells, grown axenically, contain high levels of β-D-glucosidase which catalyzes the hydrolysis of 4-methylumbelliferyl-β-D-glucopyranoside (4MUGlc) (Km, 0.9 mM), octyl-β-D-glucoside (Km, 0.17 mM), and p-nitrophenyl-β-D-glucopyranoside at relative rates of 1.00, 2.88, and 1.16, respectively (substrate concentration, 3.0 mM). When t...

Although identified as an early-diverged protozoan, Giardia lamblia shares many similarities with higher eukaryotic cells, including an internal membrane system and cytoskeleton, as well as secretory pathways. However, unlike many other eukaryotes, Giardia does not synthesize lipids de novo, but rather depends on exogenous sources for both energy p...

The current investigation evaluates the expression of phosphatidylinositol kinase (PIK) genes in the parasitic protozoan, Giardia lamblia. The G. lamblia Genome Database revealed the presence of two putative phosphatidylinositol-3-kinase (gPI3K) and one phosphatidylinositol-4-kinase (gPI4K) genes resembling the catalytic subunit of eukaryotic PIKs....

Heterotrimeric G proteins participate in signal transduction by transferring signals from cell surface receptors to intracellular effector molecules. Interestingly, recent results suggest that G proteins also interact with microtubules and participate in cell division and differentiation. It has been shown earlier that both alpha and betagamma subu...

Because of their limited lipid synthesis ability it has been postulated that Giardia lamblia trophozoites depend on lipid remodeling reactions, to generate parasite-specific phospho and glycolipids. We have shown earlier that exogenous bile acids and lipid molecules are taken up by Giardia through active transport and by other mechanisms. Another r...

Colorectal carcinoma is a leading cause of cancer related death worldwide. This deadly disease advances through a series of clinical and histopathological stages, initiated by single crypt lesions to small benign tumors and finally to malignancy. Although some progress has been made in elucidating the formation of colorectal tumors at molecular/gen...

Entamoeba, Giardia, and trichomonads are the prominent members of a group known as 'mucosal parasites'. While Entamoeba and Giardia trophozoites colonise the small intestine, trichomonads inhabit the genitourinary tracts of humans and animals. These protozoa lack mitochondria, well-developed Golgi complexes, and other organelles typical of higher e...

Recent results suggest that Giardia is able to carry out deacylation/reacylation reactions (the Lands cycle) to generate new phospholipids, effectively bypassing the de novo synthesis of the entire phospholipid molecule. The successful operation of this deacylation/reacylation cycle is important for Giardia because this protozoan parasite possesses...

Although Giardia lamblia trophozoites are unable to carry out de novo phospholipid synthesis, they can assemble complex glycophospholipids from simple lipids and fatty acids acquired from the host. Previously, we have reported that G. lamblia synthesizes GP49, an invariant surface antigen with a glycosylphosphatidylinositol (GPI) anchor. It is ther...

Giardia lamblia trophozoites are flagellated protozoa that inhabit the human small intestine, where they are exposed to various dietary lipids and fatty acids. It is believed that G. lamblia, which colonizes a lipid-rich environment of the human small intestine, is unable to synthesize phospholipids, long-chain fatty acids, and sterols de novo. The...

Gjajdja JambJia trophozoites are flagellated protozoa that inhabit the human small intestine, where they are exposed to dietary lipids, fatty acids, and cholesterol. Although Q. lamhlia is unable to synthesize phospholipids and sterols de novo; it can take up exogenous fatty acids and incorporate them into phosphatidylglycerol (PCi), or other glyce...

Giardia lamblia trophozoites colonize the human small intestine, where they are exposed to high concentrations of conjugated bile acids. Previous work has shown that bile acids enhance trophozoite survival, multiplication, and differentiation into the cyst stage. Therefore, experiments were performed to test whether carrier-mediated uptake of conju...

Giardia lamblia trophozoites are unable to carry out de novo lipid synthesis. It is therefore likely that lipids are acquired from the small intestine of the host, in which the trophozoites are exposed to free and conjugated fatty acids, various sterols, phospholipids, bile acids, and bile-lipid mixed micelles. Here we show that G. lamblia is capab...

The cyst wall ofGiardia lambliais essential for survival of the parasite outside the host.N-acetyl-d-glucosamine (GalNAc) has been reported as a major terminal sugar of cyst wall glycoproteins andN-acetyl-d-galactosamine (GalNAc) as the major sugar of the fibrous insoluble cyst wall fraction. Therefore, we measured UDP-glycosyltransferase activitie...

Giardia lamblia is a primitive protozoan and a major cause of waterborne enteric disease throughout tropical and temperate zones. The ability to grow the infective trophozoites in culture as well as the discovery of the method of in vitro encystation made it possible to study the biology of this primitive protozoan and to characterize the surface a...

Encystation of Giardia lamblia entails the appearance of a number of new antigens, as well as formation of a novel class of large encystation-specific secretory vesicles (ESV) that transport stage-specific proteins to the nascent cyst wall. The monoclonal antibody GCSA-1, which was raised against purified cyst walls, recognizes protein species of a...

Although the cyst wall of Entamoeba invadens contains chitin, synthesis of this structural polymer during encystation has not been described before. Here we report that conditions which stimulate encystation of the parasite lead to increased chitin synthase (ChS) activity, measured by incorporation of [3H]GlcNAc ([3H]N-acetylglucosamine) from UDP-G...

Since Giardia lamblia trophozoites are exposed to high concentrations of fatty acids in their human small intestinal milieu, we determined the pattern of incorporation of [3H]palmitic acid and myristic acid into G. lamblia proteins. The pattern of fatty acylation was unusually simple since greater than 90% of the Giardia protein biosynthetically la...

Giardia lamblia is the major identified cause of waterborne enteric disease in the U.S. Nonetheless, knowledge of the biological interactions between G. lamblia and the host intestinal milieu is limited. We have proposed that the great variations in the clinical manifestations and duration of giardiasis are due to parasite interactions with both im...

The Leishmania spp. are intracellular protozoal parasites which belong to the family of hemoflagellates that includes the highly infectious and well-known trypanosomes. Organisms of this genus are responsible for one of the major classes of communicable diseases of the world. The flagellated protozoan has a digenic life cycle; in the alimentary tra...

Our study demonstrates that aspirates of proximal intestinal fluid from uninfected humans kill G. lamblia trophozoites in vitro. Mucus and bile salts, however, above their critical micellar concentration (CMC) interfere with killing and may promote specific colonization of the small intestine by G. lamblia.

The cyst form of Giardia lamblia is responsible for transmission of giardiasis, a common waterborne intestinal disease. In these studies, encystation of Giardia lamblia in vitro was demonstrated by morphologic, immunologic, and biochemical criteria. In the suckling mouse model, the jejunum was shown to be a major site of encystation of the parasite...

Naegleria fowleri cells, grown axenically, contain high levels of beta-D-glucosidase which catalyzes the hydrolysis of 4-methylumbelliferyl-beta-D-glucopyranoside (4MUGlc) (Km, 0.9 mM), octyl-beta-D-glucoside (Km, 0.17 mM), and p-nitrophenyl-beta-D-glucopyranoside at relative rates of 1.00, 2.88, and 1.16, respectively (substrate concentration, 3.0...

Leishmania donovani promastigotes labelled for 2 h with 32Pi incorporated radioactivity into at least 21 different proteins, as determined by SDS/polyacrylamide-gel electrophoresis. Pulse-chase studies with 32Pi demonstrated that the labelled proteins were in a dynamic state: some radiolabelled proteins rapidly disappeared and others appeared after...

Leishmania donovani promastigotes contain intense tartrate-resistant cell surface acid phosphatase (ACP1) which blocks superoxide anion production by activated human neutrophils [A.T. Remaley et al. (1984) J. Biol. Chem, 259, 11173-11175]. An extensively purified preparation of ACP1 dephosphorylates several phosphoproteins which are phosphorylated...

Extracts of the pathogenic ameba Naegleria fowleri, prepared by freeze-thawing and sonication, were analyzed for their content of various hydrolytic enzymes that have acid pH optima. The organism is rich in acid phosphatase activity as well as a variety of glycosidases which include beta-glucosidase, beta-galactosidase, beta-fucosidase, alpha-manno...

The high-speed supernatant (100,000 g, 1 h) obtained after centrifuging a suspension of Legionella micdadei that had been freeze-thawed and sonicated contained (i) considerable acid phosphatase activity when assayed using 4-methylumbelliferyl phosphate (MUP) as the substrate, and a factor that blocked superoxide anion production by human neutrophil...

Leishmania donovani promastigotes produce large quantities of two distinct acid phosphatases; a tartrate-resistant enzyme is localized to the external surface of the plasma membrane, and a tartrate-sensitive enzyme is secreted into the growth medium. It was shown previously that preincubation of human neutrophils and macrophages with the tartrate-r...

A crude membrane fraction from promastigotes of Leishmania donovani grown in a liquid culture medium containing 20% fetal calf serum was prepared by freeze-thawing, centrifugation (200,000 X g, 30 min), and extraction with 2% (w/v) sodium cholate. After removal of the bile salt by chromatography on a Sephadex G-75 column, the solubilized membrane p...

Giardia lamblia (Gl) and Entamoeba invadens (Ei) are protozoan parasites with two morphologic stages in their life cycles. Motile trophozoites colonize the intestine of humans and reptiles respectively. Water resistant cysts, which can survive outside the host, transmit infection. In vitro cyst formation of Ei from trophozoites has been reported, a...

An extensively purified preparation of the predominant, tartrate-resistant acid phosphatase (ACP) from the external surface of Leishmania donovani promastigotes form catalyzes the dephosphorylation of several phosphoproteins; these include: pyruvate kinase, phosphorylase kinase and histones. However, the protein phosphatase activity of ACP is very...