Peptide-targeted delivery of a pH sensor for quantitative measurements of intraglycosomal pH in live Trypanosoma brucei

Measuring Dynamic Glycosomal pH Changes in Living Trypanosoma brucei

Glucose metabolism is critical for the African trypanosome, Trypanosoma brucei, as an essential metabolic process and regulator of parasite development. Little is known about the cellular responses generated when environmental glucose levels change. In both bloodstream and procyclic form (insect stage) parasites, glycosomes house most of glycolysis. These organelles are rapidly acidified in response to glucose deprivation, which likely results in the allosteric regulation of glycolytic enzymes such as hexokinase. In previous work, localizing the chemical probe used to make pH measurements was challenging, limiting its utility in other applications. This paper describes the development and use of parasites that express glycosomally localized pHluorin2, a heritable protein pH biosensor. pHluorin2 is a ratiometric pHluorin variant that displays a pH (acid)-dependent decrease in excitation at 395 nm while simultaneously yielding an increase in excitation at 475 nm. Transgenic parasites were generated by cloning the pHluorin2 open reading frame into the trypanosome expression vector pLEW100v5, enabling inducible protein expression in either lifecycle stage. Immunofluorescence was used to confirm the glycosomal localization of the pHluorin2 biosensor, comparing the localization of the biosensor to the glycosomal resident protein aldolase. The sensor responsiveness was calibrated at differing pH levels by incubating cells in a series of buffers that ranged in pH from 4 to 8, an approach we have previously used to calibrate a fluorescein-based pH sensor. We then measured pHluorin2 fluorescence at 405 nm and 488 nm using flow cytometry to determine glycosomal pH. We validated the performance of the live transgenic pHluorin2-expressing parasites, monitoring pH over time in response to glucose deprivation, a known trigger of glycosomal acidification in PF parasites. This tool has a range of potential applications, including potentially being used in high-throughput drug screening. Beyond glycosomal pH, the sensor could be adapted to other organelles or used in other trypanosomatids to understand pH dynamics in the live cell setting.

Recent advances in fluorescent probes for extracellular pH detection and imaging

Extracellular pH plays vital roles in physiological and pathological processes including tumor metastasis and chemotherapy resistance. Abnormal extracellular pH is known to be associated with various pathological states, such as those in tumors, ischemic stroke, infection, and inflammation. Specifically, dysregulated pH is regarded as a hallmark of cancer because enhanced glycolysis and poor perfusion in most solid malignant tumors create an acidic extracellular environment, which enhances tumor growth, invasion, and metastasis. Close connection between the cell functions with extracellular pH means that precise and real-time measurement of the dynamic change of extracellular pH can provide critical information for not only studying physiological and pathological processes but also diagnosis of cancer and other diseases. This review highlights the recent development of based fluorescent probes for extracellular pH measurement, including design strategies, reaction mechanism and applications for the detection and imaging of extracellular pH.

Structure, Properties, and Function of Glycosomes in Trypanosoma cruzi

Glycosomes are peroxisome-related organelles that have been identified in kinetoplastids and diplonemids. The hallmark of glycosomes is their harboring of the majority of the glycolytic enzymes. Our biochemical studies and proteome analysis of Trypanosoma cruzi glycosomes have located, in addition to enzymes of the glycolytic pathway, enzymes of several other metabolic processes in the organelles. These analyses revealed many aspects in common with glycosomes from other trypanosomatids as well as features that seem specific for T. cruzi. Their enzyme content indicates that T. cruzi glycosomes are multifunctional organelles, involved in both several catabolic processes such as glycolysis and anabolic ones. Specifically discussed in this minireview are the cross-talk between glycosomal metabolism and metabolic processes occurring in other cell compartments, and the importance of metabolite translocation systems in the glycosomal membrane to enable the coordination between the spatially separated processes. Possible mechanisms for metabolite translocation across the membrane are suggested by proteins identified in the organelle's membrane-homologs of the ABC and MCF transporter families-and the presence of channels as inferred previously from the detection of channel-forming proteins in glycosomal membrane preparations from the related parasite T. brucei. Together, these data provide insight in the way in which different parts of T. cruzi metabolism, although uniquely distributed over different compartments, are integrated and regulated. Moreover, this information reveals opportunities for the development of drugs against Chagas disease caused by these parasites and for which currently no adequate treatment is available.

PAS domain-containing phosphoglycerate kinase deficiency in Leishmania major results in increased autophagosome formation and cell death

Per-Arnt-Sim (PAS) domains are structurally conserved and present in numerous proteins throughout all branches of the phylogenetic tree. Although PAS domain-containing proteins are major players for the adaptation to environmental stimuli in both prokaryotic and eukaryotic organisms, these types of proteins are still uncharacterized in the trypanosomatid parasites, Trypanosome and Leishmania In addition, PAS-containing phosphoglycerate kinase (PGK) protein is uncharacterized in the literature. Here, we report a PAS domain-containing PGK (LmPAS-PGK) in the unicellular pathogen Leishmania The modeled structure of N-terminal of this protein exhibits four antiparallel β sheets centrally flanked by α helices, which is similar to the characteristic signature of PAS domain. Activity measurements suggest that acidic pH can directly stimulate PGK activity. Localization studies demonstrate that the protein is highly enriched in the glycosome and its presence can also be seen in the lysosome. Gene knockout, overexpression and complement studies suggest that LmPAS-PGK plays a fundamental role in cell survival through autophagy. Furthermore, the knockout cells display a marked decrease in virulence when host macrophage and BALB/c mice were infected with them. Our work begins to clarify how acidic pH-dependent ATP generation by PGK is likely to function in cellular adaptability of Leishmania.

A FRET flow cytometry method for monitoring cytosolic and glycosomal glucose in living kinetoplastid parasites

The bloodstream lifecycle stage of the kinetoplastid parasite Trypanosoma brucei relies solely on glucose metabolism for ATP production, which occurs in peroxisome-like organelles (glycosomes). Many studies have been conducted on glucose uptake and metabolism, but none thus far have been able to monitor changes in cellular and organellar glucose concentration in live parasites. We have developed a non-destructive technique for monitoring changes in cytosolic and glycosomal glucose levels in T. brucei using a fluorescent protein biosensor (FLII¹²Pglu-700μδ6) in combination with flow cytometry. T. brucei parasites harboring the biosensor allowed for observation of cytosolic glucose levels. Appending a type 1 peroxisomal targeting sequence caused biosensors to localize to glycosomes, which enabled observation of glycosomal glucose levels. Using this approach, we investigated cytosolic and glycosomal glucose levels in response to changes in external glucose or 2-deoxyglucose concentration. These data show that procyclic form and bloodstream form parasites maintain different glucose concentrations in their cytosol and glycosomes. In procyclic form parasites, the cytosol and glycosomes maintain indistinguishable glucose levels (3.4 ± 0.4mM and 3.4 ± 0.5mM glucose respectively) at a 6.25mM external glucose concentration. In contrast, bloodstream form parasites maintain glycosomal glucose levels that are ~1.8-fold higher than the surrounding cytosol, equating to 1.9 ± 0.6mM in cytosol and 3.5 ± 0.5mM in glycosomes. While the mechanisms of glucose transport operating in the glycosomes of bloodstream form T. brucei remain unresolved, the methods described here will provide a means to begin to dissect the cellular machinery required for subcellular distribution of this critical hexose.

African sleeping sickness is caused by Trypanosoma brucei. Tens of millions of people living in endemic areas are at risk for the disease. Within the mammalian bloodstream, T. brucei parasites sustain all their energy needs by metabolizing glucose present in the host’s blood within specialized organelles known as glycosomes. In vitro, bloodstream parasites rapidly die if glucose is removed from their environment. This reliance on glucose for survival has made glucose metabolism in T. brucei an important area of study with the aim to develop targeted therapeutics that disrupt glucose metabolism. However, there have previously been no reported methods to study glucose uptake and distribution dynamics in intact glycosomes in live T. brucei. Here we describe development of approaches for observing changes in glucose concentration in glycosomes in live T. brucei. Results obtained using these methods provide new insights into how T. brucei acquires and transports glucose to sustain cell survival.

A targeted delivery strategy for the development of potent trypanocides

A new drug delivery strategy was investigated for the development of potent anti-parasitic compounds against Trypanosoma brucei, the causative agent of African sleeping sickness. Thus, potent in vitro hexokinase inhibitors were rendered cytotoxic by appending a tripeptide peroxosomal targeting sequence that facilitated delivery of the molecular cargo to the appropriate organelle in the parasite.

Aggregate Formation of Oligonucleotides that Assist Molecular Imaging for Tracking of the Oxygen Status in Tumor Tissue

The use of DNA aggregates could be a promising strategy for the molecular imaging of biological functions. Herein, phosphorescent oligodeoxynucleotides were designed with the aim of visualizing oxygen fluctuation in tumor cells. DNA-ruthenium conjugates (DRCs) that consisted of oligodeoxynucleotides, a phosphorescent ruthenium complex, a pyrene unit for high oxygen responsiveness, and a nitroimidazole unit as a tumor-targeting unit were prepared. In general, oligonucleotides have low cell permeability because of their own negative charges; however, the DRC formed aggregates in aqueous solution due to the hydrophobic pyrene and nitroimidazole groups, and smoothly penetrated the cellular membrane to accumulate in tumor cells in a hypoxia-selective manner. The oxygen-dependent phosphorescence of DRC in cells was also observed. In vivo experiments revealed that aggregates of DRC accumulated in hypoxic tumor tissue that was transplanted into the left leg of mice, and showed that oxygen fluctuations in tumor tissue could be monitored by tracking of the phosphorescence emission of DRC.

pH regulation in glycosomes of procyclic form Trypanosoma brucei

Here we report the use of a fluorescein-tagged peroxisomal targeting sequence peptide (F-PTS1, acetyl-C{K(FITC)}GGAKL) for investigating pH regulation of glycosomes in live procyclic form Trypanosoma brucei When added to cells, this fluorescent peptide is internalized within vesicular structures, including glycosomes, and can be visualized after 30-60 min. Using F-PTS1 we are able to observe the pH conditions inside glycosomes in response to starvation conditions. Previous studies have shown that in the absence of glucose, the glycosome exhibits mild acidification from pH 7.4 ± 0.2 to 6.8 ± 0.2. Our results suggest that this response occurs under proline starvation as well. This pH regulation is found to be independent from cytosolic pH and requires a source of Na⁺ ions. Glycosomes were also observed to be more resistant to external pH changes than the cytosol; placement of cells in acidic buffers (pH 5) reduced the pH of the cytosol by 0.8 ± 0.1 pH units, whereas glycosomal pH decreases by 0.5 ± 0.1 pH units. This observation suggests that regulation of glycosomal pH is different and independent from cytosolic pH regulation. Furthermore, pH regulation is likely to work by an active process, because cells depleted of ATP with 2-deoxyglucose and sodium azide were unable to properly regulate pH. Finally, inhibitor studies with bafilomycin and EIPA suggest that both V-ATPases and Na⁺/H⁺ exchangers are required for glycosomal pH regulation.

Evaluation of substituted ebselen derivatives as potential trypanocidal agents

Human African trypanosomiasis is a disease of sub-Saharan Africa, where millions are at risk for the illness. The disease, commonly referred to as African sleeping sickness, is caused by an infection by the eukaryotic pathogen, Trypanosoma brucei. Previously, a target-based high throughput screen revealed ebselen (EbSe), and its sulfur analog, EbS, to be potent in vitro inhibitors of the T. brucei hexokinase 1 (TbHK1). These molecules also exhibited potent trypanocidal activity in vivo. In this manuscript, we synthesized a series of sixteen EbSe and EbS derivatives bearing electron-withdrawing carboxylic acid and methyl ester functional groups, and evaluated the influence of these substituents on the biological efficacy of the parent scaffold. With the exception of one methyl ester derivative, these modifications ablated or blunted the potent TbHK1 inhibition of the parent scaffold. Nonetheless, a few of the methyl ester derivatives still exhibited trypanocidal effects with single-digit micromolar or high nanomolar EC₅₀ values.

A Two-Photon Ratiometric Fluorescent Probe for Imaging Carboxylesterase 2 in Living Cells and Tissues

In this study, a two-photon ratiometric fluorescent probe NCEN has been designed and developed for highly selective and sensitive sensing of human carboxylesterase 2 (hCE2) based on the catalytic properties and substrate preference of hCE2. Upon addition of hCE2, the probe could be readily hydrolyzed to release 4-amino-1,8-naphthalimide (NAH), which brings remarkable red-shift in fluorescence (90 nm) spectrum. The newly developed probe exhibits good specificity, ultrahigh sensitivity, and has been successfully applied to determine the real activities of hCE2 in complex biological samples such as cell and tissue preparations. NCEN has also been used for two-photon imaging of intracellular hCE2 in living cells as well as in deep-tissues for the first time, and the results showed that the probe exhibited high ratiometric imaging resolution and deep-tissue imaging depth. All these findings suggested that this probe holds great promise for applications in bioimaging of endogenous hCE2 in living cells and in exploring the biological functions of hCE2 in complex biological systems.

Hysteresis and positive cooperativity as possible regulatory mechanisms of Trypanosoma cruzi hexokinase activity

In Trypanosoma cruzi, the causal agent of Chagas disease, the first six or seven steps of glycolysis are compartmentalized in glycosomes, which are authentic but specialized peroxisomes. Hexokinase (HK), the first enzyme in the glycolytic pathway, has been an important research object, particularly as a potential drug target. Here we present the results of a specific kinetics study of the native HK from T. cruzi epimastigotes; a sigmoidal behavior was apparent when the velocity of the reaction was determined as a function of the concentration of its substrates, glucose and ATP. This behavior was only observed at low enzyme concentration, while at high concentration classical Michaelis-Menten kinetics was displayed. The progress curve of the enzyme's activity displays a lag phase of which the length is dependent on the protein concentration, suggesting that HK is a hysteretic enzyme. The hysteretic behavior may be attributed to slow changes in the conformation of T. cruzi HK as a response to variations of glucose and ATP concentrations in the glycosomal matrix. Variations in HK's substrate concentrations within the glycosomes may be due to variations in the trypanosome's environment. The hysteretic and cooperative behavior of the enzyme may be a form of regulation by which the parasite can more readily adapt to these environmental changes, occurring within each of its hosts, or during the early phase of transition to a new host.