Physiological and morphological evidences for the presence acidocalcisomes in Trypanosoma evansi: single cell fluorescence and 31P NMR studies

Advances in the cellular biology, biochemistry, and molecular biology of acidocalcisomes

SUMMARYAcidocalcisomes are organelles conserved during evolution and closely related to the so-called volutin granules of bacteria and archaea, to the acidocalcisome-like vacuoles of yeasts, and to the lysosome-related organelles of animal species. All these organelles have in common their acidity and high content of polyphosphate and calcium. They are characterized by a variety of functions from storage of phosphorus and calcium to roles in Ca2+ signaling, osmoregulation, blood coagulation, and inflammation. They interact with other organelles through membrane contact sites or by fusion, and have several enzymes, pumps, transporters, and channels.

ATG5-Mediated Autophagy May Inhibit Pyroptosis to Ameliorate Oleic Acid-Induced Hepatocyte Steatosis

Despite activated autophagy ameliorating hepatocyte steatosis and metabolic associated fatty liver disease (MAFLD), mechanisms underlying the beneficial roles of autophagy in hepatic deregulation of lipid metabolism remain undefined. We explored whether autophagy can ameliorate oleic acid (OA)-induced hepatic steatosis by suppressing pyroptosis. Pyroptosis is involved in hepatocyte steatosis induced by OA. In addition, autophagy flux was blocked in OA-treated hepatocytes. Treatment with OA induced lipid accumulation in liver cell line L-02, which was attenuated by rapamycin (Rap), an autophagy agonist, while aggravated by autophagy inhibitor bafilomycin A1 (Baf A1). Inversely, treatment with pyroptotic agonist Nigericin aggravated OA-induced hepatic steatosis, while pyroptosis antagonist disulfiram ameliorated this effect. Mechanistically, treatment with Rap downregulated the expression of pyroptosis-related proteins, including NLRP3, Caspase-1, IL-18, GSDMD expression evoked by OA, thus improving pyroptosis in hepatic steatosis. Significantly, overexpression of ATG5 obviously downregulated cleaved caspase-1 expressions without altering the total caspase1 expressions in hepatic cell steatosis. Taken together, our studies strongly demonstrated that the activation of ATG5 inhibits pyroptosis to improve hepatic steatosis and suggest autophagy activation as a potential therapeutic strategy for pyroptosis-mediated MAFLD.

Albumin-based nanoparticles as contrast medium for MRI: vascular imaging, tissue and cell interactions, and pharmacokinetics of second-generation nanoparticles

This multidisciplinary study examined the pharmacokinetics of nanoparticles based on albumin-DTPA-gadolinium chelates, testing the hypothesis that these nanoparticles create a stronger vessel signal than conventional gadolinium-based contrast agents and exploring if they are safe for clinical use. Nanoparticles based on human serum albumin, bearing gadolinium and designed for use in magnetic resonance imaging, were used to generate magnet resonance images (MRI) of the vascular system in rats ("blood pool imaging"). At the low nanoparticle doses used for radionuclide imaging, nanoparticle-associated metals were cleared from the blood into the liver during the first 4 h after nanoparticle application. At the higher doses required for MRI, the liver became saturated and kidney and spleen acted as additional sinks for the metals, and accounted for most processing of the nanoparticles. The multiple components of the nanoparticles were cleared independently of one another. Albumin was detected in liver, spleen, and kidneys for up to 2 days after intravenous injection. Gadolinium was retained in the liver, kidneys, and spleen in significant concentrations for much longer. Gadolinium was present as significant fractions of initial dose for longer than 2 weeks after application, and gadolinium clearance was only complete after 6 weeks. Our analysis could not account quantitatively for the full dose of gadolinium that was applied, but numerous organs were found to contain gadolinium in the collagen of their connective tissues. Multiple lines of evidence indicated intracellular processing opening the DTPA chelates and leading to gadolinium long-term storage, in particular inside lysosomes. Turnover of the stored gadolinium was found to occur in soluble form in the kidneys, the liver, and the colon for up to 3 weeks after application. Gadolinium overload poses a significant hazard due to the high toxicity of free gadolinium ions. We discuss the relevance of our findings to gadolinium-deposition diseases.

Pyrophosphate Stimulates the Phosphate-Sodium Symporter of Trypanosoma brucei Acidocalcisomes and Saccharomyces cerevisiae Vacuoles

Acidocalcisomes, first described in trypanosomes and known to be present in a variety of cells, have similarities with S. cerevisiae vacuoles in their structure and composition. Both organelles share a Na⁺/P_i symporter involved in P_i release to the cytosol, where it is needed for biosynthetic reactions. Here we show that PP_i, at physiological cytosolic concentrations, stimulates the symporter expressed in either Xenopus oocytes or yeast vacuoles via its SPX domain, revealing a signaling role of this molecule.

Inorganic pyrophosphate (PP_i) is a by-product of biosynthetic reactions and has bioenergetic and regulatory roles in a variety of cells. Here we show that PP_i and other pyrophosphate-containing compounds, including polyphosphate (polyP), can stimulate sodium-dependent depolarization of the membrane potential and P_i conductance in Xenopus oocytes expressing a Saccharomyces cerevisiae or Trypanosoma brucei Na⁺/P_i symporter. PP_i is not taken up by Xenopus oocytes, and deletion of the TbPho91 SPX domain abolished its depolarizing effect. PP_i generated outward currents in Na⁺/P_i-loaded giant vacuoles prepared from wild-type or pho91Δ yeast strains expressing TbPHO91 but not from the pho91Δ strains. Our results suggest that PP_i, at physiological concentrations, can function as a signaling molecule releasing P_i from S. cerevisiae vacuoles and T. brucei acidocalcisomes.

IMPORTANCE Acidocalcisomes, first described in trypanosomes and known to be present in a variety of cells, have similarities with S. cerevisiae vacuoles in their structure and composition. Both organelles share a Na⁺/P_i symporter involved in P_i release to the cytosol, where it is needed for biosynthetic reactions. Here we show that PP_i, at physiological cytosolic concentrations, stimulates the symporter expressed in either Xenopus oocytes or yeast vacuoles via its SPX domain, revealing a signaling role of this molecule.

Rab32 is essential for maintaining functional acidocalcisomes, and for growth and infectivity of Trypanosoma cruzi

The contractile vacuole complex (CVC) of Trypanosoma cruzi, the etiologic agent of Chagas disease, collects and expels excess water as a mechanism of regulatory volume decrease after hyposmotic stress; it also has a role in cell shrinking after hyperosmotic stress. Here, we report that, in addition to its role in osmoregulation, the CVC of T. cruzi has a role in the biogenesis of acidocalcisomes. Expression of dominant-negative mutants of the CVC-located small GTPase Rab32 (TcCLB.506289.80) results in lower numbers of less-electron-dense acidocalcisomes, lower content of polyphosphate, lower capacity for acidocalcisome acidification and Ca(2+) uptake that is driven by the vacuolar proton pyrophosphatase and the Ca(2+)-ATPase, respectively, as well as less-infective parasites, revealing the role of this organelle in parasite infectivity. By using fluorescence, electron microscopy and electron tomography analyses, we provide further evidence of the active contact of acidocalcisomes with the CVC, indicating an active exchange of proteins between the two organelles.

New insights into roles of acidocalcisomes and contractile vacuole complex in osmoregulation in protists

While free-living protists are usually subjected to hyposmotic environments, parasitic protists are also in contact with hyperosmotic habitats. Recent work in one of these parasites, Trypanosoma cruzi, has revealed that its contractile vacuole complex, which usually collects and expels excess water as a mechanism of regulatory volume decrease after hyposmotic stress, has also a role in cell shrinking when the cells are submitted to hyperosmotic stress. Trypanosomes also have an acidic calcium store rich in polyphosphate (polyP), named the acidocalcisome, which is involved in their response to osmotic stress. Here, we review newly emerging insights on the role of acidocalcisomes and the contractile vacuole complex in the cellular response to hyposmotic and hyperosmotic stresses. We also review the current state of knowledge on the composition of these organelles and their other roles in calcium homeostasis and protein trafficking.

Overexpression of a cytosolic pyrophosphatase (TgPPase) reveals a regulatory role of PP(i) in glycolysis for Toxoplasma gondii

PP(i) is a critical element of cellular metabolism as both an energy donor and as an allosteric regulator of several metabolic pathways. The apicomplexan parasite Toxoplasma gondii uses PP(i) in place of ATP as an energy donor in at least two reactions: the glycolytic PP(i)-dependent PFK (phosphofructokinase) and V-H(+)-PPase [vacuolar H(+)-translocating PPase (pyrophosphatase)]. In the present study, we report the cloning, expression and characterization of cytosolic TgPPase (T. gondii soluble PPase). Amino acid sequence alignment and phylogenetic analysis indicates that the gene encodes a family I soluble PPase. Overexpression of the enzyme in extracellular tachyzoites led to a 6-fold decrease in the cytosolic concentration of PP(i) relative to wild-type strain RH tachyzoites. Unexpectedly, this subsequent reduction in PP(i) was associated with a higher glycolytic flux in the overexpressing mutants, as evidenced by higher rates of proton and lactate extrusion. In addition to elevated glycolytic flux, TgPPase-overexpressing tachyzoites also possessed higher ATP concentrations relative to wild-type RH parasites. These results implicate PP(i) as having a significant regulatory role in glycolysis and, potentially, other downstream processes that regulate growth and cell division.

Metabolomics and malaria biology

Metabolomics has ushered in a novel and multi-disciplinary realm in biological research. It has provided researchers with a platform to combine powerful biochemical, statistical, computational, and bioinformatics techniques to delve into the mysteries of biology and disease. The application of metabolomics to study malaria parasites represents a major advance in our approach towards gaining a more comprehensive perspective on parasite biology and disease etiology. This review attempts to highlight some of the important aspects of the field of metabolomics, and its ongoing and potential future applications to malaria research.

Eating at the table of another: metabolomics of host-parasite interactions

The application of metabolomics, the global analysis of metabolite levels, to the study of protozoan parasites has become an important tool for understanding the host-parasite relationship and holds promise for the development of direly needed therapeutics and improved diagnostics. Research advances over the past decade have opened the door for a systems biology approach to protozoan parasites with metabolomics, providing a crucial readout of metabolic activity. In this review, we highlight recent metabolomic approaches to protozoan parasites, including metabolite profiling, integration with genomics, transcription, and proteomic analysis, and the use of metabolic fingerprints for the diagnosis of parasitic infections.

Inorganic polyphosphate: essential for growth and survival

Inorganic polyphosphate (Poly P) is a polymer of tens to hundreds of phosphate residues linked by "high-energy" phosphoanhydride bonds as in ATP. Found in abundance in all cells in nature, it is unique in its likely role in the origin and survival of species. Here, we present extensive evidence that the remarkable properties of Poly P as a polyanion have made it suited for a crucial role in the emergence of cells on earth. Beyond that, Poly P has proved in a variety of ways to be essential for growth of cells, their responses to stresses and stringencies, and the virulence of pathogens. In this review, we pay particular attention to the enzyme, polyphosphate kinase 1 (Poly P kinase 1 or PPK1), responsible for Poly P synthesis and highly conserved in many bacterial species, including 20 or more of the major pathogens. Mutants lacking PPK1 are defective in motility, quorum sensing, biofilm formation, and virulence. Structural studies are cited that reveal the conserved ATP-binding site of PPK1 at atomic resolution and reveal that the site can be blocked with minute concentrations of designed inhibitors. Another widely conserved enzyme is PPK2, which has distinctive kinetic properties and is also implicated in the virulence of some pathogens. Thus, these enzymes, absent in yeast and animals, are novel attractive targets for treatment of many microbial diseases. Still another enzyme featured in this review is one discovered in Dictyostelium discoideum that becomes an actin-like fiber concurrent with the synthesis, step by step, of a Poly P chain made from ATP. The Poly P-actin fiber complex, localized in the cell, lengthens and recedes in response to metabolic signals. Homologs of DdPPK2 are found in pathogenic protozoa and in the alga Chlamydomonas. Beyond the immediate relevance of Poly P as a target for anti-infective drugs, a large variety of cellular operations that rely on Poly P will be considered.

Anti-VSG antibodies induce an increase in Trypanosoma evansi intracellular Ca2+ concentration

Trypanosoma evansi and Trypanosoma vivax have shown a very high immunological cross-reactivity. Anti-T. vivax antibodies were used to monitor changes in the T. evansi intracellular Ca2+ concentration ([Ca2+]i) by fluorometric ratio imaging from single parasites. A short-time exposure of T. evansi parasites to sera from T. vivax-infected bovines induced an increase in [Ca2+]i, which generated their complete lysis. The parasite [Ca2+]i boost was reduced but not eliminated in the absence of extracellular Ca2+ or following serum decomplementation. Decomplemented anti-T. evansi VSG antibodies also produced an increase in the parasite [Ca2+]i, in the presence of extracellular Ca2+. Furthermore, this Ca2+ signal was reduced following blockage with Ni2+ or in the absence of extracellular Ca2+, suggesting that this response was a combination of an influx of Ca2+ throughout membrane channels and a release of this ion from intracellular stores. The observed Ca2+ signal was specific since (i) it was completely eliminated following pre-incubation of the anti-VSG antibodies with the purified soluble VSG, and (ii) affinity-purified anti-VSG antibodies also generated an increase in [Ca2+]i by measurements on single cells or parasite populations. We also showed that an increase of the T. evansi [Ca2+]i by the calcium A-23187 ionophore led to VSG release from the parasite surface. In addition, in vivo immunofluorescence labelling revealed that anti-VSG antibodies induced the formation of raft patches of VSG on the parasite surface. This is the first study to identify a ligand that is coupled to calcium flux in salivarian trypanosomes.

An intracellular calcium store is present in Leishmania donovani glycosomes

A fourth intracellular Ca2+ pool in Leishmania donovani was identified by permeabilizing plasma membrane with digitonin. In Fura 2 loaded cells Ca2+ was released synergistically when mitochondrial function was blocked by antimycin and oligomycin. Vanadate did not have any effect if applied before incorporation of these mitochondrial poisons. However, the same inhibitor which inhibits Ca2+-ATPase activity of endoplasmic reticulum was able to release Ca2+ at a slow rate when added after antimycin and oligomycin. Alkalization of cytoplasmic pH allowed further release of Ca2+ essentially from the acidocalcisome. Purified glycosomes could mediate Ca2+ uptake mechanism in presence of vanadate whereas bafilomycin, a specific and potent inhibitor of vacuolar proton pump did not have any effect. Glycosomal Ca2+-ATPase activity was optimum at pH 7.5. The apparent Km for calciumin presence of vanadate was 12 nM. Taken together, it may be suggested that a vanadate-insensitive Ca2+-ATPase is present in the membrane of this microbody. Presence of glycosomal Ca2+ was further confirmed by imaging of Ca2+ activity in the Fura 2 loaded purified organelle using confocal laser. Results reveal that newly localized glycosomal calcium may essentially be an effective candidate to play a significant role in cellular function.

Acidocalcisomes - conserved from bacteria to man

Recent work has shown that acidocalcisomes, which are electron-dense acidic organelles rich in calcium and polyphosphate, are the only organelles that have been conserved during evolution from prokaryotes to eukaryotes. Acidocalcisomes were first described in trypanosomatids and have been characterized in most detail in these species. Acidocalcisomes have been linked with several functions, including storage of cations and phosphorus, polyphosphate metabolism, calcium homeostasis, maintenance of intracellular pH homeostasis and osmoregulation. Here, we review acidocalcisome ultrastructure, composition and function in different trypanosomatids and other organisms.

Evaluation of the Presence of a Thapsigargin-Sensitive Calcium Store in Trypanosomatids Using Trypanosoma evansi as a Model

Ca2+ plays an important role in the regulation of several important activities in different trypanosomatids. These parasites possess a Ca2+ transport system in the endoplasmic reticulum (ER) involved in Ca2+ homeostasis, which has been reported to be insensitive to thapsigargin, a classical inhibitor of the sarcoplasmic-ER Ca2+ adenosine triphosphatase (ATPase) (SERCA) in most eukaryotic cells. However, currently there is a controversy regarding the existence of a thapsigargin-sensitive ER Ca2+ store in these parasites. Therefore, we decided to explore the effect of this inhibitor using different methodological approaches. First, we selected Trypanosoma evansi as a parasite model to warrant the homogeneity of the population because this parasite has only a single life cycle, i.e., bloodstream-form trypomastigotes. Second, we compared the thapsigargin effect on Ca2+ homeostasis by spectrophotometrical Ca2+ measurements using 3 different approaches: whole-cell populations, cells that have been permeabilized by treatment with digitonin, and intact single cells. Our results demonstrate that a low concentration of thapsigargin induces Ca2+ release from intracellular Ca2+ stores in this parasite, which can be observed independently of the method used. Furthermore, the addition of thapsigargin before or after nigericin did not abolish its effect, showing that thapsigargin acts specifically on the ER. In conclusion, our results indicate the presence of a nonmitochondrial thapsigargin-sensitive Ca2+ store in T. evansi.

Identification of new leishmanicidal peptide lead structures by automated real-time monitoring of changes in intracellular ATP

Leishmanicidal drugs interacting stoichiometrically with parasite plasma membrane lipids, thus promoting permeability, have raised significant expectations for Leishmania chemotherapy due to their nil or very low induction of resistance. Inherent in this process is a decrease in intracellular ATP, either wasted by ionic pumps to restore membrane potential or directly leaked through larger membrane lesions caused by the drug. We have adapted a luminescence method for fast automated real-time monitoring of this process, using Leishmania donovani promastigotes transfected with a cytoplasmic luciferase form, previously tested for anti-mitochondrial drugs. The system was first assayed against a set of well-known membrane-active drugs [amphotericin B, nystatin, cecropin A-melittin peptide CA(1-8)M(1-18)], plus two ionophoric polyethers (narasin and salinomycin) not previously tested on Leishmania, then used to screen seven new cecropin A-melittin hybrid peptides. All membrane-active compounds showed a good correlation between inhibition of luminescence and leishmanicidal activity. Induction of membrane permeability was demonstrated by dissipation of membrane potential, SYTOX trade mark Green influx and membrane damage assessed by electron microscopy, except for the polyethers, where ATP decrease was due to inhibition of its mitochondrial synthesis. Five of the test peptides showed an ED50 around 1 microM on promastigotes. These peptides, with equal or better activity than 26-residue-long CA(1-8)M(1-18), are the shortest leishmanicidal peptides described so far, and validate our luminescence assay as a fast and cheap screening tool for membrane-active compounds.

Calcium regulation in protozoan parasites

The calcium ion (Ca(2+)) is used as a major signaling molecule in a diverse range of eukaryotic cells including several human parasitic protozoa, such as Trypanosoma cruzi, Trypanosoma brucei, Leishmania spp, Plasmodium spp, Toxoplasma gondii, Cryptosporidium parvum, Entamoeba histolytica, Giardia lamblia and Trichomonas vaginalis. Ca(2+) is critical for invasion of intracellular parasites, and its cytosolic concentration is regulated by the concerted operation of several transporters present in the plasma membrane, endoplasmic reticulum, mitochondria and acidocalcisomes. Recent findings have shed light on the function of these transporters, the roles that they play in cellular metabolism and their potential use for targeting them for new therapies.