P-type Proton ATPases are Involved in Intracellular Calcium and Proton Uptake in the Plant Parasite Phytomonas francai

Inorganic polyphosphates are stored in spherites within the midgut of Anticarsia gemmatalis and play a role in copper detoxification

Inorganic polyphosphates (PolyP) are widespread molecules that have been shown to play a role in metal detoxification and heavy-metal tolerance. In the present report, we investigated the functional role of spherites as PolyP-metal binding stores in epithelial cells of the midgut of Anticarsia gemmatalis, a lepidopteran pest of soybean. PolyP stores were detected by DAPI staining and indirect immunohistochemistry as vesicles distributed in columnar cells and around goblet cell cavities. These PolyP vesicles were identified as spherites by their elemental profile in cell lysates that were partially modulated by P- or V-ATPases. PolyP levels along the midgut were detected using a recombinant exopolyphosphatase assay. When copper was added in the diet of larva, copper detection in spherites by X-ray microanalysis correlated with an increase in the relative phosphorous X-ray signal and with an increase in PolyP levels in epithelia cell lysate. Transmission electron microscopy of chemically fixed or cryofixed and freeze substituted tissues confirmed a preferential localization of spherites around the goblet cell cavity. Taken together, these results suggest that spherites store high levels of PolyP that are modulated during metal uptake and detoxification. The similarity between PolyP granules and spherites herein described also suggest that PolyP is one of the main phosphorous source of spherites found in different biological models. This suggests physiological roles played by spherites in the midgut of arthropods and mechanisms involved in heavy metal resistance among different insect genera.

Volutin granules of Eimeria parasites are acidic compartments and have physiological and structural characteristics similar to acidocalcisomes

The structural organization of parasites has been the subject of investigation by many groups and has lead to the identification of structures and metabolic pathways that may represent targets for anti-parasitic drugs. A specific group of organelles named acidocalcisomes has been identified in a number of organisms, including the apicomplexan parasites such as Toxoplasma and Plasmodium, where they have been shown to be involved in cation homeostasis, polyphosphate metabolism, and osmoregulation. Their structural counterparts in the apicomplexan parasite Eimeria have not been fully characterized. In this work, the ultrastructural and chemical properties of acidocalcisomes in Eimeria were characterized. Electron microscopy analysis of Eimeria parasites showed the dense organelles called volutin granules similar to acidocalcisomes. Immunolocalization of the vacuolar proton pyrophosphatase, considered as a marker for acidocalcisomes, showed labeling in vesicles of size and distribution similar to the dense organelles seen by electron microscopy. Spectrophotometric measurements of the kinetics of proton uptake showed a vacuolar proton pyrophosphatase activity. X-ray mapping revealed significant amounts of Na, Mg, P, K, Ca, and Zn in their matrix. The results suggest that volutin granules of Eimeria parasites are acidic, dense organelles, and possess structural and chemical properties analogous to those of other acidocalcisomes, suggesting a similar functional role in these parasites.

Characterization of Ca2+uptake in a subcellular membrane fraction ofHerpetomonassp. promastigotes

ATP-dependent Ca2+uptake was studied in a subcellular fraction fromHerpetomonassp. prepared by mechanical disruption and using45Ca2+as a tracer. The uptake was stimulated by Ca2+with a K0·5of 0·1 μmand a Hill number (nH)=2·8±0·4. The Ca2+-dependent ATP hydrolysis was optimal at pH 7·0 and had a Ca2+dependence identical to uptake. The uptake was highly stimulated by oxalate whereas calmodulin had no activating effect. ATP stimulated Ca2+uptake with a biphasic pattern that resembled the curves described for the purified preparations of rabbit sarcoplasmic reticulum. The ATP stimulation is described as the sum of two Michaelis-Menten curves with Km1=0·25±0·19 μmand Km2=29·6±6·8 μm. GTP or UTP could also promote Ca2+uptake, but with less efficiency than ATP. Vanadate inhibited the uptake with low apparent affinity. Thapsigargin and cyclopiazonic acid were almost ineffective. The Ca2+uptake was insensitive to H+ionophores and to bafilomycin suggesting no participation of acidocalcisomes. The results are comparable to those obtained using cells permeabilized with digitonin and using arsenaze III as Ca2+indicator. The Ca2+uptake activity described here seems to belong to the endoplasmic reticulum ofHerpetomonassp. and is suitable for further studies on the mechanisms of calcium homeostasis in parasites.

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.

Molecular characterization of Trypanosoma brucei P-type H+-ATPases

Previous studies in Trypanosoma brucei have shown that intracellular pH homeostasis is affected by inhibitors of H+-ATPases, suggesting a major role for these pumps in this process (Vander-Heyden, N., Wong, J., and Docampo, R., (2000) Biochem. J. 346, 53-62). Here, we report the cloning and sequencing of three genes (TbHA1, TbHA2, and TbHA3) present in the genome of T. brucei that encode proteins with homology to fungal and plant P-type proton-pumping ATPases. Northern and Western blot analyses revealed that these genes are up-regulated in procyclic trypomastigotes. TbHA1, TbHA2, and TbHA3 complemented a Saccharomyces cerevisiae strain deficient in P-type H+-ATPase activity, providing genetic evidence for their function. Indirect immunofluorescence analysis showed that TbHA proteins are localized mainly in the plasma membrane of procyclic forms and in the plasma membrane and flagellum of bloodstream forms. T. brucei H+-ATPase genes were functionally characterized using double-stranded RNA interference methodology. The induction of double-stranded RNA (RNA interference) caused growth inhibition, which was more accentuated in procyclic forms and when expression of all TbHA proteins was decreased. Knockdown of TbHA1 and TbHA3, but not of TbHA2, resulted in cells with a lower steady-state pH(i) and a slower rate of pH(i) recovery from acidification. No evidence was found of an intracellular P-type H+-ATPase activity. These results establish that T. brucei H+-ATPases are plasma membrane enzymes essential for parasite viability.

Kinetics of pyrophosphate-driven proton uptake by acidocalcisomes of Leptomonas wallacei

In this work, we show the kinetics of pyrophosphate-driven H+ uptake by acidocalcisomes in digitonin-permeabilized promastigotes of Leptomonas wallacei. The vacuolar proton pyrophosphatase activity was optimal in the pH range of 7.5-8.0, was inhibited by imidiodiphosphate, and was completely dependent on K+ and PPi. H+ was released with the addition of Ca2+, suggesting the presence of a Ca2+/H+ antiport. In addition, X-ray elemental mapping associated with energy-filtering transmission electron microscopy showed that most of the Ca, Na, Mg, P, K, Fe, and Zn were located in acidocalcisomes. L. wallacei immunolabeled with antibodies against Trypanosoma cruzi pyrophosphatase show intense fluorescence in cytoplasmatic organelles of size and distribution similar to the acidocalcisomes. Altogether, the results show that L. wallacei acidocalcisomes possess a H+-pyrophosphatase with characteristics of type I V-H+-PPase. However, we did not find any evidence, either for the presence of H+-ATPases or for Na+/H+ exchangers in these acidocalcisomes.