Vacuolar localization of an Entamoeba histolytica homologue of the plasma membrane ATPase (PMCA)

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.

Entamoeba histolytica and pathogenesis: A calcium connection

Calcium signaling plays a key role in many essential processes in almost all eukaryotic systems. It is believed that it may also be an important signaling system of the protist parasite Entamoeba histolytica. Motility, adhesion, cytolysis, and phagocytosis/trogocytosis are important steps in invasion and pathogenesis of E. histolytica, and Ca²⁺ signaling is thought to be associated with these processes leading to tissue invasion. There are a large number of Ca²⁺-binding proteins (CaBPs) in E. histolytica, and a number of these proteins appear to be associated with different steps in pathogenesis. The genome encodes 27 EF-hand–containing CaBPs in addition to a number of other Ca²⁺-binding domain/motif-containing proteins, which suggest intricate calcium signaling network in this parasite. Unlike other eukaryotes, a typical calmodulin-like protein has not been seen in E. histolytica. Though none of the CaBPs display sequence similarity with a typical calmodulin, extensive structural similarity has been seen in spite of lack of significant functional overlap with that of typical calmodulins. One of the unique features observed in E. histolytica is the identification of CaBPs (EhCaBP1, EhCaBP3) that have the ability to directly bind actin and modulate actin dynamics. Direct interaction of CaBPs with actin has not been seen in any other system. Pseudopod formation and phagocytosis are some of the processes that require actin dynamics, and some of the amoebic CaBPs (EhC2Pk, EhCaBP1, EhCaBP3, EhCaBP5) participate in this process. None of these E. histolytica CaBPs have any homolog in organisms other than different species of Entamoeba, suggesting a novel Ca²⁺ signaling pathway that has evolved in this genus.

Vacuolar Ca(2+) uptake

Calcium transporters that mediate the removal of Ca(2+) from the cytosol and into internal stores provide a critical role in regulating Ca(2+) signals following stimulus induction and in preventing calcium toxicity. The vacuole is a major calcium store in many organisms, particularly plants and fungi. Two main pathways facilitate the accumulation of Ca(2+) into vacuoles, Ca(2+)-ATPases and Ca(2+)/H(+) exchangers. Here I review the biochemical and regulatory features of these transporters that have been characterised in yeast and plants. These Ca(2+) transport mechanisms are compared with those being identified from other vacuolated organisms including algae and protozoa. Studies suggest that Ca(2+) uptake into vacuoles and other related acidic Ca(2+) stores occurs by conserved mechanisms which developed early in evolution.

Entamoeba histolytica: ouabain-insensitive Na(+)-ATPase activity

Our aim was to determine the presence of sodium pumps in Entamoeba histolytica. It is shown through the measurement of ouabain-sensitive ATPase activity and immunoblotting that E. histolytica does not express (Na(+)+K(+))ATPase. On the other hand, we observed a Na(+)-ATPase with the following characteristics: (1) stimulated by Na(+) or K(+), but these effects are not addictive; (2) the apparent affinity is similar for Na(+) and K(+) (K(0.5) = 13.3 +/- 3.7 and 15.4 +/- 3.1mM, respectively), as well as the V(max) (24.9 +/- 1.5 or 27.5 +/- 1.6 nmol Pi mg(-1)min(-1), respectively); (3) insensitive up to 2mM ouabain; and (4) inhibited by furosemide with an IC(50) of 0.12 +/- 0.004 mM. Furthermore, this enzyme forms a Na(+)- or K(+)-stimulated, furosemide- and hydroxylamine-sensitive ATP-driven acylphosphate phosphorylated intermediate.

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.

Trypanosoma brucei Plasma Membrane-Type Ca2+-ATPase 1 (TbPMC1) and 2 (TbPMC2) Genes Encode Functional Ca2+-ATPases Localized to the Acidocalcisomes and Plasma Membrane, and Essential for Ca2+ Homeostasis and Growth

Trypanosoma brucei adaptation and survival in its host involve integrated regulation of Ca(2+) pumps (Ca(2+)-ATPases), which are essential in calcium ion homeostasis. Here we report the cloning and sequencing of two genes (TbPMC1 and TbPMC2) encoding plasma membrane-type Ca(2+)-ATPases (PMCAs) of T. brucei, an agent of African trypanosomiasis. Indirect immunofluorescence analysis using antibodies against the proteins and against epitope tags introduced into each protein showed that TbPMC1 co-localized with the vacuolar H(+)-pyrophosphatase to the acidocalcisomes while TbPMC2 localized to the plasma membrane. Northern and Western blot analyses revealed that TbPMC1 and TbPMC2 are up-regulated during blood stages. TbPMC1 and TbPMC2 suppressed the Ca(2+) hypersensitivity of a mutant of S. cerevisiae that has a defect in vacuolar Ca(2+) accumulation. T. brucei Ca(2+)-ATPase genes were functionally characterized by using double-stranded RNA interference (RNAi) methodology to produce inducible Ca(2+)-ATPase-deficient procyclic forms. Similar results were obtained with bloodstream form trypomastigotes, except that the RNAi system was leaky and mRNA and protein levels recovered with time. The induction of dsRNA (RNAi) caused gross morphological alterations, and growth inhibition of procyclic forms. Induction of RNAi against TbPMC1 but not against TbPMC2 caused elevated levels of cytosolic Ca(2+) and decreased mobilization of Ca(2+) from intracellular stores following ionophore addition. These results establish that T. brucei PMCA-Ca(2+)-ATPases are essential for parasite viability and validate them as targets for drug development.

TcSCA complements yeast mutants defective in Ca2+ pumps and encodes a Ca2+-ATPase that localizes to the endoplasmic reticulum of Trypanosoma cruzi

Intracellular Ca(2+) in Trypanosoma cruzi is mainly located in an acidic compartment named the acidocalcisome, which among other pumps and exchangers possesses a plasma membrane-type Ca(2+)-ATPase. Evidence for an endoplasmic reticulum-located Ca(2+) uptake has been more elusive and based on indirect results. Here we report the cloning and sequencing of a gene encoding a sarcoplasmic-endoplasmic reticulum-type Ca(2+)-ATPase from T. cruzi. The protein (TcSCA) predicted from the nucleotide sequence of the gene has 1006 amino acids and a molecular mass of 109.7 kDa. Several sequence motifs found in sarcoplasmic-endoplasmic reticulum-type Ca(2+)-ATPases were present in TcSCA. Expression of TcSCA in yeast mutants deficient in the Golgi and vacuolar Ca(2+) pumps (pmr1 pmc1 cnb 1) restored growth on EGTA. Membranes were isolated from the pmr1 pmc1 cnb1 mutant transformed with TcSCA, and it was found that the TcSCA polypeptide formed a Ca(2+)-dependent and hydroxylamine-sensitive (32)P-labeled phosphoprotein of 110 kDa in the presence of [gamma-(32)P]ATP. Cyclopiazonic acid, but not thapsigargin, blocked this phosphoprotein formation. Transgenic parasites expressing constructs of TcSCA with green fluorescent protein exhibited co-localization of TcSCA with the endoplasmic reticulum proteins BiP and calreticulin. An endoplasmic reticulum location was also found in amastigotes and trypomastigotes using a polyclonal antibody against a COOH-terminal region of the protein. The ability of TcSCA to restore growth of mutant pmr1 pmc1 cnb 1 on medium containing Mn(2+) suggests that TcSCA may also regulate Mn(2+) homeostasis by pumping Mn(2+) into the endoplasmic reticulum of T. cruzi.

A plasma membrane-type Ca(2+)-ATPase co-localizes with a vacuolar H(+)-pyrophosphatase to acidocalcisomes of Toxoplasma gondii

Ca(2+)-ATPases are likely to play critical roles in the biochemistry of Toxoplasma gondii, since these protozoa are obligate intracellular parasites and the Ca(2+) concentration in their intracellular location is three orders of magnitude lower than in the extracellular medium. Here, we report the cloning and sequencing of a gene encoding a plasma membrane-type Ca(2+)-ATPase (PMCA) of T.gondii (TgA1). The predicted protein (TgA1) exhibits 32-36% identity to vacuolar Ca(2+)-ATPases of Trypanosoma cruzi, Saccharomyces cerevisiae, Entamoeba histolytica and Dictyostelium discoideum. Sequencing of both cDNA and genomic DNA from T.gondii indicated that TgA1 contains two introns near the C-terminus. A hydropathy profile of the protein suggests 10 transmembrane domains. TgA1 suppresses the Ca(2+) hypersensitivity of a mutant of S.cerevisiae that has a defect in vacuolar Ca(2+) accumulation. Indirect immunofluorescence and immunoelectron microscopy analysis indicate that TgA1 localizes to the plasma membrane and co-localizes with the vacuolar H(+)-pyrophosphatase to intracellular vacuoles identified morphologically and by X-ray microanalysis as the acidocalcisomes. This vacuolar-type Ca(2+)-ATPase could play an important role in Ca(2+) homeostasis in T.gondii.

Molecular epidemiology of Entamoeba spp.: evidence of a bottleneck (Demographic sweep) and transcontinental spread of diploid parasites

Entamoeba histolytica causes amebic colitis and liver abscess in developing countries such as Mexico and India. Entamoeba dispar is morphologically identical but is not associated with disease. Here we determined the ploidy of E. histolytica and developed PCR-based methods for distinguishing field isolates of E. histolytica or E. dispar. Fluorescence in situ hybridization showed that E. histolytica trophozoites are diploid for five "single-copy" probes tested. Intergenic sequences between superoxide dismutase and actin 3 genes of clinical isolates of E. histolytica from the New and Old Worlds were identical, as were those of E. dispar. These results suggest a bottleneck or demographic sweep in entamoebae which infect humans. In contrast, E. histolytica and E. dispar genes encoding repeat antigens on the surface of trophozoites (Ser-rich protein) or encysting parasites (chitinase) were highly polymorphic. chitinase alleles suggested that the early axenized strains of E. histolytica, HM-1 from Mexico City, Mexico, and NIH-200 from Calcutta, India, are still present and that similar E. dispar parasites can be identified in both the New and Old Worlds. Ser-rich protein alleles, which suggested the presence of the HM-1 strain in Mexico City, included some E. histolytica genes that predicted Ser-rich proteins with very few repeats. These results, which suggest diversifying selection at chitinase and Ser-rich protein loci, demonstrate the usefulness of these alleles for distinguishing clinical isolates of E. histolytica and E. dispar.