A mechanism of intracellular P2X receptor activation

VPS8D, a CORVET subunit, is required to maintain the contractile vacuole complex in Tetrahymena thermophila

Contractile vacuole complexes (CVCs) are complex osmoregulatory organelles, with vesicular (bladder) and tubular (spongiome) subcompartments. The mechanisms that underlie their formation and maintenance within the eukaryotic endomembrane network are poorly understood. In the Ciliate Tetrahymena thermophila, six differentiated CORVETs (class C core vacuole/endosome tethering complexes), with Vps8 subunits designated A-F, are likely to direct endosomal trafficking. Vps8Dp localizes to both bladder and spongiome. We show by inducible knockdown that VPS8D is essential to CVC organization and function. VPS8D knockdown increased susceptibility to osmotic shock, tolerated in the wildtype but triggering irreversible lethal swelling in the mutant. The knockdown rapidly triggered contraction of the spongiome and lengthened the period of the bladder contractile cycle. More prolonged knockdown resulted in disassembly of both the spongiome and bladder, and dispersal of proteins associated with those compartments. In stressed cells where the normally singular bladder is replaced by numerous vesicles bearing bladder markers, Vps8Dp concentrated conspicuously at long-lived inter-vesicle contact sites, consistent with tethering activity. Similarly, Vps8Dp in cell-free preparations accumulated at junctions formed after vacuoles came into close contact. Also consistent with roles for Vps8Dp in tethering and/or fusion were the emergence in knockdown cells of multiple vacuole-related structures, replacing the single bladder.

The old and the new about the contractile vacuole of Trypanosoma cruzi

Osmoregulation is a conserved cellular process required for the survival of all organisms. In protists, the need for robust compensatory mechanisms that can maintain cell volume and tonicity within physiological range is even more relevant, as their life cycles are often completed in different environments. Trypanosoma cruzi, the protozoan pathogen responsible for Chagas disease, is transmitted by an insect vector to multiple types of mammalian hosts. The contractile vacuole complex (CVC) is an organelle that senses and compensates osmotic changes in the parasites, ensuring their survival upon ionic and osmotic challenges. Recent work shows that the contractile vacuole is also a key component of the secretory and endocytic pathways, regulating the selective targeting of surface proteins during differentiation. Here we summarize our current knowledge of the mechanisms involved in the osmoregulatory processes that take place in the vacuole, and we explore the new and exciting functions of this organelle in cell trafficking and signaling.

Matrix metalloproteinases, purinergic signaling, and epigenetics: hubs in the spinal neuroglial network following peripheral nerve injury

Activation of glial cells (reactive gliosis) and the purinergic pathway, together with metalloproteinase (MMP)-induced remodeling of the neural extracellular matrix (nECM), drive maladaptive changes in the spinal cord following peripheral nerve injury (PNI). We evaluated the effects on spinal maladaptive plasticity through administration of oxidized ATP (oxATP), an antagonist of P2X receptors (P2XR), and/or GM6001, an inhibitor of MMPs, in rats following spared nerve injury (SNI) of the sciatic nerve. With morpho-molecular techniques, we demonstrated a reduction in spinal reactive gliosis and changes in the neuro-glial-nECM crosstalk via expression remodeling of P2XR, nerve growth factor (NGF) receptors (TrkA and p75), and histone deacetylase 2 (HDAC2) after treatments with oxATP/GM6001. Altogether, our data suggest that MMPs and purinergic inhibition have a modulatory impact on key proteins in the neuro-glial-nECM network, acting at different levels from intracellular signaling to epigenetic modifications.

A polycystin-type transient receptor potential (Trp) channel that is activated by ATP

ATP and ADP are ancient extra-cellular signalling molecules that in Dictyostelium amoebae cause rapid, transient increases in cytosolic calcium due to an influx through the plasma membrane. This response is independent of hetero-trimeric G-proteins, the putative IP3 receptor IplA and all P2X channels. We show, unexpectedly, that it is abolished in mutants of the polycystin-type transient receptor potential channel, TrpP. Responses to the chemoattractants cyclic-AMP and folic acid are unaffected in TrpP mutants. We report that the DIF morphogens, cyclic-di-GMP, GABA, glutamate and adenosine all induce strong cytoplasmic calcium responses, likewise independently of TrpP. Thus, TrpP is dedicated to purinergic signalling. ATP treatment causes cell blebbing within seconds but this does not require TrpP, implicating a separate purinergic receptor. We could detect no effect of ATP on chemotaxis and TrpP mutants grow, chemotax and develop almost normally in standard conditions. No gating ligand is known for the human homologue of TrpP, polycystin-2, which causes polycystic kidney disease. Our results now show that TrpP mediates purinergic signalling in Dictyostelium and is directly or indirectly gated by ATP.

Summary: We show that a Trp channel related to the mammalian polycystin channel, rather than a P2X receptor, is responsible for the purinergic stimulation of cytosolic calcium levels in Dictyostelium cells.

Comparative study of the P2X gene family in animals and plants

P2X receptors are ligand-gated ion channels that can bind with the adenosine triphosphate (ATP) and have diverse functional roles in neuropathic pain, inflammation, special sense, and so on. In this study, 180 putative P2X genes, including 176 members in 32 animal species and 4 members in 3 species of lower plants, were identified. These genes were divided into 13 groups, including 7 groups in vertebrates and 6 groups in invertebrates and lower plants, through phylogenetic analysis. Their gene organization and motif composition are conserved in most predicted P2X members, while group-specific features were also found. Moreover, synteny relationships of the putative P2X genes in vertebrates are conserved while simultaneously experiencing a series of gene insertion, inversion, and transposition. Recombination signals were detected in almost all of the vertebrates and invertebrates, suggesting that intragenic recombination may play a significant role in the evolution of P2X genes. Selection analysis also identified some positively selected sites that acted on the evolution of most of the predicted P2X proteins. The phenomenon of alternative splicing occurred commonly in the putative P2X genes of vertebrates. This article explored in depth the evolutional relationship among different subtypes of P2X genes in animal and plants and might serve as a solid foundation for deciphering their functions in further studies.

Evidence for Extracellular ATP as a Stress Signal in a Single-Celled Organism

ATP is omnipresent in biology and acts as an extracellular signaling molecule in mammals. Information regarding the signaling function of extracellular ATP in single-celled eukaryotes is lacking. Here, we explore the role of extracellular ATP in cell volume recovery during osmotic swelling in the amoeba Dictyostelium. Release of micromolar ATP could be detected during cell swelling and regulatory cell volume decrease (RVD) phases during hypotonic challenge. Scavenging ATP with apyrase caused profound cell swelling and loss of RVD. Apyrase-induced swelling could be rescued by 100 μM βγ-imidoATP. N-Ethylmalemide (NEM), an inhibitor of vesicular exocytosis, caused heightened cell swelling, loss of RVD, and inhibition of ATP release. Amoebas with impaired contractile vacuole (CV) fusion (drainin knockout [KO] cells) displayed increased swelling but intact ATP release. One hundred micromolar Gd(3+) caused cell swelling while blocking any recovery by βγ-imidoATP. ATP release was 4-fold higher in the presence of Gd(3+). Cell swelling was associated with an increase in intracellular nitric oxide (NO), with NO-scavenging agents causing cell swelling. Swelling-induced NO production was inhibited by both apyrase and Gd(3+), while NO donors rescued apyrase- and Gd(3+)-induced swelling. These data suggest extracellular ATP released during cell swelling is an important signal that elicits RVD. Though the cell surface receptor for ATP in Dictyostelium remains elusive, we suggest ATP operates through a Gd(3+)-sensitive receptor that is coupled with intracellular NO production.

P2X7 receptors: role in bone cell formation and function

The role of the P2X7 receptor (P2X7R) is being explored with intensive interest in the context of normal bone physiology, bone-related diseases and, to an extent, bone cancer. In this review, we cover the current understanding of P2X7R regulation of bone cell formation, function and survival. We will discuss how the P2X7R drives lineage commitment of undifferentiated bone cell progenitors, the vital role of P2X7R activation in bone mineralisation and its relatively unexplored role in osteocyte function. We also review how P2X7R activation is imperative for osteoclast formation and its role in bone resorption via orchestrating osteoclast apoptosis. Variations in the gene for the P2X7R (P2RX7) have implications for P2X7R-mediated processes and we review the relevance of these genetic variations in bone physiology. Finally, we highlight how targeting P2X7R may have therapeutic potential in bone disease and cancer.

Rab11 regulates trafficking of trans-sialidase to the plasma membrane through the contractile vacuole complex of Trypanosoma cruzi

Trypanosoma cruzi is the etiologic agent of Chagas disease. Although this is not a free-living organism it has conserved a contractile vacuole complex (CVC) to regulate its osmolarity. This obligate intracellular pathogen is, in addition, dependent on surface proteins to invade its hosts. Here we used a combination of genetic and biochemical approaches to delineate the contribution of the CVC to the traffic of glycosylphosphatidylinositol (GPI)-anchored proteins to the plasma membrane of the parasite and promote host invasion. While T. cruzi Rab11 (GFP-TcRab11) localized to the CVC, a dominant negative (DN) mutant tagged with GFP (GFP-TcRab11DN) localized to the cytosol, and epimastigotes expressing this mutant were less responsive to hyposmotic and hyperosmotic stress. Mutant parasites were still able to differentiate into metacyclic forms and infect host cells. GPI-anchored trans-sialidase (TcTS), mucins of the 60-200 KDa family, and trypomastigote small surface antigen (TcTSSA II) co-localized with GFP-TcRab11 to the CVC during transformation of intracellular amastigotes into trypomastigotes. Mucins of the gp35/50 family also co-localized with the CVC during metacyclogenesis. Parasites expressing GFP-TcRab11DN prevented TcTS, but not other membrane proteins, from reaching the plasma membrane, and were less infective as compared to wild type cells. Incubation of these mutants in the presence of exogenous recombinant active, but not inactive, TcTS, and a sialic acid donor, before infecting host cells, partially rescued infectivity of trypomastigotes. Taking together these results reveal roles of TcRab11 in osmoregulation and trafficking of trans-sialidase to the plasma membrane, the role of trans-sialidase in promoting infection, and a novel unconventional mechanism of GPI-anchored protein secretion.

P2X4 forms functional ATP-activated cation channels on lysosomal membranes regulated by luminal pH

P2X receptors are commonly known as plasma membrane cation channels involved in a wide variety of cell functions. The properties of these channels have been extensively studied on the plasma membrane. However, studies in amoeba suggest that P2X receptors are also present intracellularly and involved in vesicle fusion with the plasma membrane. Recently, it was shown that in addition to plasma membrane expression, mammalian P2X4 was also localized intracellularly in lysosomes. However, it was not clear whether the lysosomal P2X4 receptors function as channels and how they are activated and regulated. In this paper, we show that both P2X4 and its natural ligand, ATP, are enriched in lysosomes of COS1 and HEK293 cells. By directly recording membrane currents from enlarged lysosomal vacuoles, we demonstrated that lysosomal P2X4 formed channels activated by ATP from the luminal side in a pH-dependent manner. While the acidic pH at the luminal side inhibited P2X4 activity, increasing the luminal pH in the presence of ATP caused P2X4 activation. We further showed that, as for the plasma membrane P2X4, the lysosomal P2X4 was potentiated by ivermectin but insensitive to suramin and PPADS, and it permeated the large cation N-methyl-d-glucamine upon activation. Our data suggest that P2X4 forms functional ATP-activated cation channels on lysosomal membranes regulated by luminal pH. Together with the reported fusion effect of intracellular P2X in lower organisms, we speculate that the lysosome-localized P2X4 may play specific roles in membrane trafficking of acidic organelles in mammalian cells.

Calcium-dependent regulation of Rab activation and vesicle fusion by an intracellular P2X ion channel

Rab GTPases play key roles in the delivery, docking and fusion of intracellular vesicles. However, the mechanism by which spatial and temporal regulation of Rab GTPase activity is controlled is poorly understood. Here we describe a mechanism by which localized calcium release through a vesicular ion channel controls Rab GTPase activity. We show that activation of P2XA, an intracellular ion channel localized to the Dictyostelium discoideum contractile vacuole system, results in calcium efflux required for downregulation of Rab11a activity and efficient vacuole fusion. Vacuole fusion and Rab11a downregulation require the activity of CnrF, an EF-hand-containing Rab GAP found in a complex with Rab11a and P2XA. CnrF Rab GAP activity for Rab11a is enhanced by the presence of calcium and the EF-hand domain. These findings suggest that P2XA activation results in vacuolar calcium release, which triggers activation of CnrF Rab GAP activity and subsequent downregulation of Rab11a to allow vacuole fusion.

Primitive ATP-activated P2X receptors: discovery, function and pharmacology

Adenosine 5-triphosphate (ATP) is omnipresent in biology. It is therefore no surprise that organisms have evolved multifaceted roles for ATP, exploiting its abundance and restriction of passive diffusion across biological membranes. A striking role is the emergence of ATP as a bona fide transmitter molecule, whereby the movement of ATP across membranes serves as a chemical message through a direct ligand-receptor interaction. P2X receptors are ligand-gated ion channels that mediate fast responses to the transmitter ATP in mammalian cells including central and sensory neurons, vascular smooth muscle, endothelium, and leukocytes. Molecular cloning of P2X receptors and our understanding of structure-function relationships has provided sequence information with which to query an exponentially expanding wealth of genome sequence information including protist, early animal and human pathogen genomes. P2X receptors have now been cloned and characterized from a number of simple organisms. Such work has led to surprising new cellular roles for the P2X receptors family and an unusual phylogeny, with organisms such as Drosophila and C. elegans notably lacking P2X receptors despite retaining ionotropic receptors for other common transmitters that are present in mammals. This review will summarize current work on the evolutionary biology of P2X receptors and ATP as a signaling molecule, discuss what can be drawn from such studies when considering the action of ATP in higher animals and plants, and outline how simple organisms may be exploited experimentally to inform P2X receptor function in a wider context.

The trafficking and targeting of P2X receptors

The functional expression of P2X receptors at the plasma membrane is dependent on their trafficking along secretory and endocytic pathways. There are seven P2X receptor subunits, and these differ in their subcellular distributions because they have very different trafficking properties. Some are retained within the endoplasmic reticulum (ER), while others are predominantly at the cell surface or within endosomes and lysosomes. Changes in recruitment of receptors to and from the plasma membrane provides a way of rapidly up- or down-regulating the cellular response to adenosine triphosphate (ATP). An additional layer of regulation is the targeting of these receptors within the membranes of each compartment, which affects their stability, function and the nature of the effector proteins with which they form signaling complexes. The trafficking and targeting of P2X receptors is regulated by their interactions with other proteins and with lipids and we can expect this to vary in a cell-type specific manner and in response to changes in the environment giving rise to differences in receptor activity and function.

Effect of P2X4 and P2X7 receptor antagonism on the pressure diuresis relationship in rats

Reduced glomerular filtration, hypertension and renal microvascular injury are hallmarks of chronic kidney disease, which has a global prevalence of ~10%. We have shown previously that the Fischer (F344) rat has lower GFR than the Lewis rat, and is more susceptible to renal injury induced by hypertension. In the early stages this injury is limited to the pre-glomerular vasculature. We hypothesized that poor renal hemodynamic function and vulnerability to vascular injury are causally linked and genetically determined. In the present study, normotensive F344 rats had a blunted pressure diuresis relationship, compared with Lewis rats. A kidney microarray was then interrogated using the Endeavour enrichment tool to rank candidate genes for impaired blood pressure control. Two novel candidate genes, P2rx7 and P2rx4, were identified, having a 7− and 3− fold increased expression in F344 rats. Immunohistochemistry localized P2X₄ and P2X₇ receptor expression to the endothelium of the pre-glomerular vasculature. Expression of both receptors was also found in the renal tubule; however there was no difference in expression profile between strains. Brilliant Blue G (BBG), a relatively selective P2X₇ antagonist suitable for use in vivo, was administered to both rat strains. In Lewis rats, BBG had no effect on blood pressure, but increased renal vascular resistance, consistent with inhibition of some basal vasodilatory tone. In F344 rats BBG caused a significant reduction in blood pressure and a decrease in renal vascular resistance, suggesting that P2X₇ receptor activation may enhance vasoconstrictor tone in this rat strain. BBG also reduced the pressure diuresis threshold in F344 rats, but did not alter its slope. These preliminary findings suggest a physiological and potential pathophysiological role for P2X₇ in controlling renal and/or systemic vascular function, which could in turn affect susceptibility to hypertension-related kidney damage.

Functional properties of five Dictyostelium discoideum P2X receptors

The Dictyostelium discoideum genome encodes five proteins that share weak sequence similarity with vertebrate P2X receptors. Unlike vertebrate P2X receptors, these proteins are not expressed on the surface of cells, but populate the tubules and bladders of the contractile vacuole. In this study, we expressed humanized cDNAs of P2XA, P2XB, P2XC, P2XD, and P2XE in human embryonic kidney cells and altered the ionic and proton environment in an attempt to reflect the situation in amoeba. Recording of whole-cell membrane currents showed that four receptors operated as ATP-gated channels (P2XA, P2XB, P2XD, and P2XE). At P2XA receptors, ATP was the only effective agonist of 17 structurally related putative ligands that were tested. Extracellular sodium, compared with potassium, strongly inhibited ATP responses in P2XB, P2XD, and P2XE receptors. Increasing the proton concentration (pH 6.2) accelerated desensitization at P2XA receptors and decreased currents at P2XD receptors, but increased the currents at P2XB and P2XE receptors. Dictyostelium lacking P2XA receptors showed impaired regulatory volume decrease in hypotonic solution. This phenotype was readily rescued by overexpression of P2XA and P2XD receptors, partially rescued by P2XB and P2XE receptors, and not rescued by P2XC receptors. The failure of the nonfunctional receptor P2XC to restore the regulatory volume decrease highlights the importance of ATP activation of P2X receptors for a normal response to hypo-osmotic shock, and the weak rescue by P2XB and P2XE receptors indicates that there is limited functional redundancy among Dictyostelium P2X receptors.

ATP-Binding Cassette Transporter B4 Anchors the Cell Adhesion Molecule DdCAD-1 to Cell Membrane in Dictyostelium discoideum

In Dictyostelium, soluble cell adhesion molecule, DdCAD-1, regulates cell-cell interaction through an unknown anchoring protein on the plasma membrane. Far western blot analysis using different probes revealed that the potential DdCAD-1 interacting protein was between 64 and 98 kDa. To isolate and identify the anchoring protein, GST-DdCAD-1 and anchoring protein were cross-linked in vivo by chemical cross-linker and stable protein complex was isolated by co-immunoprecipitation assays. The protein cross-linked to DdCAD-1 was extracted from the gel slice and trypsinized. The peptides were subjected to analysis by mass spectrometry, which showed that the putative anchoring protein belongs to ATP-binding cassette transporter family.

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.

Intracellular P2X receptors as novel calcium release channels and modulators of osmoregulation in Dictyostelium: a comparison of two common laboratory strains

P2X receptors are calcium permeable ligand-gated ion channels activated by ATP. Their role as cell surface receptors for extracellular ATP released physiologically by mammalian cells is well established. However, the cellular function of P2X receptor subtypes that populate the membranes of intracellular compartments is not defined. An initial report described how intracellular P2X receptors control the function of the contractile vacuole, an osmoregulatory organelle in Dictyostelium and other protists, and that genetic disruption of P2X receptors severely impaired cell volume control during hypotonic stress. However, later studies refuted a functional role of intracellular P2X receptors in Dictyostelium. Here we provide evidence that the discrepancies reported between the studies are due to the laboratory strain of Dictyostelium employed, which display different phenotypes in response to hypotonic stress and a varied dependency upon P2X receptors for osmoregulation. We use the recent discovery that intracellular P2X receptors are novel calcium release channels to provide some mechanistic insight in an effort to explain why the strain variance may exist.

Cloning and characterization of a P2X receptor expressed in the central nervous system of Lymnaea stagnalis

P2X receptors are membrane ion channels gated by extracellular ATP. Mammals possess seven distinct P2X subtypes (P2X1-7) that have important functions in a wide array of physiological processes including roles in the central nervous system (CNS) where they have been linked to modulation of neurotransmitter release. We report here the cloning and functional characterization of a P2X receptor from the mollusc Lymnaea stagnalis. This model organism has a relatively simple CNS consisting of large readily identifiable neurones, a feature which together with a well characterized neuronal circuitry for important physiological processes such as feeding and respiration makes it an attractive potential model to examine P2X function. Using CODEHOP PCR we identified a single P2X receptor (LymP2X) in Lymnaea CNS which was subsequently cloned by RT-PCR. When heterologously expressed in Xenopus oocytes, LymP2X exhibited ATP evoked inward currents (EC(50) 6.2 µM) which decayed during the continued presence of agonist. UTP and ADP did not activate the receptor whereas αβmeATP was a weak agonist. BzATP was a partial agonist with an EC(50) of 2.4 µM and a maximal response 33% smaller than that of ATP. The general P2 receptor antagonists PPADS and suramin both inhibited LymP2X currents with IC(50) values of 8.1 and 27.4 µM respectively. LymP2X is inhibited by acidic pH whereas Zn(2+) and Cu(2+) ions exhibited a biphasic effect, potentiating currents up to 100 µM and inhibiting at higher concentrations. Quantitative RT-PCR and in situ hybridization detected expression of LymP2X mRNA in neurones of all CNS ganglia suggesting this ion channel may have widespread roles in Lymnaea CNS function.