Calcium-Dependent Signaling and Kinases in Apicomplexan Parasites
2009; Cell Press; Volume: 5; Issue: 6 Linguagem: Inglês
10.1016/j.chom.2009.05.017
ISSN1934-6069
AutoresOliver Billker, Sebastian Lourido, L. David Sibley,
Tópico(s)Trypanosoma species research and implications
ResumoCalcium controls many critical events in the complex life cycles of apicomplexan parasites including protein secretion, motility, and development. Calcium levels are normally tightly regulated and rapid release of calcium into the cytosol activates a family of calcium-dependent protein kinases (CDPKs), which are normally characteristic of plants. CDPKs present in apicomplexans have acquired a number of unique domain structures likely reflecting their diverse functions. Calcium regulation in parasites is closely linked to signaling by cyclic nucleotides and their associated kinases. This Review summarizes the pivotal roles that calcium- and cyclic nucleotide-dependent kinases play in unique aspects of parasite biology. Calcium controls many critical events in the complex life cycles of apicomplexan parasites including protein secretion, motility, and development. Calcium levels are normally tightly regulated and rapid release of calcium into the cytosol activates a family of calcium-dependent protein kinases (CDPKs), which are normally characteristic of plants. CDPKs present in apicomplexans have acquired a number of unique domain structures likely reflecting their diverse functions. Calcium regulation in parasites is closely linked to signaling by cyclic nucleotides and their associated kinases. This Review summarizes the pivotal roles that calcium- and cyclic nucleotide-dependent kinases play in unique aspects of parasite biology. The Apicomplexa is an ancient phylum of some 5000 diverse eukaryotic species that are largely parasitic on marine invertebrates, insects, and vertebrates, where they have gained notoriety because of their role in animal and human diseases. Five Plasmodium species are responsible for significant mortality and morbidity due to malaria, the most serious form of which is caused by P. falciparum (Snow et al., 2005Snow R.W. Guerra C.A. Noor A.M. Myint H.Y. Hay S.I. The global distribution of clinical episodes of Plasmodium falciparum malaria.Nature. 2005; 434: 214-217Crossref PubMed Scopus (2087) Google Scholar). Related organisms such as Toxoplasma gondii (Joynson and Wreghitt, 2001Joynson D.H. Wreghitt T.J. Toxoplasmosis: A comprehensive clinical guide. Cambridge University Press, Cambridge, UK2001Crossref Google Scholar) and Cryptosporidium spp. (Tzipori and Widmer, 2008Tzipori S. Widmer G. A hundred-year retrospective on cryptosporidiosis.Trends Parasitol. 2008; 24: 184-189Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar) cause opportunistic infections of considerable importance in immunocompromised individuals. Highly divergent from well-studied organisms such as yeast, flies, and worms, apicomplexans are most closely related to ciliates and dinoflagellates (Baldauf, 2003Baldauf S.L. The deep roots of eukaryotes.Science. 2003; 300: 1703-1706Crossref PubMed Scopus (593) Google Scholar). As a result of this extreme evolutionary divergence, much of their basic biology is distinct from what we know of model organisms. Apicomplexans often contain plant-like features, owing to two events: (1) very early branching that likely predates the animal-plant split (Baldauf, 2003Baldauf S.L. The deep roots of eukaryotes.Science. 2003; 300: 1703-1706Crossref PubMed Scopus (593) Google Scholar), and (2) acquisition of a secondary endosymbiont derived from engulfment of an algal cell (Waller and McFadden, 2005Waller R.F. McFadden G.I. The apicoplast: a review of the derived plastid of apicomplexan parasites.Curr. Issues Mol. Biol. 2005; 7: 57-79PubMed Google Scholar). As such, signaling pathways in apicomplexans contain both conserved and unique features. Apicomplexans have highly polarized cells that are specialized for regulated secretion and directed entry into their host cells (Cowman and Crabb, 2006Cowman A.F. Crabb B.S. Invasion of red blood cells by malaria parasites.Cell. 2006; 124: 755-766Abstract Full Text Full Text PDF PubMed Scopus (622) Google Scholar, Sibley, 2004Sibley L.D. Invasion strategies of intracellular parasites.Science. 2004; 304: 248-253Crossref PubMed Scopus (340) Google Scholar). Calcium controls a number of critical events in the life cycle including secretion of adhesins, gliding motility, cell invasion, and egress (Moreno and Docampo, 2003Moreno S.N.J. Docampo R. Calcium regulation in protozoan parasites.Curr. Opin. Microbiol. 2003; 6: 359-364Crossref PubMed Scopus (119) Google Scholar). Calcium also influences developmental processes that occur at distinct stages in their complex life cycles (Alano and Billker, 2005Alano P. Billker O. Gametocytes and Gametes.in: Sherman I.W. Molecular Approaches to Malaria. ASM Press, Washington, D.C.2005: 191-219Google Scholar). In this Review, we focus on the role of calcium signaling in two important apicomplexan parasites—Toxoplasma gondii and Plasmodium spp. Aside from its significance as a pathogen, T. gondii has gained importance as model for cellular and biochemical studies of an otherwise intractable group of parasites (Sibley, 2004Sibley L.D. Invasion strategies of intracellular parasites.Science. 2004; 304: 248-253Crossref PubMed Scopus (340) Google Scholar). In contrast, cellular studies are more challenging in Plasmodium spp., yet application of genetics has been useful to explore the role of stage-specific genes (Carvalho and Menard, 2005Carvalho T.G. Menard R. Manipulating the Plasmodium genome.Curr. Issues Mol. Biol. 2005; 7: 39-55PubMed Google Scholar). Together, studies conducted in these two systems provide a framework for understanding how calcium regulates several important classes of protein kinases, which in turn control much of the unique biology of these organisms. In this Review, we outline findings that have been developed separately in each of these systems, calling attention to cases where parallel studies have been informative and highlighting areas where further comparative studies are needed. Apicomplexans are united by a conserved set of features that define their apically polarized cellular forms, which are specialized for migration and cell invasion (Morrissette and Sibley, 2002Morrissette N.S. Sibley L.D. Cytoskeleton of apicomplexan parasites.Microbiol. Mol. Biol. Rev. 2002; 66: 21-38Crossref PubMed Scopus (305) Google Scholar). The apical pole is defined by a unique microtubular organizing center, which in Toxoplasma is composed of a conoid and two polar rings, from which a group of singlet microtubules emanates and subtends the membrane of the elongated organism (Morrissette and Sibley, 2002Morrissette N.S. Sibley L.D. Cytoskeleton of apicomplexan parasites.Microbiol. Mol. Biol. Rev. 2002; 66: 21-38Crossref PubMed Scopus (305) Google Scholar). Beneath the plasma membrane, an inner membrane comprised of flattened vesicles derived from the endoplasmic reticulum surrounds the parasite. A meshwork of filaments formed from articulin-like proteins (Gould et al., 2008Gould S.B. Tham W.H. Cowman A.F. McFadden G.I. Waller R.F. Alveolins, a new family of cortical proteins that define the protist infrakingdom Alveolata.Mol. Biol. Evol. 2008; 25: 1219-1230Crossref PubMed Scopus (109) Google Scholar) provides structural support for the inner membrane complex that extends beneath the entire surface of the cell, except at its very apex. Three sets of secretory organelles discharge their contents from the apical end of the parasite during cell invasion (Carruthers and Sibley, 1997Carruthers V.B. Sibley L.D. Sequential protein secretion from three distinct organelles of Toxoplasma gondii accompanies invasion of human fibroblasts.Eur. J. Cell Biol. 1997; 73: 114-123PubMed Google Scholar). These features are conserved among sporozoites of most apicomplexans including malaria, except for the absence of a conoid—and while they are less well defined in merozoites, these forms also share a common mechanism of cell invasion. Apicomplexans rely on an actin-myosin motor for gliding motility, tissue migration, and cell invasion (Cowman and Crabb, 2006Cowman A.F. Crabb B.S. Invasion of red blood cells by malaria parasites.Cell. 2006; 124: 755-766Abstract Full Text Full Text PDF PubMed Scopus (622) Google Scholar, Sibley, 2004Sibley L.D. Invasion strategies of intracellular parasites.Science. 2004; 304: 248-253Crossref PubMed Scopus (340) Google Scholar). Motility and invasion are highly dependent on assembly of filamentous actin in the parasite (Dobrowolski and Sibley, 1996Dobrowolski J.M. Sibley L.D. Toxoplasma invasion of mammalian cells is powered by the actin cytoskeleton of the parasite.Cell. 1996; 84: 933-939Abstract Full Text Full Text PDF PubMed Scopus (508) Google Scholar). The force for motility is generated by a small myosin, best studied in T. gondii but also conserved in Plasmodium, which is anchored in the inner membrane (Gaskins et al., 2004Gaskins E. Gilk S. DeVore N. Mann T. Ward G.E. Beckers C. Identification of the membrane receptor of a class XIV myosin Toxoplasma gondii.J. Cell Biol. 2004; 165: 383-393Crossref PubMed Scopus (187) Google Scholar, Meissner et al., 2002Meissner M. Schluter D. Soldati D. Role of Toxoplasma gondii myosin A in powering parasite gliding and host cell invasion.Science. 2002; 298: 837-840Crossref PubMed Scopus (343) Google Scholar). Cell invasion is also critically dependent on secretion of adhesins, which are stored in apical secretory organelles called micronemes. Among the best studied of these are TRAP, which is essential for gliding motility and invasion of Plasmodium (Sultan et al., 1997Sultan A.A. Thathy V. Frevert U. Robson K.J.H. Crisanti A. Nussenzweig V. Nussenzweig R.S. Menard R. TRAP is necessary for gliding motility and infectivity of Plasmodium sporozoites.Cell. 1997; 90: 511-522Abstract Full Text Full Text PDF PubMed Scopus (484) Google Scholar), and its ortholog MIC2, which is likewise essential in T. gondii (Huynh and Carruthers, 2006Huynh M.H. Carruthers V.B. Toxoplasma MIC2 is a major determinant of invasion and virulence.PLoS Pathog. 2006; 2: 753-762Google Scholar). TRAP and MIC2 contain several extracellular domains that interact with the substratum, a transmembrane domain, and a short cytoplasmic tail. The cytoplasmic domains of TRAP and MIC2 bind specifically to aldolase through a combination of charge and hydrophobic interactions, thereby linking to filamentous actin, and coupling adhesion with actin-based motility (Bosch et al., 2007Bosch J. Buscaglia C.A. Krumm B. Ingason B.P. Lucas R. Roach C. Cardozo T. Nussenzweig V. Hol W.G. Aldolase provides an unusual binding site for thrombospondin-related anonymous protein in the invasion machinery of the malaria parasite.Proc. Natl. Acad. Sci. USA. 2007; 104: 7015-7020Crossref PubMed Scopus (67) Google Scholar, Buscaglia et al., 2003Buscaglia C.A. Coppens I. Hol W.G.J. Nussenzweig V. Site of interaction between aldolase and thrombospondin-related anonymous protein in Plasmodium.Mol. Biol. Cell. 2003; 14: 4947-4957Crossref PubMed Scopus (139) Google Scholar, Jewett and Sibley, 2003Jewett T.J. Sibley L.D. Aldolase forms a bridge between cell surface adhesins and the actin cytoskeleton in apicomplexan parasites.Mol. Cell. 2003; 11: 885-894Abstract Full Text Full Text PDF PubMed Scopus (258) Google Scholar, Starnes et al., 2006Starnes G.L. Jewett T.J. Carruthers V.B. Sibley L.D. Two separate, conserved acidic amino acid domains within the Toxoplasma gondii MIC2 cytoplasmic tail are required for parasite survival.J. Biol. Chem. 2006; 281: 30745-30754Crossref PubMed Scopus (37) Google Scholar, Starnes et al., 2009Starnes G.L. Coincon M. Sygusch J. Sibley L.D. Aldolase is essential for energy production and bridging adhesin-cytoskeletal interactions during parasite invasion of host cells.Cell Host Microbe. 2009; 5: 353-364Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar). Following apical secretion, adhesins are translocated by the actin-myosin motor, traveling along the cell surface to the posterior end, thus driving net forward movement. This process culminates with trimming from the surface by a family of rhomboid proteins that clip the adhesins within their transmembrane domains (Brossier et al., 2005Brossier F. Jewett T.J. Sibley L.D. Urban S. A spatially-localized rhomboid protease cleaves cell surface adhesins essential for invasion by Toxoplasma.Proc. Natl. Acad. Sci. USA. 2005; 102: 4146-4151Crossref PubMed Scopus (170) Google Scholar, O'Donnell et al., 2006O'Donnell R.A. Hackett F. Howell S.A. Treeck M. Struck N. Krnajski Z. Withers-Martinez C. Gilberger T.W. Blackman M.J. Intramembrane proteolysis mediates shedding of a key adhesin during erythrocyte invasion by the malaria parasite.J. Cell Biol. 2006; 174: 1023-1033Crossref PubMed Scopus (149) Google Scholar). Actin-myosin based gliding motility is both conserved within the group and unique to apicomplexans, which otherwise lack locomotory organelles such as cilia and flagella (other than in the male gamete). The requirements for intracellular calcium have been extensively studied in T. gondii due to the availability of quantitative in vitro assays for motility, protein secretion, host cell invasion, and egress (Figure 1). Calcium levels in T. gondii are maintained at very low levels (i.e., 100 nM) in the cytosol of resting parasites, as is typical of eukaryotic cells (Moreno and Docampo, 2003Moreno S.N.J. Docampo R. Calcium regulation in protozoan parasites.Curr. Opin. Microbiol. 2003; 6: 359-364Crossref PubMed Scopus (119) Google Scholar). Intracellular storage sites of calcium include the endoplasmic reticulum (ER), mitochondrion, and acidocalcisomes (Moreno and Docampo, 2003Moreno S.N.J. Docampo R. Calcium regulation in protozoan parasites.Curr. Opin. Microbiol. 2003; 6: 359-364Crossref PubMed Scopus (119) Google Scholar). The primary mobilizable store for calcium is likely the ER, and intracellular calcium provides the key signal for triggering a number of important events (Lovett and Sibley, 2003Lovett J.L. Sibley L.D. Intracellular calcium stores in Toxoplasma gondii govern invasion of host cells.J. Cell Sci. 2003; 116: 3009-3016Crossref PubMed Scopus (156) Google Scholar). Apicomplexans contain a conserved sacroplasmic-endoplasmic reticulum calcium ATPase (SERCA) that pumps calcium into the lumen of the ER, hence refilling this store and decreasing cytoplasmic levels (Figure 2). The SERCA pump in T. gondii has been localized to the ER and is sensitive to the plant compounds thapsigargin and artemisinin (Nagamune et al., 2007Nagamune K. Beatty W.L. Sibley L.D. Artemisinin induces calcium-dependent secretion in Toxoplasma gondii.Eukaryot. Cell. 2007; 6: 2147-2156Crossref PubMed Scopus (86) Google Scholar). Thapsigargin and artemisinin also affect the activity of the Plasmodium SERCA when expressed in Xenopus (Eckstein-Ludwig et al., 2003Eckstein-Ludwig U. Webb R.J. van Goethem I.D.A. East J.M. Lee A.G. Kimura M. O'Neill P.M. Bray P.G. Ward S.A. Krishna S. Artemisinins target the SERCA of Plasmodium falciparum.Nature. 2003; 424: 957-961Crossref PubMed Scopus (830) Google Scholar), and this molecular target may partially explain the effectiveness of artemisinins as antimalarial agents. In addition, T. gondii contains several plasma membrane Ca2+ ATPases, one of which has been genetically disrupted leading to decreased infectivity (Luo et al., 2005Luo S. Ruiz F.A. Moreno S.N. The acidocalcisome Ca2+ ATPase (TgA1) of Toxoplasma gondii is required for polyphosphate storage, intracellular calcium homeostasis and virulence.Mol. Microbiol. 2005; 55: 1034-1045Crossref PubMed Scopus (50) Google Scholar). Apicomplexans also contain two PMR1-like transporters, which in yeast are Golgi-type Ca2+ ATPases, and a single Ca2+/H+ exchanger (Nagamune et al., 2008bNagamune K. Moreno S.N. Chini E.N. Sibley L.D. Calcium regulation and signaling in apicomplexan parasites.Subcell. Biochem. 2008; 47: 70-81Crossref PubMed Scopus (88) Google Scholar). Collectively, these pumps control resting calcium levels and affect changes in calcium in response to environmental cues.Figure 2Calcium-Response Pathways Controlling Secretion and Motility in T. gondiiShow full captionThe primary mobilizable store for calcium in T. gondii is the endoplasmic reticulum (ER), which is filled by the action of a Ca2+ ATPase called SERCA. Stimulation of yet unidentified receptors (R) at the plasma membrane generates inositol triphosphate (IP3) and diacyl glycerol (DAG) from phopsphatidyl inositol bisphosphate (PIP2) through the activation of phospholipase C (PLC). Signaling, through a unique or common receptor, also results in activation of ADP ribose (ADPR) cyclase to produce cADPR. Phamacological and biochemical evidence supports the existence of calcium release channels that respond to cADPR (ryanodine receptors (RyR) and through IP3 receptors (IP3R) (Lovett et al., 2002Lovett J.L. Marchesini N. Moreno S.N. Sibley L.D. Toxoplasma gondii microneme secretion involves intracellular Ca2+ release from IP3 / ryanodine sensitive stores.J. Biol. Chem. 2002; 277: 25870-25876Crossref PubMed Scopus (124) Google Scholar). Released calcium activates calcium-dependent protein kinases such as TgCDPK1, which has previously been implicated in microneme (MIC) secretion in T. gondii (Kieschnick et al., 2001Kieschnick H. Wakefield T. Narducci C.A. Beckers C. Toxoplasma gondii attachment to host cells is regulated by a calmodulin- like domain protein kinase.J. Biol. Chem. 2001; 276: 12369-12377Crossref PubMed Scopus (122) Google Scholar). Independently, activation of guanylyl cyclase to generate cyclic GMP (cGMP) is important for activating protein kinase G (PKG), which also controls microneme secretion (Wiersma et al., 2004Wiersma H.I. Galuska S.E. Tomley F.M. Sibley L.D. Liberator P.A. Donald R.G.K. A role for coccidian cGMP-dependent protein kinase in motility and invasion.Int. J. Parasitol. 2004; 34: 369-380Crossref PubMed Scopus (110) Google Scholar). Other studies in P. falciparum indicate that CDPKs may act on the myosin motor complex (Green et al., 2008Green J.L. Rees-Channer R.R. Howell S.A. Martin S.R. Knuepfer E. Taylor H.M. Grainger M. Holder A.A. The motor complex of Plasmodium falciparum: phosphorylation by a calcium-dependent protein kinase.J. Biol. Chem. 2008; 283: 30980-30989Crossref PubMed Scopus (131) Google Scholar), which is anchored in the inner membrane complex (IMC).View Large Image Figure ViewerDownload Hi-res image Download (PPT) The primary mobilizable store for calcium in T. gondii is the endoplasmic reticulum (ER), which is filled by the action of a Ca2+ ATPase called SERCA. Stimulation of yet unidentified receptors (R) at the plasma membrane generates inositol triphosphate (IP3) and diacyl glycerol (DAG) from phopsphatidyl inositol bisphosphate (PIP2) through the activation of phospholipase C (PLC). Signaling, through a unique or common receptor, also results in activation of ADP ribose (ADPR) cyclase to produce cADPR. Phamacological and biochemical evidence supports the existence of calcium release channels that respond to cADPR (ryanodine receptors (RyR) and through IP3 receptors (IP3R) (Lovett et al., 2002Lovett J.L. Marchesini N. Moreno S.N. Sibley L.D. Toxoplasma gondii microneme secretion involves intracellular Ca2+ release from IP3 / ryanodine sensitive stores.J. Biol. Chem. 2002; 277: 25870-25876Crossref PubMed Scopus (124) Google Scholar). Released calcium activates calcium-dependent protein kinases such as TgCDPK1, which has previously been implicated in microneme (MIC) secretion in T. gondii (Kieschnick et al., 2001Kieschnick H. Wakefield T. Narducci C.A. Beckers C. Toxoplasma gondii attachment to host cells is regulated by a calmodulin- like domain protein kinase.J. Biol. Chem. 2001; 276: 12369-12377Crossref PubMed Scopus (122) Google Scholar). Independently, activation of guanylyl cyclase to generate cyclic GMP (cGMP) is important for activating protein kinase G (PKG), which also controls microneme secretion (Wiersma et al., 2004Wiersma H.I. Galuska S.E. Tomley F.M. Sibley L.D. Liberator P.A. Donald R.G.K. A role for coccidian cGMP-dependent protein kinase in motility and invasion.Int. J. Parasitol. 2004; 34: 369-380Crossref PubMed Scopus (110) Google Scholar). Other studies in P. falciparum indicate that CDPKs may act on the myosin motor complex (Green et al., 2008Green J.L. Rees-Channer R.R. Howell S.A. Martin S.R. Knuepfer E. Taylor H.M. Grainger M. Holder A.A. The motor complex of Plasmodium falciparum: phosphorylation by a calcium-dependent protein kinase.J. Biol. Chem. 2008; 283: 30980-30989Crossref PubMed Scopus (131) Google Scholar), which is anchored in the inner membrane complex (IMC). Recording of calcium transients from Fluo-4 labeled tachyzoites of T. gondii during gliding motility revealed an oscillating pattern of intracellular calcium that was abruptly dampened during cell invasion (Lovett and Sibley, 2003Lovett J.L. Sibley L.D. Intracellular calcium stores in Toxoplasma gondii govern invasion of host cells.J. Cell Sci. 2003; 116: 3009-3016Crossref PubMed Scopus (156) Google Scholar). The pattern of calcium waves correlates with periods of microneme secretion, which also abruptly terminate following successful invasion of host cells. How these calcium transients are controlled and how they are shut off is presently unclear. However, agents that alter the pattern or amplitude of oscillations also affect microneme secretion and cell invasion. For example, treatment with calmidazolium increases the frequency of calcium transients while decreasing the amplitude of the waves, resulting in enhanced microneme secretion and gliding (Wetzel et al., 2004Wetzel D.M. Chen L.A. Ruiz F.A. Moreno S.N.J. Sibley L.D. Calcium-mediated protein secretion potentiates motility by Toxoplasma gondii.J. Cell Sci. 2004; 117: 5739-5748Crossref PubMed Scopus (87) Google Scholar). Other compounds, such as thapsigargin and artemisinin, oppose this pattern by increasing the amplitude but decreasing the frequency (Nagamune et al., 2007Nagamune K. Beatty W.L. Sibley L.D. Artemisinin induces calcium-dependent secretion in Toxoplasma gondii.Eukaryot. Cell. 2007; 6: 2147-2156Crossref PubMed Scopus (86) Google Scholar). Hence, the timing of release and spatial pattern of calcium may control activation of downstream effectors important in mediating secretion and/or activating the motor complex. Cytoplasmic calcium in T. gondii controls microneme secretion as shown by pharmacological agents such as ionophores that raise calcium and stimulate secretion of micronemal proteins; conversely, chelation of intracellular calcium using BAPTA-AM (0,0′-Bis [2-aminophenyl] ethyleneglycol-N,N,N′,N′-tetraacetic acid, acetoxymethyl ester) prevents microneme secretion and disrupts motility and cell invasion (Carruthers et al., 1999Carruthers V.B. Giddings O.K. Sibley L.D. Secretion of micronemal proteins is associated with Toxoplasma invasion of host cells.Cell. Microbiol. 1999; 1: 225-236Crossref PubMed Scopus (260) Google Scholar, Carruthers and Sibley, 1999Carruthers V.B. Sibley L.D. Mobilization of intracellular calcium stimulates microneme discharge in Toxoplasma gondii.Mol. Microbiol. 1999; 31: 421-428Crossref PubMed Scopus (274) Google Scholar). While not as amenable to experimental analyses, studies suggest a similar requirement for calcium in the control of microneme discharge from sporozoites of Cryptosporidium parvum (Chen et al., 2004Chen X.M. O'Hara S.P. Huang B.Q. Nelson J.B. Lin J.J.C. Zhu G. Ward H.D. LaRusso N.F. Apical organelle discharge by Cryptosporidium parvum is temperature, cytoskeleton, and intracellular calcium dependent and required for host cell invasion.Infect. Immun. 2004; 72: 6806-6816Crossref PubMed Scopus (59) Google Scholar), Eimeria tenella (Wiersma et al., 2004Wiersma H.I. Galuska S.E. Tomley F.M. Sibley L.D. Liberator P.A. Donald R.G.K. A role for coccidian cGMP-dependent protein kinase in motility and invasion.Int. J. Parasitol. 2004; 34: 369-380Crossref PubMed Scopus (110) Google Scholar), and P. berghei (Gantt et al., 2000Gantt S. Persson C. Rose K. Birkett A.J. Abagyan B. Nussenzweig V. Antibodies against thrombospondin-related anonymous protein do not inhibit Plasmodium sporozoite infectivity in vivo.Infect. Immun. 2000; 68: 3667-3673Crossref PubMed Scopus (84) Google Scholar). Biochemical studies in T. gondii reveal the presence of both cyclase and hydrolase activities involved in production of the second messenger cyclic ADP ribose (cADPR) (Chini et al., 2005Chini E.N. Nagamune K. Wetzel D.M. Sibley L.D. Evidence that the cADPR signaling pathway controls calcium-mediated secretion in Toxoplasma gondii.Biochem. J. 2005; 389: 269-277Crossref PubMed Scopus (48) Google Scholar). A separate pathway generates inositol triphosphate (IP3), presumably through phospholipase C (Lovett et al., 2002Lovett J.L. Marchesini N. Moreno S.N. Sibley L.D. Toxoplasma gondii microneme secretion involves intracellular Ca2+ release from IP3 / ryanodine sensitive stores.J. Biol. Chem. 2002; 277: 25870-25876Crossref PubMed Scopus (124) Google Scholar), which has been characterized in vitro using recombinant protien (Fang et al., 2006Fang J. Marchesini N. Moreno S.N.J. A Toxoplasma gondii phosphoinositde phospholipase C (TgPI-PLC) with high affinity for phosphatidylinositol.Biochem. J. 2006; 394: 417-425Crossref PubMed Scopus (35) Google Scholar). Pharmacological evidence also indicates the presence of calcium release channels that respond to IP3 and cADPR (Lovett et al., 2002Lovett J.L. Marchesini N. Moreno S.N. Sibley L.D. Toxoplasma gondii microneme secretion involves intracellular Ca2+ release from IP3 / ryanodine sensitive stores.J. Biol. Chem. 2002; 277: 25870-25876Crossref PubMed Scopus (124) Google Scholar) (Figure 2). Similar to plant cells, the molecular basis of these calcium-release channels has not been defined and the genomes of apicomplexans do not contain obvious orthologs of the IP3 or ryanodine-responsive channels that have been characterized in mammalian cells (Nagamune et al., 2008bNagamune K. Moreno S.N. Chini E.N. Sibley L.D. Calcium regulation and signaling in apicomplexan parasites.Subcell. Biochem. 2008; 47: 70-81Crossref PubMed Scopus (88) Google Scholar). Additionally, the receptors that sense environmental changes to generate IP3 or cADPR have not been identified, despite biochemical and pharmacological evidence that both second messengers are required for motility and invasion (Lovett et al., 2002Lovett J.L. Marchesini N. Moreno S.N. Sibley L.D. Toxoplasma gondii microneme secretion involves intracellular Ca2+ release from IP3 / ryanodine sensitive stores.J. Biol. Chem. 2002; 277: 25870-25876Crossref PubMed Scopus (124) Google Scholar) (Figure 2). Elevated calcium also participates in activating egress of T. gondii from the cell, a process that depends on microneme secretion and active parasite motility (Figure 1). Artificial elevation of calcium using calcium ionophore triggers egress, a process that also relies on active motility (Endo et al., 1982Endo T. Sethi K.K. Piekarski G. Toxoplasma gondii: calcium ionophore A23187-mediated exit of trophozoites from infected murine macrophages.Exp. Parasitol. 1982; 53: 179-188Crossref PubMed Scopus (110) Google Scholar). Treatment with calcium ionophore will trigger egress of T. gondii throughout the process of intracellular development, perhaps due to its unusual mode of binary cell division (Morrissette and Sibley, 2002Morrissette N.S. Sibley L.D. Cytoskeleton of apicomplexan parasites.Microbiol. Mol. Biol. Rev. 2002; 66: 21-38Crossref PubMed Scopus (305) Google Scholar), which leaves the parasite ready to reinvade throughout the entire cell cycle. Recent studies indicate that a plant-like pathway for controlling cADPR production through abscisic acid (ABA) controls this pathway in T. gondii (Nagamune et al., 2008aNagamune K. Hicks L.M. Fux B. Broissier F. Chini E.N. Sibley L.D. Abscisic acid controls calcium-dependent egress and development in Toxoplasma gondii.Nature. 2008; 451: 207-211Crossref PubMed Scopus (170) Google Scholar). This process is similar to the role of ABA in controlling cADPR production and elevated calcium in guard cells in plants (Wu et al., 1997Wu Y. Kuzma J. Marechal E. Graeff R. Lee H.C. Foster R. Chua N.H. Abscisic acid signaling through cyclic ADP ribose in plants.Science. 1997; 278: 2126-2130Crossref PubMed Scopus (314) Google Scholar). In T. gondii, the signaling pathway resembles quorum sensing in that ABA accumulates during intracellular replication, ultimately triggering increases in cADPR and resulting in elevated calcium that stimulates cell egress (Nagamune et al., 2008aNagamune K. Hicks L.M. Fux B. Broissier F. Chini E.N. Sibley L.D. Abscisic acid controls calcium-dependent egress and development in Toxoplasma gondii.Nature. 2008; 451: 207-211Crossref PubMed Scopus (170) Google Scholar). ABA is derived from the isoprenoid pathway, presumably present in apicomplexans due to their ancestral acquisition of a secondary endosymbiont of algal origin (Ralph et al., 2004Ralph S.A. van Dooren G.G. Waller R.F. Crawford M.J. Fraunholz M. Foth B.J. Tonkin C.J. Roos D. McFadden G.I. Metabolic maps and functions of the Plasmodium falciparum apicoplast.Nature. 2004; 2: 203-216Google Scholar). Blockade of this pathway with the plant herbicide fluoridone (FLU) prevents cell egress and results in a developmental switch to slow-growing tissue cysts (Figure 1). Egress also requires microneme secretion, in part due to release of a perforin-like protein called TgPLP1 that participates in lysis of the parasite-containing vacuole (Kafsack et al., 2009Kafsack B.F. Pena J.D. Coppens I. Ravindran S. Boothroyd J.C. Carruthers V.B. Rapid membrane disruption by a perforin-like protein facilitates parasite exit
Referência(s)