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To Migrate or to Invade: Those Are the Options

2007; Cell Press; Volume: 2; Issue: 5 Linguagem: Inglês

10.1016/j.chom.2007.10.008

1934-6069

Miguel Prudêncio, Maria M. Mota,

Toxoplasma gondii Research Studies

Plasmodium sporozoites penetrate and migrate through multiple cells in the host before productively invading a hepatocyte and forming a parasitophorous vacuole. In this issue of Cell Host & Microbe, Coppi and colleagues show that sporozoite interaction with the highly sulfated heparan sulfate proteoglycans on liver cells induces proteolytic cleavage of the major sporozoite surface molecule. They conclude that this interaction is the primary trigger that activates sporozoites for productive invasion. Plasmodium sporozoites penetrate and migrate through multiple cells in the host before productively invading a hepatocyte and forming a parasitophorous vacuole. In this issue of Cell Host & Microbe, Coppi and colleagues show that sporozoite interaction with the highly sulfated heparan sulfate proteoglycans on liver cells induces proteolytic cleavage of the major sporozoite surface molecule. They conclude that this interaction is the primary trigger that activates sporozoites for productive invasion. The liver stage is the first, obligatory step of any natural malaria infection. Plasmodium sporozoites reach the liver after being deposited into the skin of their vertebrate host through the bite of an infected female Anopheles mosquito (Figure 1). After injection into dermal tissue, sporozoites use gliding motility to travel through the skin and into dermal vessels (Prudencio et al., 2006Prudencio M. Rodriguez A. Mota M.M. Nat. Rev. Microbiol. 2006; 4: 849-856Crossref PubMed Scopus (302) Google Scholar). Once inside the circulatory system, sporozoites rapidly reach the liver sinusoids. The marked tropism displayed by sporozoites toward the hepatocyte suggests the occurrence of specific interactions between Plasmodium surface protein(s) and host molecule(s). The highly sulfated heparan sulfate proteoglycans (HSPGs) on liver cells were shown to play a crucial role in these processes through their interactions with the circumsporozoite protein (CSP), the sporozoite's major surface protein (Pinzon-Ortiz et al., 2001Pinzon-Ortiz C. Friedman J. Esko J. Sinnis P. J. Biol. Chem. 2001; 276: 26784-26791Crossref PubMed Scopus (106) Google Scholar, Sinnis and Coppi, 2007Sinnis P. Coppi A. Parasitol. Int. 2007; 56: 171-178Crossref PubMed Scopus (51) Google Scholar). HSPGs are a family of glycoproteins present on the cell surfaces and in the extracellular matrix of most mammalian cells where they function as coreceptors for growth factors and matrix proteins. In this issue of Cell Host & Microbe, Coppi et al., 2007Coppi A. Tewari R. Bishop J.R. Bennett B.L. Lawrence R. Esko J.D. Billker O. Sinnis P. Cell Host & Microbe. 2007; 2 (this issue): 316-327Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar reveal a previously unknown role for these liver HSPGs during Plasmodium infection. During their journey to the liver, sporozoites likely traverse several cell types, namely the dermal cells in the vicinity of their injection site, endothelial cells that line blood vessels, and Kupffer cells in the liver sinusoids (Figure 1). In addition, sporozoites traverse several hepatocytes in the liver before productively invading a final one, with formation of a parasitophorous vacuole (Mota et al., 2001Mota M.M. Pradel G. Vanderberg J.P. Hafalla J.C. Frevert U. Nussenzweig R.S. Nussenzweig V. Rodriguez A. Science. 2001; 291: 141-144Crossref PubMed Scopus (378) Google Scholar). The molecular mechanisms that govern the choice between the sporozoites migratory behavior and “productive invasion” are still not fully understood. Now, Coppi et al., 2007Coppi A. Tewari R. Bishop J.R. Bennett B.L. Lawrence R. Esko J.D. Billker O. Sinnis P. Cell Host & Microbe. 2007; 2 (this issue): 316-327Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar show that the degree of sulfation of HSPGs constitutes a molecular signal for the switch between these parasite behaviors. They demonstrate that processing of CSP by proteolytic cleavage, which is known to occur when sporozoites contact hepatocytes (Coppi et al., 2005Coppi A. Pinzon-Ortiz C. Hutter C. Sinnis P. J. Exp. Med. 2005; 201: 27-33Crossref PubMed Scopus (148) Google Scholar), is induced by highly sulfated HSPGs. Furthermore, they show that the invasion process involves signaling by a calcium-dependent protein kinase, probably acting in concert with other signaling events. Together, the findings of Coppi et al. constitute a valuable contribution to the understanding of the molecular events that take place during malaria liver stage infection. Coppi and colleagues show that inhibition of productive invasion by treatment with an inhibitor of CSP processing leads to a significantly extended sporozoite migration through cells. This observation not only confirms the requirement of CSP cleavage for invasion with parasitophorous vacuole formation but also indicates that sporozoites continue to migrate in the absence of a molecular trigger for productive invasion. Indeed, an inverse correlation was found between sporozoite migration and both CSP proteolytic cleavage and invasion, whereas a positive correlation was shown to exist between the latter two events. The link found between the sulfation degree of HSPGs, productive invasion, and CSP processing is demonstrated in a series of elegantly designed experiments. The authors show that migration is significantly increased by chemical inhibition of sulfation or by incubation of sporozoites with cells that bear a specific deficiency in the degree of sulfation of their heparan sulfate chains, thus convincingly establishing a link between sulfation and productive invasion. Since the latter requires the processing of CSP, it would be natural to assume that CSP cleavage might be dependent upon the sulfation degree of HSPGs. Coppi et al., 2007Coppi A. Tewari R. Bishop J.R. Bennett B.L. Lawrence R. Esko J.D. Billker O. Sinnis P. Cell Host & Microbe. 2007; 2 (this issue): 316-327Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar firmly establish the connection between these two events by demonstrating that, not only soluble heparin but, most importantly, highly sulfated HSPGs on the hepatocyte surface, induce the proteolytic cleavage of CSP and, hence, modulate sporozoite infectivity. When sporozoites interact with the highly sulfated liver HSPGs, it can be assumed that a chain of signaling events follows that leads to CSP cleavage and productive invasion. This assumption is supported by the observation of Coppi et al., 2007Coppi A. Tewari R. Bishop J.R. Bennett B.L. Lawrence R. Esko J.D. Billker O. Sinnis P. Cell Host & Microbe. 2007; 2 (this issue): 316-327Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar that treatment of sporozoites with a protein kinase (PK) inhibitor leads to an increase in migration and to the concomitant inhibition of CSP processing and invasion. In their quest for the PK(s) likely to be involved in this process, the authors identified a Plasmodium gene, which they named CDPK-6, whose product plays a crucial role in sporozoite invasion of hepatocytes. Such a role is confirmed by the observation that the migratory activity of CDPK-6 knockout P. berghei parasites was significantly enhanced whereas their ability to productively invade Hepa 1-6 cells was significantly decreased relative to wild-type parasites. Furthermore, CDPK-6 mutant parasites display a marked decrease in their ability to cleave CSP, further supporting a role for CDPK-6 in invasion. Whether or not there is an absolute requirement for sporozoite migration through hepatocytes prior to productive invasion is, at present, not entirely clear. It has been shown that migration leads to the release of hepatocyte growth factor (HGF), which acts as a mediator of signals that make the host cell more permissive to development of infection (Carrolo et al., 2003Carrolo M. Giordano S. Cabrita-Santos L. Corso S. Vigario A.M. Silva S. Leiriao P. Carapau D. Armas-Portela R. Comoglio P.M. et al.Nat. Med. 2003; 9: 1363-1369Crossref PubMed Scopus (116) Google Scholar). Another study showed that the treatment of sporozoites with ionomycin, a Ca2+ ionophore that stimulates the secretion of apical organelles, leads to enhanced infectivity and inhibition of migration. Also, incubation of sporozoites with lysates of hepatocytes had a similar effect, suggesting that host cells carry molecules that can activate sporozoites for infection (Mota et al., 2002Mota M.M. Hafalla J.C. Rodriguez A. Nat. Med. 2002; 8: 1318-1322Crossref PubMed Scopus (149) Google Scholar). More recently, it has been shown that exposure of sporozoites to the intracellular K+ concentration enhances sporozoite infectivity, while decreasing cell migration activity (Kumar et al., 2007Kumar K.A. Garcia C.R. Chandran V.R. Van Rooijen N. Zhou Y. Winzeler E. Nussenzweig V. Mol. Biochem. Parasitol. 2007; 156: 32-40Crossref PubMed Scopus (49) Google Scholar). However, Ishino et al., 2004Ishino T. Yano K. Chinzei Y. Yuda M. PLoS Biol. 2004; 2: e4https://doi.org/10.1371/journal.pbio.0020004Crossref PubMed Scopus (187) Google Scholar demonstrated that parasites deficient in a protein named “sporozoite protein essential for cell traversal” (SPECT) do not migrate through cells and yet retain their ability to accomplish infection in vitro, thus questioning the requirement of migration for completion of infection. By showing that a high degree of HSPG sulfation promotes the switch to an invasive phenotype, Coppi et al., 2007Coppi A. Tewari R. Bishop J.R. Bennett B.L. Lawrence R. Esko J.D. Billker O. Sinnis P. Cell Host & Microbe. 2007; 2 (this issue): 316-327Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar have identified what seems to be the primary trigger to initiate sporozoite activation for productive invasion. However, as these authors point out, despite the fact that hepatocytes contain highly sulfated HSPGs, sporozoites do migrate through several of them before productively invading a final one, suggesting that activation is likely a continuous process that involves the contributions of multiple signaling factors. The observations of Coppi et al., 2007Coppi A. Tewari R. Bishop J.R. Bennett B.L. Lawrence R. Esko J.D. Billker O. Sinnis P. Cell Host & Microbe. 2007; 2 (this issue): 316-327Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar, however, do not fully resolve the apparent discrepancy between the various observations of increased infectivity as a result of cell migration and the spect mutant phenotype. One possible explanation for the spect mutants' ability to invade without prior migration through cells is that, rather than being unable to traverse cells, they might be, “by default,” activated for invasion. Whichever the case, it seems clear that, under physiological conditions, wild-type sporozoites do migrate through several hepatocytes before productively invading one. Whether this reflects the need for additional sporozoite activation signals or a delay in the signaling events induced by highly sulfated HSPGs remains to be fully elucidated. The liver stage of Plasmodium infection is clearly the least understood of the two stages that comprise the parasite's life cycle within its vertebrate host. This is somehow paradoxical if we consider its potential as a target for prophylactic antimalarial intervention. One of the reasons for the current lack of knowledge is the complexity of the molecular events that take place in the infected liver. Furthermore, it is becoming increasingly apparent that the vertebrate host does not play a passive role in the infection process and that, on the contrary, it actively influences the fate of infection. In this context, the paper by Coppi et al., 2007Coppi A. Tewari R. Bishop J.R. Bennett B.L. Lawrence R. Esko J.D. Billker O. Sinnis P. Cell Host & Microbe. 2007; 2 (this issue): 316-327Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar represents a crucial piece of this immense jigsaw.