Toxoplasma gondii is an obligate intracellular protozoan parasite that invades and replicates within most nucleated cells of warm-blooded animals. The basis for this wide host cell tropism is unknown but could be because parasites invade host cells using distinct pathways and/or repertoires of host factors. Using synchronized parasite invasion assays, we found that host microtubule disruption significantly reduces parasite invasion into host cells early after stimulating parasite invasion but not at later time points. Host microtubules are specifically associated with the moving junction, which is the site of contact between the host cell and the invading parasite. Host microtubules are specifically associated with the moving junction of those parasites invading early after stimulating invasion but not with those invading later. Disruption of host microtubules has no effect on parasite contact, attachment, motility, or rate of penetration. Rather, host microtubules hasten the time before parasites commence invasion. This effect on parasite invasion is distinct from the role that host microtubules play in bacterial and viral infections, where they function to traffic the pathogen or pathogenderived material from the host cell's periphery to its interior. These data indicate that the host microtubule cytoskeleton is a structure used by Toxoplasma to rapidly infect its host cell and highlight a novel function for host microtubules in microbial pathogenesis.Toxoplasma gondii is an obligate intracellular protozoan parasite that is capable of causing disease in fetuses and immunocompromised individuals (23). The parasite infects a wide range of nucleated cells of most warm-blooded animals. The mechanisms underlying this wide tropism are not known but could be due to either the parasite infecting cells using a ubiquitously expressed host receptor and associated machinery, inserting its own receptor into the host cell's plasma membrane, or using multiple host cell receptors/machinery (5).Toxoplasma invasion is a multistep, complex process consisting of parasite contact to host cells, intimate attachment, parasite motility, and then penetration (5). Host cell contact is a loose, low-affinity interaction that is mediated by parasite surface antigens. An unknown signal then triggers the release of proteins from a specialized secretory organelle called micronemes whose contents include proteins that function as adhesins. This is then followed by parasite gliding motility on the host cell surface. At some point, proteins from a second secretory organelle, named rhoptries, are exocytosed. Among these rhoptry proteins, several (RON2, RON4, RON5, and RON8) are part of a preformed complex that binds the previously secreted AMA1 microneme protein (1,2,20,33). Together, these proteins form the moving junction complex, which defines the parasite entry site on the host cell plasma membrane. Parasite penetration occurs by the parasite propel-

Toxoplasma gondii is an intracellular protozoan parasite that can cause devastating disease in fetuses and immune-compromised individuals. We previously reported that the ␣ subunit of the host cell transcription factor, hypoxia-inducible factor-1 (HIF-1), is up-regulated by infection and necessary for Toxoplasma growth. Under basal conditions, HIF-1␣ is constitutively expressed but rapidly targeted for proteasomal degradation after two proline residues are hydroxylated by a family of prolyl hydroxylases (PHDs). The PHDs are ␣-ketoglutarate-dependent dioxygenases that have low K m values for oxygen, making them important cellular oxygen sensors. Thus, when oxygen levels decrease, HIF-1␣ is not hydroxylated, and HIF-1 is activated. How Toxoplasma activates HIF-1 under normoxic conditions remains unknown. Here, we report that Toxoplasma infection increases HIF-1␣ stability by preventing HIF-1␣ prolyl hydroxylation. Infection significantly decreases PHD2 abundance, which is the key prolyl hydroxylase for regulating HIF-1␣. The effects of Toxoplasma on HIF-1␣ abundance and prolyl hydroxylase activity require activin-like receptor kinase signaling. Finally, parasite growth is severely diminished when signaling from this family of receptors is inhibited. Together, these data indicate that PHD2 is a key host cell factor for T. gondii growth and represent a novel mechanism by which a microbial pathogen subverts host cell signaling and transcription to establish its replicative niche.Toxoplasma gondii is an obligate intracellular protozoan parasite that can cause devastating disease in immune-compromised people and fetuses (1, 2). The ability for the parasite to cause disease is directly linked to its replication inside a parasitophorous vacuole within its host cell. From this vacuole, parasites scavenge nutrients from its host cell while causing reorganization of host organelles and cytoskeletal elements, preventing host cell apoptosis, and altering host gene expression (3-5). Therefore, Toxoplasma has developed mechanisms to manipulate host cell processes that permit it to grow. Identifying the parasite factors that cause these changes and the specific host pathways modulated by these factors is critical for developing new drugs to treat and prevent Toxoplasma infections and disease.Previously, our laboratory demonstrated that Toxoplasma activates the host cell transcription factor hypoxia-inducible factor-1 (HIF-1) 3 (6). HIF-1 is activated by a parasite-derived, secreted factor independent of direct contact between the parasite and host cell. This is in contrast to other Toxoplasma factors that control host cell signaling by virtue of their secretion into the host cell cytoplasm (7-10). Significantly, HIF-1 is important for parasite growth at both normoxic and physiological O 2 levels but dispensable for host cell replication at either O 2 levels (6).HIF-1 is a heterodimer composed of ␣ and ␤ subunits and is the master regulator of the cellular response to decreased O 2 availability (11). Although both subunits are co...

Toxoplasma gondii is a ubiquitous apicomplexan parasite that can cause severe disease in fetuses and immune-compromised patients. Rhoptries, micronemes, and dense granules, which are secretory organelles unique to Toxoplasma and other apicomplexan parasites, play critical roles in parasite growth and virulence. To understand how these organelles modulate infected host cells, we sought to identify host cell transcription factors triggered by their release. Early growth response 2 (EGR2) is a host cell transcription factor that is rapidly upregulated and activated in Toxoplasma-infected cells but not in cells infected with the closely related apicomplexan parasite Neospora caninum. EGR2 upregulation occurred only when live parasites were in direct contact with the host cell and not from exposure to cell extracts that contain dense granule or micronemal proteins. When microneme-mediated attachment was blocked by pretreating parasites with a calcium chelator, EGR2 expression was significantly reduced. In contrast, when host cells were infected with parasites in the presence of cytochalasin D, which allows rhoptry secretion but prevents parasite invasion, EGR2 was activated. Finally, we demonstrate that Toxoplasma activation of host p38 mitogen-activated protein kinase is necessary but not sufficient for EGR2 activation. Collectively, these data indicate that EGR2 is specifically upregulated by a parasite-derived secreted factor that is most likely a resident rhoptry protein.

In Dictyostelium, sporulation occurs synchronously as prespore cells approach the apex of the aerial stalk during culmination. Each prespore cell becomes surrounded by its own coat comprised of a core of crystalline cellulose and a branched heteropolysaccharide sandwiched between heterogeneous cysteine-rich glycoproteins. The function of the heteropolysaccharide, which consists of galactose and N-acetylgalactosamine, is unknown. Two glycosyltransferase-like genes encoding multifunctional proteins, each with predicted features of a heteropolysaccharide synthase, were identified in the Dictyostelium discoideum genome. pgtB and pgtC transcripts were modestly upregulated during early development, and pgtB was further intensely upregulated at the time of heteropolysaccharide accumulation. Disruption of either gene reduced synthase-like activity and blocked heteropolysaccharide formation, based on loss of cytological labeling with a lectin and absence of component sugars after acid hydrolysis. Cell mixing experiments showed that heteropolysaccharide expression is spore cell autonomous, suggesting a physical association with other coat molecules during assembly. Mutant coats expressed reduced levels of crystalline cellulose based on chemical analysis after acid degradation, and cellulose was heterogeneously affected based on flow cytometry and electron microscopy. Mutant coats also contained elevated levels of selected coat proteins but not others and were sensitive to shear. Mutant spores were unusually susceptible to hypertonic collapse and damage by detergent or hypertonic stress. Thus, the heteropolysaccharide is essential for spore integrity, which can be explained by a role in the formation of crystalline cellulose and regulation of the protein content of the coat.In Dictyostelium, spores are the only surviving cell type produced by starvation-induced multicellular development. During this process, solitary amoebae aggregate to form a migratory slug composed of prespore and prestalk cells, which then culminate to form a fruiting body consisting of spores perched on top of a 1 to 2 mm tall cellular stalk. As each prespore cell becomes a spore, it dehydrates, accumulates trehalose, and encloses itself in a specialized cell wall (37). This physicochemical barrier protects the enclosed amoeba from external stress and probably actively regulates terminal sporulation and spore germination. The spore coat is formed de novo from four known sources: (i) an early-formed pool of proteins and (ii) a galactose-rich polysaccharide (GPS) stored together in prespore vesicles (PSVs) of the slug, (iii) a late-formed pool of protein(s), including SP65 (23), and (iv) cellulose formed de novo at the cell surface. These components are separately deposited at the cell surface, where they organize into an asymmetrical trilaminar "sandwich" with proteins on either side enclosing the polysaccharides in the interior (see Fig. 5B).Cellulose is the primary structural component of the middle layer and is required for organization of the prote...