Apicomplexan parasites include a number of pathogens that cause a wide array of human disease such as malaria, toxoplasmosis, and cryptosporidiosis. Together, these parasites account for nearly 400 million infections each year and almost 500,000 deaths. The pathogenesis of the infection with these parasites is reliant on their capacity to replicate within their host cells. However, the molecular mechanisms and signals that sustain their growth within host cells remain unclear. Our laboratory found evidence of the presence and essentiality of the inositol pyrophosphates (IPPs) or diphosphoinositol polyphosphates (PP-IPs) signaling pathway in Toxoplasma gondii. These peculiar molecules are characterized by a high-energy phosphoanhydride bonds (pyro or diphospho) and their participation in diverse nuclear and cytoplasmic functions. The synthesis pathway for PP-IPs starts with the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by a phosphoinositide phospholipase C (PI-PLC) to inositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DAG), both important second messengers. IP3 can be further metabolized to other inositol polyphosphates by kinases and phosphatases to form inositol tetrakisphosphate (IP4), inositol pentakisphosphate (IP5), inositol hexakisphosphate (IP6) and pyrophosphorylated derivatives like PP-IPs, also known as inositol pyrophosphates. These molecules possess one or more high-energy pyrophosphate moiety like inositol heptakisphosphate (IP7) which can be further phosphorylated to inositol octakisphosphate (IP8). In T. gondii there are orthologues for four kinases that likely catalyze the conversion of IP3 to IP8, and there is evidence that three of the biosynthetic enzymes, PI-PLC, inositol phosphate multikinase or IPMK and IP6 kinase are essential. PP-IPs transduce signals by interacting with proteins and modifying their function. One mechanism involves the transference of a b-phosphoryl group from PP-IPs into a phosphorylated serine surrounded by acidic residues. This posttranslational modification was termed protein pyrophosphorylation and is Mg2+-dependent and non-enzymatic. PP-IPs can also alter a target protein allosterically through interaction with specific domains such as pleckstrin homology (PH) or SPX domains. SPX domains are present in the Vacuolar Transporter Chaperone (VTCs) complex of yeasts, which synthesizes polyphosphate (polyP). PP-IPs regulate the synthesis of polyP through their association with SPX domains. T. gondii possesses orthologues of the VTC complex as well as the phosphate transporter XPR1 and these proteins possess SPX domains. Our preliminary data show that in T. gondii both the synthesis of polyPs and PP-IPs is essential and we propose to focus on the characterization of the PP-IPs pathway in T. gondii, an untapped subject, which very likely impact the activation and generation of molecules that are part of the pathways involved in the pathogenicity (lytic cycle) and life cycle decisions of the parasite.
Millions of people are infected with Toxoplasma gondii and we need to understand how it causes disease. We discovered that the inositol pyrophosphate synthesis pathway is essential for the parasite survival. Our hypothesis is that this pathway generates signaling molecules that are key for the pathogenesis of Toxoplasma.
