Signaling by inositol pyrophosphates in Toxoplasma gondii (RESUBMISSION) - Abstract
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.
