The triumvirate of signaling molecules controlling Toxoplasma microneme exocytosis: Cyclic GMP, calcium, and phosphatidic acid

Abstract

To elicit effective invasion and egress from infected cells, obligate intracellular parasites of the phylum Apicomplexa rely on the timely and spatially controlled exocytosis of specialized secretory organelles termed the micronemes. The effector molecules and signaling events underpinning this process are intricate; however, recent advances within the field of Toxoplasma gondii research have facilitated a broader understanding as well as a more integrated view of this complex cascade of events and have unraveled the importance of phosphatidic acid (PA) as a lipid mediator at multiple steps in this process.

Fig 1. Schematic of the signaling cascade underpinning cGMP, calcium, and PA generation at the parasite pellicle.

Activation of GC at the parasite plasma membrane in response to DGK2 activation and subsequent PA production promotes the formation of cGMP. cGMP serves to activate PKG, which in turn promotes the formation of PI-PLC substrates. cGMP production is regulated by PDE, which is regulated by the activity of the PKAc1. PKAc1 is itself regulated by PKA regulatory domain, which binds AC-generated cAMP. PI-PLC converts PI(_4,5)P2 to IP3 and DAG. IP3 is thought to stimulate the release of calcium, likely from ER stores, whereas DAG is converted by DGK1 into PA. PA bound by APH facilitates DOC2.1-mediated fusing of the micronemes to the parasite surface and thus their exocytosis. AC, adenylate cyclase; APH, acylated pleckstrin homology domain–containing protein; BIPPO, 5-benzyl-3-isopropyl-1H-pyrazolo[4,3-d]pyrimidin-7(6H)-one; C1, compound 1; cAMP, cyclic adenosine monophosphate; CDC50.1, cell division control protein 50.1; CDPK, calcium-dependent protein kinase; cGMP, cyclic guanosine monophosphate; DAG, diacylglycerol; DGK, DAG kinase; ER, endoplasmic reticulum; DOC2.1, double C2 domain–containing protein 1; GC, guanylate cyclase; GTP, guanosine triphosphate; IP3, inositol triphosphate; PA, phosphatidic acid; PAP, PA phosphatase; PDE, phosphodiesterase; PI, phosphoinositol; PI(_4,5)P2, phosphatidylinositol 4,5-bisphosphate; PI4K, phosphatidylinositol 4-kinase; PI₄P, phosphatidylinositol 4-phosphate; PI4P5K, phosphatidylinositol 4-phosphate 5-kinase; PI-PLC, phosphoinositide-phospholipase C; PKAc1, protein kinase A catalytic 1 domain; PKAr, PKA regulatory subunit; PKG, protein kinase G; UGO, unique GC organizer.