title: Serum Albumin Stimulates Protein Kinase G-dependent Microneme Secretion in Toxoplasma gondii.
abstract: Microneme secretion is essential for motility, invasion, and egress in apicomplexan parasites. Although previous studies indicate that Ca(2+) and cGMP control microneme secretion, little is known about how these pathways are naturally activated. Here we have developed genetically encoded indicators for Ca(2+) and microneme secretion to better define the signaling pathways that regulate these processes in Toxoplasma gondii We found that microneme secretion was triggered in vitro by exposure to a single host protein, serum albumin. The natural agonist serum albumin induced microneme secretion in a protein kinase G-dependent manner that correlated with increased cGMP levels. Surprisingly, serum albumin acted independently of elevated Ca(2+) and yet it was augmented by artificial agonists that raise Ca(2+), such as ethanol. Furthermore, although ethanol elevated intracellular Ca(2+), it alone was unable to trigger secretion without the presence of serum or serum albumin. This dichotomy was recapitulated by zaprinast, a phosphodiesterase inhibitor that elevated cGMP and separately increased Ca(2+) in a protein kinase G-independent manner leading to microneme secretion. Taken together, these findings reveal that microneme secretion is centrally controlled by protein kinase G and that this pathway is further augmented by elevation of intracellular Ca(2.)
authors.affiliation: From the Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Brown
authors.affiliation: From the Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Lourido
authors.affiliation: From the Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri 63110 sibley@wusm.wustl.edu.
authors.forename: L David
authors.initials: LD
authors.lastname: Sibley
mesh_terms: D020013:Calcium Signaling / Q000187:drug effects* / Q000235:genetics; D006152:Cyclic GMP / Q000235:genetics / Q000378:metabolism*; D017869:Cyclic GMP-Dependent Protein Kinases / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D015800:Protozoan Proteins / Q000235:genetics / Q000378:metabolism*; D011688:Purinones / Q000494:pharmacology; D012709:Serum Albumin / Q000494:pharmacology*; D014122:Toxoplasma / Q000235:genetics / Q000378:metabolism*
chemical_list: D015800:Protozoan Proteins; D011688:Purinones; D012709:Serum Albumin; D017869:Cyclic GMP-Dependent Protein Kinases; C011145:zaprinast; D006152:Cyclic GMP
keywords: calcium; calcium imaging; calcium intracellular release; cyclic GMP (cGMP); protein kinase G (PKG); protein secretion; second messenger