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 Ca²⁺ and cGMP control microneme secretion, little is known about how these pathways are naturally activated. Here we have developed genetically encoded indicators for Ca²⁺ 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 Ca²⁺ and yet it was augmented by artificial agonists that raise Ca²⁺, such as ethanol. Furthermore, although ethanol elevated intracellular Ca²⁺, 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 Ca²⁺ 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 Ca²⁺.