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A positive feedback loop mediates crosstalk between calcium, cyclic nucleotide and lipid signaling in Toxoplasma gondii.

ABSTRACT: Fundamental processes of obligate intracellular parasites, such as Plasmodium falciparum and Toxoplasma gondii are controlled by a set of plant-like calcium dependent kinases (CDPKs), the conserved cAMP- and cGMP-dependent protein kinases (PKA and PKG), second messengers and lipid signalling. While some major components of the signalling networks have been identified, how these are connected is largely not known. Here, we compare the phospho-signalling networks during Toxoplasma egress from its host cell by artificially raising cGMP or calcium levels to activate PKG or CDPKs, respectively. We show that both these inducers trigger near identical signalling pathways and provide evidence for a feedback loop involving CDPK3. We measure phospho- and lipid signalling in parasites treated with the Ca2+ ionophore A23187 in a sub-minute timecourse and show CDPK3-dependent regulation of diacylglycerol levels and increased phosphorylation of four phosphodiesterases (PDEs), suggesting their function in the feedback loop. Disruption of CDPK3 leads to elevated cAMP levels and inhibition of PKA signalling rescues the egress defect of ΔCDPK3 parasites treated with A23187. Biochemical analysis of the four PDEs identifies PDE2 as the only cAMP-specific PDE among these candidates while the other PDEs are cGMP specific; two of which are inhibited by the predicted PDE inhibitor BIPPO. Conditional deletion of the 4 PDEs supports an important, but non-essential role of PDE1 and PDE2 for growth, while only the latter plays a role in controlling A23187-mediated egress. In summary, we uncover a positive feedback loop that potentiates signalling during egress and links several signalling pathways together.

INSTRUMENT(S): Orbitrap Fusion Lumos

ORGANISM(S): Homo Sapiens (human) Toxoplasma Gondii Rh

TISSUE(S): Epithelial Cell, Foreskin Fibroblast Cell Line, Cell Culture

DISEASE(S): Toxoplasmosis

SUBMITTER: Malgorzata Broncel

LAB HEAD: Moritz Treeck

PROVIDER: PXD030057 | Pride | 2022-10-17

REPOSITORIES: Pride

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Project description:Acrodysostosis represents a group of rare genetic disorders characterized by defective skeletal development and is often accompanied by intellectual disabilities. Mutations in the 3’5’cyclic AMP (cAMP)-dependent Protein Kinase (PKA) type I regulatory subunit isoform α (RIα) and phosphodiesterase (PDE) PDE4D have both been implicated in impaired PKA regulation in Acrodysostosis. How mutations on PDEs and RIα interfere with regulation of cAMP-PKA signaling is not understood. cAMP-PKA signaling can be described in two phases. In the activation phase, cAMP binding to RIα dissociates the free C-subunit. PDEs hydrolyze cAMP bound to RIα, priming the cAMP-free RIα for reassociation with the C-subunit, thereby completing one PKA activation cycle. Signal termination is thus critical for resetting PKA to its basal state and promoting adaptation to hormonal hyperstimulation. This proceeds through formation of a transient signal termination RIα:PDE complex that facilitates cAMP channeling from the cAMP-binding domain of RIα to the catalytic site of PDE. Signal termination of cAMP-PKA proceeds in three steps: Step 1) Channeling: Translocation of cAMP from the CNB of RIα to the PDE catalytic site for hydrolysis Step 2) Processivity: Binding of free cAMP from the cytosol at both CNBs of RIα and Step 3) Product (5’AMP) release from the PDE hydrolysis site through competitive displacement by a new molecule of cAMP that trigger subsequent activation cycles of PKA. We have identified the molecular basis for two acrodysostosis mutants, (PDE8 T690P) and RIα (T207A) that both allosterically impair cAMP-PKA signal termination. A combination of amide hydrogen/deuterium exchange mass spectrometry (HDXMS) and Fluorescence Polarization (FP) reveal that PDE8 T690P and RIα T207A both blocked processive hydrolysis of cAMP by interfering with competitive displacement of product 5’AMP release from the nucleotide channel at the end of each round of cAMP hydrolysis. While T690P blocked product 5’AMP release from the PDE, T207A greatly slowed release of substrate from RIα. These results highlight the role of processivity in cAMP hydrolysis by RIα:PDE termination complexes for adaptation to cAMP from GPCR hyperstimulation. Impairment of the signal termination process provides an alternate molecular basis for acrodysostosis.

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A positive feedback loop mediates crosstalk between calcium, cyclic nucleotide and lipid signaling in Toxoplasma gondii.

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