abscisic-acid and Toxoplasmosis

The protozoan Toxoplasma gondii (T. gondii) is a zoonotic disease agent causing systemic infection in warm-blooded intermediate hosts including humans. During the acute infection, the parasite infects host cells and multiplies intracellularly in the asexual tachyzoite stage. In this stage of the life cycle, invasion, multiplication, and egress are the most critical events in parasite replication. T. gondii features diverse cell organelles to support these processes, including the apicoplast, an endosymbiont-derived vestigial plastid originating from an alga ancestor. Previous studies have highlighted that phytohormones can modify the calcium-mediated secretion, e.g., of adhesins involved in parasite movement and cell invasion processes. The present study aimed to elucidate the influence of different plant hormones on the replication of asexual tachyzoites in a human foreskin fibroblast (HFF) host cell culture. T. gondii replication was measured by the determination of T. gondii DNA copies via qPCR. Three selected phytohormones, namely abscisic acid (ABA), gibberellic acid (GIBB), and kinetin (KIN) as representatives of different plant hormone groups were tested. Moreover, the influence of typical cell culture media components on the phytohormone effects was assessed. Our results indicate that ABA is able to induce a significant increase of T. gondii DNA copies in a typical supplemented cell culture medium when applied in concentrations of 20 ng/μl or 2 ng/μl, respectively. In contrast, depending on the culture medium composition, GIBB may potentially serve as T. gondii growth inhibitor and may be further investigated as a potential treatment for toxoplasmosis.

Topics: Abscisic Acid; DNA; Humans; Plant Growth Regulators; Toxoplasma; Toxoplasmosis

Autophagy has been shown to contribute to defense against intracellular bacteria and parasites. In comparison, the ability of such pathogens to manipulate host cell autophagy to their advantage has not been examined. Here we present evidence that infection by Toxoplasma gondii, an intracellular protozoan parasite, induces host cell autophagy in both HeLa cells and primary fibroblasts, via a mechanism dependent on host Atg5 but independent of host mammalian target of rapamycin suppression. Infection led to the conversion of LC3 to the autophagosome-associated form LC3-II, to the accumulation of LC3-containing vesicles near the parasitophorous vacuole, and to the relocalization toward the vacuole of structures labeled by the phosphatidylinositol 3-phosphate indicator YFP-2xFYVE. The autophagy regulator beclin 1 was concentrated in the vicinity of the parasitophorous vacuole in infected cells. Inhibitor studies indicated that parasite-induced autophagy is dependent on calcium signaling and on abscisic acid. At physiologically relevant amino acid levels, parasite growth became defective in Atg5-deficient cells, indicating a role for host cell autophagy in parasite recovery of host cell nutrients. A flow cytometric analysis of cell size as a function of parasite content revealed that autophagy-dependent parasite growth correlates with autophagy-dependent consumption of host cell mass that is dependent on parasite progression. These findings indicate a new role for autophagy as a pathway by which parasites may effectively compete with the host cell for limiting anabolic resources.

Topics: Abscisic Acid; Animals; Apoptosis Regulatory Proteins; Autophagy; Autophagy-Related Protein 5; Beclin-1; Calcium Signaling; Carrier Proteins; HeLa Cells; Humans; Membrane Proteins; Mice; Microtubule-Associated Proteins; Phagosomes; Phosphotransferases (Alcohol Group Acceptor); Proteins; TOR Serine-Threonine Kinases; Toxoplasma; Toxoplasmosis

Calcium controls a number of critical events, including motility, secretion, cell invasion and egress by apicomplexan parasites. Compared to animal and plant cells, the molecular mechanisms that govern calcium signalling in parasites are poorly understood. Here we show that the production of the phytohormone abscisic acid (ABA) controls calcium signalling within the apicomplexan parasite Toxoplasma gondii, an opportunistic human pathogen. In plants, ABA controls a number of important events, including environmental stress responses, embryo development and seed dormancy. ABA induces production of the second-messenger cyclic ADP ribose (cADPR), which controls release of intracellular calcium stores in plants. cADPR also controls intracellular calcium release in the protozoan parasite T. gondii; however, previous studies have not revealed the molecular basis of this pathway. We found that addition of exogenous ABA induced formation of cADPR in T. gondii, stimulated calcium-dependent protein secretion, and induced parasite egress from the infected host cell in a density-dependent manner. Production of endogenous ABA within the parasite was confirmed by purification (using high-performance liquid chromatography) and analysis (by gas chromatography-mass spectrometry). Selective disruption of ABA synthesis by the inhibitor fluridone delayed egress and induced development of the slow-growing, dormant cyst stage of the parasite. Thus, ABA-mediated calcium signalling controls the decision between lytic and chronic stage growth, a developmental switch that is central in pathogenesis and transmission. The pathway for ABA production was probably acquired with an algal endosymbiont that was retained as a non-photosynthetic plastid known as the apicoplast. The plant-like nature of this pathway may be exploited therapeutically, as shown by the ability of a specific inhibitor of ABA synthesis to prevent toxoplasmosis in the mouse model.

Topics: Abscisic Acid; Animals; Calcium; Calcium Signaling; Cyclic ADP-Ribose; Disease Models, Animal; Mice; Mice, Inbred BALB C; Plant Growth Regulators; Protozoan Proteins; Pyridones; Toxoplasma; Toxoplasmosis