A Toxoplasma gondii patatin-like phospholipase contributes to host cell invasion

Disruption of a Plasmodium falciparum patatin-like phospholipase delays male gametocyte exflagellation

An essential process in transmission of the malaria parasite to the Anopheles vector is the conversion of mature gametocytes into gametes within the mosquito gut, where they egress from the red blood cell (RBC). During egress, male gametocytes undergo exflagellation, leading to the formation of eight haploid motile microgametes, while female gametes retain their spherical shape. Gametocyte egress depends on sequential disruption of the parasitophorous vacuole membrane and the host cell membrane. In other life cycle stages of the malaria parasite, phospholipases have been implicated in membrane disruption processes during egress, however their importance for gametocyte egress is relatively unknown. Here, we performed comprehensive functional analyses of six putative phospholipases for their role during development and egress of Plasmodium falciparum gametocytes. We localize two of them, the prodrug activation and resistance esterase (PF3D7_0709700) and the lysophospholipase 1 (PF3D7_1476700), to the parasite plasma membrane. Subsequently, we show that disruption of most of the studied phospholipase genes does neither affect gametocyte development nor egress. The exception is the putative patatin-like phospholipase 3 (PF3D7_0924000), whose gene deletion leads to a delay in male gametocyte exflagellation, indicating an important, albeit not essential, role of this enzyme in male gametogenesis.

Analysis of CDPK1 targets identifies a trafficking adaptor complex that regulates microneme exocytosis in Toxoplasma

Apicomplexan parasites use Ca2+-regulated exocytosis to secrete essential virulence factors from specialized organelles called micronemes. Ca2+-dependent protein kinases (CDPKs) are required for microneme exocytosis; however, the molecular events that regulate trafficking and fusion of micronemes with the plasma membrane remain unresolved. Here, we combine sub-minute resolution phosphoproteomics and bio-orthogonal labeling of kinase substrates in Toxoplasma gondii to identify 163 proteins phosphorylated in a CDPK1-dependent manner. In addition to known regulators of secretion, we identify uncharacterized targets with predicted functions across signaling, gene expression, trafficking, metabolism, and ion homeostasis. One of the CDPK1 targets is a putative HOOK activating adaptor. In other eukaryotes, HOOK homologs form the FHF complex with FTS and FHIP to activate dynein-mediated trafficking of endosomes along microtubules. We show the FHF complex is partially conserved in T. gondii, consisting of HOOK, an FTS homolog, and two parasite-specific proteins (TGGT1_306920 and TGGT1_316650). CDPK1 kinase activity and HOOK are required for the rapid apical trafficking of micronemes as parasites initiate motility. Moreover, parasites lacking HOOK or FTS display impaired microneme protein secretion, leading to a block in the invasion of host cells. Taken together, our work provides a comprehensive catalog of CDPK1 targets and reveals how vesicular trafficking has been tuned to support a parasitic lifestyle.

Phospholipases A and Lysophospholipases in protozoan parasites

Phospholipases (PLs) and Lysophospholipases (LysoPLs) are a diverse group of esterases responsible for phospholipid or lysophospholipid hydrolysis. They are involved in several biological processes, including lipid catabolism, modulation of the immune response and membrane maintenance. PLs are classified depending on their site of hydrolysis as PLA1, PLA2, PLC and PLD. In many pathogenic microorganisms, from bacteria to fungi, PLAs and LysoPLs have been described as critical virulence and/or pathogenicity factors. In protozoan parasites, a group containing major human and animal pathogens, growing literature show that PLAs and LysoPLs are also involved in the host infection. Their ubiquitous presence and role in host-pathogen interactions make them particularly interesting to study. In this review, we summarize the literature on PLAs and LysoPLs in several protozoan parasites of medical relevance, and discuss the growing interest for them as potential drug and vaccine targets.

Analysis of CDPK1 targets identifies a trafficking adaptor complex that regulates microneme exocytosis in Toxoplasma

Apicomplexan parasites use Ca2+-regulated exocytosis to secrete essential virulence factors from specialized organelles called micronemes. Ca2+-dependent protein kinases (CDPKs) are required for microneme exocytosis; however, the molecular events that regulate trafficking and fusion of micronemes with the plasma membrane remain unresolved. Here, we combine sub-minute resolution phosphoproteomics and bio-orthogonal labeling of kinase substrates in Toxoplasma gondii to identify 163 proteins phosphorylated in a CDPK1-dependent manner. In addition to known regulators of secretion, we identify uncharacterized targets with predicted functions across signaling, gene expression, trafficking, metabolism, and ion homeostasis. One of the CDPK1 targets is a putative HOOK activating adaptor. In other eukaryotes, HOOK homologs form the FHF complex with FTS and FHIP to activate dynein-mediated trafficking of endosomes along microtubules. We show the FHF complex is partially conserved in T. gondii, consisting of HOOK, an FTS homolog, and two parasite-specific proteins (TGGT1_306920 and TGGT1_316650). CDPK1 kinase activity and HOOK are required for the rapid apical trafficking of micronemes as parasites initiate motility. Moreover, parasites lacking HOOK or FTS display impaired microneme protein secretion, leading to a block in the invasion of host cells. Taken together, our work provides a comprehensive catalog of CDPK1 targets and reveals how vesicular trafficking has been tuned to support a parasitic lifestyle.

Two small-molecule inhibitors of Toxoplasma gondii proliferation in vitro

Background

Toxoplasmosis caused by Toxoplasma gondii is a globally distributed zoonosis. Most infections appear asymptomatic in immunocompetent individuals, but toxoplasmosis can be fatal in fetuses and immunocompromised adults. There is an urgent need to research and develop effective and low-toxicity anti-T. gondii drugs because of some defects in current clinical anti-T. gondii drugs, such as limited efficacy, serious side effects and drug resistance.

Methods

In this study, 152 autophagy related compounds were evaluated as anti-T. gondii drugs. The activity of β-galactosidase assay based on luminescence was used to determine the inhibitory effect on parasite growth. At the same time, MTS assay was used to further detect the effects of compounds with over 60% inhibition rate on host cell viability. The invasion, intracellular proliferation, egress and gliding abilities of T. gondii were tested to assess the inhibitory effect of the chosen drugs on the distinct steps of the T. gondii lysis cycle.

Results

The results showed that a total of 38 compounds inhibited parasite growth by more than 60%. After excluding the compounds affecting host cell activity, CGI-1746 and JH-II-127 were considered for drug reuse and further characterized. Both CGI-1746 and JH-II-127 inhibited tachyzoite growth by 60%, with IC50 values of 14.58 ± 1.52 and 5.88 ± 0.23 μM, respectively. TD50 values were 154.20 ± 20.15 and 76.39 ± 14.32 μM, respectively. Further research found that these two compounds significantly inhibited the intracellular proliferation of tachyzoites. Summarize the results, we demonstrated that CGI-1746 inhibited the invasion, egress and especially the gliding abilities of parasites, which is essential for the successful invasion of host cells, while JH-II-127 did not affect the invasion and gliding ability, but seriously damaged the morphology of mitochondria which may be related to the damage of mitochondrial electron transport chain.

Discussion

Taken together, these findings suggest that both CGI-1746 and JH-II-127 could be potentially repurposed as anti-T. gondii drugs, lays the groundwork for future therapeutic strategies.

How Apicomplexa Parasites Secrete and Build Their Invasion Machinery

Apicomplexa are obligatory intracellular parasites that sense and actively invade host cells. Invasion is a conserved process that relies on the timely and spatially controlled exocytosis of unique specialized secretory organelles termed micronemes and rhoptries. Microneme exocytosis starts first and likely controls the intricate mechanism of rhoptry secretion. To assemble the invasion machinery, micronemal proteins-associated with the surface of the parasite-interact and form complexes with rhoptry proteins, which in turn are targeted into the host cell. This review covers the molecular advances regarding microneme and rhoptry exocytosis and focuses on how the proteins discharged from these two compartments work in synergy to drive a successful invasion event. Particular emphasis is given to the structure and molecular components of the rhoptry secretion apparatus, and to the current conceptual framework of rhoptry exocytosis that may constitute an unconventional eukaryotic secretory machinery closely related to the one described in ciliates. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Investigation of Antiparasitic Activity of Two Marine Natural Products, Estradiol Benzoate, and Octyl Gallate, on Toxoplasma gondii In Vitro

Toxoplasmosis, caused by Toxoplasma gondii, is a common disease worldwide and could be severe and even fatal in immunocompromised individuals and fetuses. Limitation in current available treatment options drives the need to develop novel therapeutics. This study assessed the anti-T. gondii potential of 103 marine natural products. A luminescence-based β-galactosidase activity assay was used to screen the marine natural products library. Afterward, those compounds that displayed over 70% parasite inhibition ratio were further chosen to assess their cytotoxicity. Compounds exhibiting low cytotoxicity (≥80% cell viability) were applied to evaluate the inhibition efficacy on discrete steps of the T. gondii lytic cycle, including invasion, intracellular growth, and egress abilities as well as the cell cycle. We found that both estradiol benzoate and octyl gallate caused >70% inhibition of tachyzoite growth with IC₅₀ values of 4.41 ± 0.94 and 5.66 ± 0.35 μM, respectively, and displayed low cytotoxicity with TD₅₀ values of 34.11 ± 2.86 and 26.4 ± 0.98 μM, respectively. Despite their defects in inhibition of invasion and egress of tachyzoite, the two compounds markedly inhibited the tachyzoite intracellular replication. Flow cytometric analyses further suggested that the anti-T. gondii activity of estradiol benzoate, rather than octyl gallate, may be linked to halting cell cycle progression of tachyzoite from G1 to S phase. Taken together, these findings suggest that both estradiol benzoate and octyl gallate are potential inhibitors for anti-T. gondii infection and support the further exploration of marine natural products as a thinkable source of alternative and active agents against T. gondii.

Unraveling the Elusive Rhoptry Exocytic Mechanism of Apicomplexa

Apicomplexan parasites are unicellular eukaryotes that invade the cells in which they proliferate. The development of genetic tools in Toxoplasma, and then in Plasmodium, in the 1990s allowed the first description of the molecular machinery used for motility and invasion, revealing a crucial role for two different secretory organelles, micronemes and rhoptries. Rhoptry proteins are injected directly into the host cytoplasm not only to promote invasion but also to manipulate host functions. Nonetheless, the injection machinery has remained mysterious, a major conundrum in the field. Here we review recent progress in uncovering structural components and proteins implicated in rhoptry exocytosis and explain how revisiting early findings and considering the evolutionary origins of Apicomplexa contributed to some of these discoveries.