Toxoplasma gondii does not simply make all the membrane lipids it needs. This study shows that the parasite manufactures one form of phosphatidylethanolamine, but scavenges another from its host—and disrupting that balance can stop infection.

Toxoplasma gondii is a prevalent zoonotic pathogen infecting livestock as well as humans, and it can infect many types of nucleated cells in various vertebrates. The fast-dividing tachyzoite stage reproduces in other tissues and can eventually cause necrosis through perpetual lytic cycles.

For intracellular reproduction within host cells, the parasite must produce sufficient membrane biomass through synthesis and salvage pathways. Phosphatidylethanolamine is the second most common phospholipid in tachyzoites, and it appears in ester- and ether-linked forms.

Several routes can produce ester-linked phosphatidylethanolamine, including PtdSer decarboxylases, the CDP-ethanolamine, or Kennedy, pathway in the endoplasmic reticulum, and P4-ATPase-mediated flipping in the plasma membrane. The conical shape of ester-linked phosphatidylethanolamine contributes to membrane curvature and regulates budding, fusion, and fission events.

Ether-linked phosphatidylethanolamine allows stronger intermolecular hydrogen bonding between headgroups, resulting in decreased membrane fluidity. The study examined the biogenesis and physiological relevance of ester- and ether-linked phosphatidylethanolamine during the lytic cycle of Toxoplasma tachyzoites.

ECT catalyzes ethanolamine kinase-derived phosphoethanolamine to CDP-ethanolamine in the parasite cytosol. Conditional depletion of ECT impairs the amount and synthesis of ester-linked phosphatidylethanolamine, whereas ether-linked phosphatidylethanolamine remains unaffected.

Intracellular parasites could salvage ether-linked phosphatidylethanolamine from host cells. TgECT is a cytosolic protein that was refractory to genetic deletion in tachyzoites, suggesting that the protein is indispensable for tachyzoite survival.

The researchers generated a transgenic parasite strain expressing ECT fused to a C-terminal spaghetti monster, or ten-times-HA, by three-prime genomic tagging. Western blotting confirmed an expected one-hundred-eighty kilodalton TgECT-smHA band in the transgenic strain but not in the parental strain.

Immunofluorescent staining showed a punctate distribution of ECT-smHA within the parasite, co-localizing with the cytosolic marker aldolase. A conditional mutant enabled evaluation of ECT during the parasite’s lytic cycle in HFF cells, using plaque assays that indicate overall growth fitness.

The parental strain grew normally regardless of IAA, and the TgECT-AID-three-times-HA mutant also grew normally without auxin. In contrast, IAA-induced depletion of ECT arrested parasite growth, as shown by the lack of plaques in the host-cell monolayer. These results ascertain the physiologically critical role of ECT in Toxoplasma gondii.

Figure two establishes and validates a conditional TgECT depletion strain. The authors show CRISPR-mediated AID and three-times-HA tagging, PCR confirmation, and co-localization with TgHSP90; auxin treatment then removes the tagged protein over time.

In plaque assays, parental parasites form plaques with or without IAA, whereas the IAA-treated TgECT-AID-three-times-HA strain produces no detectable plaques, indicating that ECT is required for the tachyzoite lytic cycle. The study examined replication, cell budding, egress, invasion, and gliding motility to investigate the lethal phenotype in ECT-depleted tachyzoite cultures.

Replication was quantified by enumerating parasites per vacuole and calculating the fraction of vacuoles with different numbers of progeny. Unlike the parental strain, the ECT mutant had a much higher fraction of small vacuoles containing one to eight parasites after auxin treatment at both time points.

Large vacuoles with more than sixteen parasites were barely present under ECT-depleted conditions, and daughter-cell formation was strongly impaired in the auxin-treated mutant. Figure three tests how depleting TgECT affects tachyzoite development and motility-related infection processes.

Across replication, budding, natural egress, invasion, gliding motility, and parasite yield, the inducible TgECT-AID-3xHA strain is compared with the parental strain with or without IAA; zaprinast-induced egress is assessed separately. The strong defects in IAA-treated TgECT cells, particularly for invasion and yield, support the authors’ conclusion that TgECT is required for cell division and motility-dependent infection.

Loss of ECT impairs the inner membrane complex, which contains the actin-myosin motor, or glideosome, that drives motility-dependent invasion and egress. After twenty-four and forty-eight hours with IAA, mutant parasites were evidently narrow and shortened compared with the parental strain.

Measurements of TgGAP45-labeled inner membrane complex showed a significant decrease in length and width at both time points after IAA treatment. The inner membrane complex length shrank by ten percent, while tachyzoite width declined by twenty-five percent after auxin exposure.

Figure five shows that auxin-induced depletion of TgECT broadly disrupts Toxoplasma phospholipid composition. In panel a, the mutant’s overall phospholipid abundance falls to roughly half of the untreated reference, while panel b shows significant decreases across major classes including phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, phosphatidylthreonine, and phosphatidylglycerol.

Panel c further separates phosphatidylethanolamine into ester- and ether-linked pools, revealing a pronounced reduction in the ester-linked pool. Total lipids were isolated from parasites, resolved, and analyzed by HPLC-MS; compared with parental and untreated mutant controls, the ECT-depleted strain showed about a fifty percent decline in phospholipid content.

All major phospholipid classes significantly decreased in the IAA-treated mutant, while no change was apparent in the parental strain. ECT knockdown caused a decline in ester-linked phosphatidylethanolamine but not in ether-linked lipid.

The ester-to-ether phosphatidylethanolamine ratio changed from two-point-six to one in untreated cultures to one-point-one to one in the ECT-depleted mutant. Figure six shows that IAA exposure produced little threshold-crossing change in the parental strain, while the TgECT-AID-3xHA strain displayed many phospholipid species meeting the twofold fold-change and false-discovery-rate-corrected p-value thresholds.

The volcano plots color significant species by lipid class and scale circles by abundance; the heatmaps then show selected species across minus IAA and plus IAA conditions, including ester and ether forms of phosphatidylcholine and phosphatidylethanolamine. This matters because it links ECT depletion to selective remodeling of parasite phospholipid composition.

No lipid species in the parental strain were affected above the threshold of at least two-fold change with a p value of at most zero-point-zero-one. The TgECT-AID-three-times-HA strain showed substantial regulation of phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, phosphatidylthreonine, lysophosphatidylcholine, and lysophosphatidylinositol species.

Among downregulated ester phospholipids, phosphatidylethanolamine, phosphatidylserine, and phosphatidylthreonine species were most evident. Ether lipids belonging to phosphatidylethanolamine and phosphatidylcholine were upregulated or unaltered in the mutant.

Figure seven links lipid abundance to isotope-tracing experiments. Panel a shows that depleting ECT with IAA alters specific ester- and ether-linked PtdEtn species, while panel b outlines labeling either parasite synthesis with carbon-thirteen ethanolamine or host-derived PtdEtn for salvage.

Panels c and d then measure isotope enrichment in extracellular and intracellular tachyzoites, supporting a functional Kennedy pathway for selected ester-PtdEtn species and the ability to salvage ether-linked PtdEtn from the host environment. Ether-linked phosphatidylethanolamine species were increased or unaffected after ECT depletion, and bioinformatic analysis did not find a suitable enzyme for ether-phosphatidylethanolamine synthesis in the parasite genome.

The proposed model was that the Kennedy pathway provides ester-linked phosphatidylethanolamine, while ether-lipid production depends on the parasite milieu. Stable isotope labeling with thirteen-carbon, two-ethanolamine followed by lipidomic analysis distinguished phosphatidylethanolamine species synthesized by the parasite from those salvaged from host cells.

Extracellular parasites showed enrichment of thirteen-carbon, two-ethanolamine in several ester-linked phosphatidylethanolamine species, including C34:1 at more than five-fold, C34:2 at about eleven-fold, and C36:2 at about ten-fold. The same analysis found about six-fold enrichment for C38:4 and C40:5, confirming a functional CDP-ethanolamine pathway.

IAA-mediated ECT depletion impaired labeling of C34:1, C34:2, and C36:2, while labeling of C38:4 and C40:5 was not affected by IAA. Thirteen-carbon, two-ethanolamine-labeled host cells did not produce isotopic enrichment in any ester-linked phosphatidylethanolamine species, ruling out salvage of those lipids.

By contrast, A34:2 and A36:5 were enriched about four- to five-fold in intracellularly grown parasites but not in host-free parasites. A38:5, A38:6, and A40:6 were labeled equally in both settings at about two- to three-fold, and ECT depletion did not change ether-phosphatidylethanolamine labeling.

The data indicate that ester-linked phosphatidylethanolamine is synthesized primarily de novo through the CDP-ethanolamine pathway, whereas ether-linked phosphatidylethanolamine is salvaged from the host cell. ECT is essential for the lytic cycle, and distinct reactions appear to drive the synthesis of discrete ester-linked phosphatidylethanolamine species.

The ester-linked species encoded by the Kennedy pathway cannot be compensated for by other means, leading to a lethal phenotype in the ECT mutant. Lipid biosynthesis, trafficking, salvage, sensing, and signaling remain poorly appreciated in Apicomplexa.

Toxoplasma tachyzoites deploy distinct routes to fulfill their need for ester- and ether-linked phosphatidylethanolamine species. Ester-linked lipids are generated through the Kennedy pathway, while ether-linked lipids are salvaged from host cells. ECT is an essential protein facilitating parasite invasion and replication during the lytic cycle.

Its phylogenetic divergence from mammalian homologs makes TgECT an excellent drug target for therapeutic inhibition of acute toxoplasmosis. TgECT is a vital enzyme contributing to the synthesis of ester-linked phosphatidylethanolamine, and its conditional depletion impairs membrane biogenesis, replication, and invasion in tachyzoites.

The work reveals collaboration among endogenous synthesis, interconversion, and salvage pathways to produce phosphatidylethanolamine. The data show previously unknown scavenging of host-derived ether-linked phosphatidylethanolamine and provide a basis for therapeutic targeting of phosphatidylethanolamine synthesis in Toxoplasma gondii.

The central finding is a division of labor: the parasite’s CDP-ethanolamine pathway makes ester-linked phosphatidylethanolamine, while host cells supply ether-linked forms. Because this pathway supports membrane structure, invasion, and replication, TgECT emerges as a possible therapeutic target.