Metabolic adaptability and nutrient scavenging in Toxoplasma gondii: insights from ingestion pathway-deficient mutants.

The obligate intracellular parasite Toxoplasma gondii replicates within a specialized compartment called the parasitophorous vacuole (PV). Recent work showed that despite living within a PV, Toxoplasma endocytoses proteins from the cytosol of infected host cells via a so-called ingestion pathway. The ingestion pathway is initiated by dense granule protein GRA14, which binds host endosomal sorting complex required for transport (ESCRT) machinery to bud vesicles into the lumen of the PV. The protein-containing vesicles are internalized by the parasite and trafficked to the plant vacuole-like compartment (PLVAC), where cathepsin protease L (CPL) degrades the cargo, and the chloroquine resistance transporter (CRT) exports the resulting peptides and amino acids to the parasite cytosol. However, although the ingestion pathway was proposed to be a conduit for nutrients, there is limited evidence for this hypothesis. We reasoned that if Toxoplasma uses the ingestion pathway to acquire nutrients, then parasites lacking GRA14, CPL, or CRT should rely more on biosynthetic pathways or alternative scavenging pathways. To explore this, we conducted a genome-wide clustered regularly interspaced short palindromic repeats (CRISPR) screen in wild-type (WT) parasites and Δgra14, Δcpl, and Δcrt mutants to identify genes that become more fitness conferring in ingestion-deficient parasites. Our screen revealed a significant overlap of genes that potentially become more fitness conferring in the ingestion mutants compared to WT. Pathway analysis indicated that Δcpl and Δcrt mutants relied more on pyrimidine biosynthesis, fatty acid biosynthesis, tricarboxylic acid (TCA) cycle, and lysine degradation. Bulk metabolomic analysis showed reduced levels of glycolytic intermediates and amino acids in the ingestion mutants compared to WT, highlighting the pathway's potential role in host resource scavenging. Interestingly, Δcpl and Δcrt showed an exacerbated growth defect when cultured in amino acid-depleted media, suggesting that disrupting proteolysis or the export of proteolytic products from the PLVAC affects parasite survival during nutrient scarcity.

Importance: Toxoplasma gondii is an obligate intracellular pathogen that infects virtually any nucleated cell in most warm-blooded animals. Infections are asymptomatic in most cases, but people with weakened immunity can experience severe disease. For the parasite to replicate within the host, it must efficiently acquire essential nutrients, especially as it is unable to make several key metabolites. Understanding the mechanisms by which Toxoplasma scavenges nutrients from the host is crucial for identifying potential therapeutic targets. Our study suggests that the ingestion pathway contributes to sustaining parasite metabolites and parasite replication under amino acid-limiting conditions. This work advances our understanding of the metabolic adaptability of Toxoplasma.