Toxoplasma TgATG9 is critical for autophagy and long-term persistence in tissue cysts

Abstract

Many of the world’s warm-blooded species are chronically infected with Toxoplasma gondii tissue cysts, including an estimated one-third of the global human population. The cellular processes that permit long-term persistence within the cyst are largely unknown for T. gondii and related coccidian parasites that impact human and animal health. Herein, we show that genetic ablation of TgATG9 substantially reduces canonical autophagy and compromises bradyzoite viability. Transmission electron microscopy revealed numerous structural abnormalities occurring in ∆atg9 bradyzoites. Intriguingly, abnormal mitochondrial networks were observed in TgATG9-deficient bradyzoites, some of which contained numerous different cytoplasmic components and organelles. ∆atg9 bradyzoite fitness was drastically compromised in vitro and in mice, with very few brain cysts identified in mice 5 weeks post-infection. Taken together, our data suggests that TgATG9, and by extension autophagy, is critical for cellular homeostasis in bradyzoites and is necessary for long-term persistence within the cyst of this coccidian parasite.

An Uninvited Seat at the Dinner Table: How Apicomplexan Parasites Scavenge Nutrients from the Host

“…This was corroborated in a following study from Smith et al [130] that demonstrated that autophagy is constitutively induced in bradyzoites and essential for cyst survival. Inhibition of this process through TgAtg9 gene deletion resulted in bradyzoite death and a dramatic decrease in the brain cyst burden in challenged mice, mirroring the results obtained for the TgCPL knockout strain [128,130]. It still remains to be determined whether autophagy and VAC digestion following encystation are required to renew organelles during the chronic stage (and thus is critical for cellular homeostasis), or if it reflects a starvation response to provide essential substrates to sustain the bradyzoite.…”

“…CPL-deficient bradyzoites were also shown to accumulate autophagosomes, indicating autophagy could be essential to chronic persistence [128]. This was corroborated in a following study from Smith et al [130] that demonstrated that autophagy is constitutively induced in bradyzoites and essential for cyst survival. Inhibition of this process through TgAtg9 gene deletion resulted in bradyzoite death and a dramatic decrease in the brain cyst burden in challenged mice, mirroring the results obtained for the TgCPL knockout strain [128,130].…”

Toxoplasma TgAtg8-TgAtg3 Interaction Primarily Contributes to Apicoplast Inheritance and Parasite Growth in Tachyzoite

“…These findings indicate that apicomplexan Atg8 homologs seem to be clearly different from yeast and mammalian Atg8s, which are not strictly necessary for cell growth under nutrient-rich conditions ( 45 , 46 ). However, a recent work has shown that, in Toxoplasma bradyzoites,TgAtg8 also benefits bradyzoite viability due to its abilities of efficient autophagosome formation and transportation of autophagic materials to VAC ( 16 ). Therefore, the dual functions of TgAtg8, apicoplast biogenesis in tachyzoites and viability in bradyzoites, suggest that it may be leveraged for the development of drugs against both acute and chronic Toxoplasma infection.…”

“…It has previously been shown that autophagosome-like structures can be observed in response to nutrient starvation and endoplasmic reticulum stress in tachyzoites ( 14 , 15 ) and bradyzoites ( 16 , 17 ) of Toxoplasma . However, besides its cytosolic and autophagosomal localizations, the Toxoplasma gondii Atg8 (TgAtg8) homologue also localizes to the outermost membrane of the apicoplast, a nonphotosynthetic plastid-like organelle shared by most members of Apicomplexa ( 18 ).…”

The Autophagy Machinery in Human-Parasitic Protists; Diverse Functions for Universally Conserved Proteins

“…In the chronic stage parasites (bradyzoites), cathepsin L-depleted cells accumulate TgATG8-positive puncta, suggesting an arrest of autophagy [103]. The recycling of amino acids by autophagy may be crucial for the persistence of Toxoplasma in its host, as demonstrated by specific inactivation of ATG9 in this stage [104]. While Apicomplexa of the coccidia subclass (including Toxoplasma) possess an ATG9 homolog that seems essential for canonical autophagy akin to other eukaryotic models, other Apicomplexa like Plasmodium do not, which raises the question of how autophagosomes can be generated in these parasites.…”