Toxoplasma gondii bradyzoite-specific BAG1 is nonessential for cyst formation due to compensation by other heat-shock proteins

Toxoplasma gondii is an opportunistic pathogenic protozoan that infects all warm-blooded animals, including humans, and causes zoonotic toxoplasmosis. The bradyzoite antigen 1 (BAG1), known as heat-shock protein (HSP)30, is a specific antigen expressed during the early stage of T. gondii tachyzoite-bradyzoite conversion.

The data suggested that when bag1 was absent, the stress response of T. gondii was partially compensated by increased levels of other HSPs, resulting in the formation of fewer cysts. This highlighted a complex regulatory network beyond BAG1 influencing the parasite's transformation into bradyzoites, emphasizing the vital compensatory function of HSPs in the T. gondii life cycle adaptation.

A bag1 gene knockout strain based on the T. gondii type II ME49 was constructed and designated as ME49Δbag1. The invasion, proliferation, and cyst formation efficiency in the cell model and survival in the mouse model were compared between the ME49 and ME49Δbag1 strains after infection. Quantitative polymerase chain reaction (qPCR) was used to detect the transcriptional level of important genes, and western-blot was used to detect protein levels.

ME49Δbag1 displayed significantly inhibited cyst formation, although it was not completely blocked. During early differentiation induced by alkaline and starvation conditions in vitro, the proliferation of ME49Δbag1 was significantly accelerated relative to the ME49 strain. Meanwhile, the transcription of the HSP family and bradyzoite formation deficient 1 (bfd1) were significantly enhanced. The observed upregulation suggests a compensatory mechanism to counterbalance the impaired stress responses of T. gondii following bag1 knockout. On the other hand, the elevated transcription levels of several HSP family members, including HSP20, HSP21, HSP40, HSP60, HSP70, and HSP90, along with BFD1, implied the involvement of alternative regulatory factors in bradyzoite differentiation aside from BAG1. Conclusions: The data suggested that when bag1 was absent, the stress response of T. gondii was partially compensated by increased levels of other HSPs, resulting in the formation of fewer cysts. This highlighted a complex regulatory network beyond BAG1 influencing the parasite's transformation into bradyzoites, emphasizing the vital compensatory function of HSPs in the T. gondii life cycle adaptation.