Toxoplasma bradyzoites exhibit physiological plasticity of calcium and energy stores controlling motility and egress

Abstract

Toxoplasma gondii has evolved different developmental stages for disseminating during acute infection (i.e. tachyzoites) and for establishing chronic infection (i.e. bradyzoites). Calcium ion (Ca2+) signaling tightly regulates the lytic cycle of tachyzoites by controlling microneme secretion and motility to drive egress and cell invasion. However, the roles of Ca2+ signaling pathways in bradyzoites remain largely unexplored. Here we show that Ca2+ responses are highly restricted in bradyzoites and that they fail to egress in response to agonists. Development of dual-reporter parasites revealed dampened Ca2+ responses and minimal microneme secretion by bradyzoites induced in vitro or harvested from infected mice and tested ex vivo. Ratiometric Ca2+ imaging demonstrated lower Ca2+ basal levels, reduced magnitude, and slower Ca2+ kinetics in bradyzoites compared with tachyzoites stimulated with agonists. Diminished responses in bradyzoites were associated with down-regulation of Ca2+-ATPases involved in intracellular Ca2+ storage in the endoplasmic reticulum (ER) and acidocalcisomes. Once liberated from cysts by trypsin digestion, bradyzoites incubated in glucose plus Ca2+ rapidly restored their intracellular Ca2+ and ATP stores leading to enhanced gliding. Collectively, our findings indicate that intracellular bradyzoites exhibit dampened Ca2+ signaling and lower energy levels that restrict egress, and yet upon release they rapidly respond to changes in the environment to regain motility.

A Signaling Factor Linked to Toxoplasma gondii Guanylate Cyclase Complex Controls Invasion and Egress during Acute and Chronic Infection

“…According to transcriptional profiling of T. gondii from ToxoDB, SLF, GC, UGO, and CDC50.1 are also expressed during chronic infection ( 48 ). Additionally, bradyzoite gliding motility was recently shown to depend on calcium signaling ( 33 ), suggesting that GC could operate to control bradyzoites’ invasion, motility, or egress. To determine if bradyzoites have the capacity to actively egress and reinvade host cells, contributing to cyst burden, we assessed the role of SLF and GC in bradyzoites using the mouse model of infection.…”

“…A recent study unveiled that although bradyzoites store less calcium than tachyzoites, the calcium signaling pathway plays a critical role in gliding motility of bradyzoites, as demonstrated by inhibition of key players (PKG and CDPK1). Bradyzoites within in vivo cysts became motile but failed to egress upon treatment with A23187 or zaprinast treatment ( 33 ). It seems that the thick cyst wall of ex vivo cysts restrict the bradyzoites’ ability to egress.…”

“…These observations could indicate a dynamic nature of bradyzoites. Recent work demonstrated that calcium signaling controls bradyzoite gliding motility ( 33 ). Upon oral ingestion, bradyzoites are released from tissue cysts and invade intestinal epithelium cells ( 34 ).…”

The role of Ca2+ in the injury of host cells during the schizogenic stage of E. tenella

“…Furthermore, Ca 2+ plays a crucial role in parasite differentiation, cell scaffold dynamics, migration and colonization and is also a relevant factor in the interaction between the parasite and the host cell ( Docampo and Moreno, 2021 ). The intracellular free Ca 2+ of the host cell increases significantly during the process of host cell development of various intracellular parasites, such as Trypanosoma cruzi, Plasmodium falciparum, Leishmania , and Toxoplasma gondii ( Docampo and Huang, 2021 ; Fu et al, 2021 ; Orrego et al, 2021 ; Scarpelli et al, 2021 ). Therefore, the process of intracellular parasites damaging host cells may be closely related to Ca 2+ signal transduction.…”

Essential Functions of Calmodulin and Identification of Its Proximal Interacting Proteins in Tachyzoite-Stage Toxoplasma gondii via BioID Technology

“…Ca 2+ fluctuation in the cytosol of the parasite and host cell affects some essential processes in T. gondii , including secretion, adhesion, invasion, motility, and egress ( 7 – 11 ). Compared with toxoplasma tachyzoites, bradyzoites exhibit suppressed Ca 2+ signaling, thus limiting egress and rapid response to external stimuli ( 12 ). T. gondii can increase cytoplasmic Ca 2+ content by acquiring Ca 2+ from external and intracellular calcium reserves (including the endoplasmic reticulum, mitochondria, acidocalcisome, and plant-like vacuole) ( 6 , 13 ).…”