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 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 downregulation 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.

Keywords: calcium signaling; cell biology; chronic infection; dormancy; exocytosis; infectious disease; microbiology; reactivation; tissue cyst.