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

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 (Ca ²⁺ ) signaling tightly regulates the lytic cycle of tachyzoites by controlling microneme secretion and motility to drive egress and cell invasion. However, the roles of Ca ²⁺ signaling pathways in bradyzoites remain largely unexplored. Here we show that Ca ²⁺ responses are highly restricted in bradyzoites and that they fail to egress in response to agonists. Development of dual-reporter parasites revealed dampened calcium responses and minimal microneme secretion by bradyzoites induced in vitro or harvested from infected mice and tested ex vivo. Ratiometric Ca ²⁺ imaging demonstrated lower Ca ²⁺ basal levels, reduced magnitude, and slower Ca ²⁺ kinetics in bradyzoites compared with tachyzoites stimulated with agonists. Diminished responses in bradyzoites were associated with down-regulation of calcium ATPases involved in intracellular Ca ²⁺ storage in the endoplasmic reticulum (ER) and acidocalcisomes. Once liberated from cysts by trypsin digestion, bradyzoites incubated in glucose plus calcium rapidly restored their intracellular Ca ²⁺ and ATP stores leading to enhanced gliding. Collectively, our findings indicate that intracellular bradyzoites exhibit dampened Ca ²⁺ signaling and lower energy levels that restrict egress, and yet upon release they rapidly respond to changes in the environment to regain motility.