<i>Toxoplasma</i> bradyzoites exhibit physiological plasticity of calcium and energy stores controlling motility and egress.

Abstract

<i>Toxoplasma gondii</i> 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.