Toxoplasma bradyzoites exhibit physiological plasticity of calcium and energy stores controlling motility and egress
Yong Fu, Kevin M Brown, Nathaniel G Jones, Silvia NJ Moreno, L David Sibley
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 (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 Ca 2+ 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 downregulation of Ca 2+ -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 Ca 2+ 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.