Ionophore-resistant mutant of Toxoplasma gondii reveals involvement of a sodium/hydrogen exchanger in calcium regulation

Calcium is a critical mediator of many intracellular processes in eukaryotic cells. In the obligate intracellular parasite Toxoplasma gondii, for example, a rise in [Ca(2+)] is associated with significant morphological changes and rapid egress from host cells. To understand the mechanisms behind such dramatic effects, we isolated a mutant that is altered in its responses to the Ca(2+) ionophore A23187 and found the affected gene encodes a homologue of Na(+)/H(+) exchangers (NHEs) located on the parasite's plasma membrane. We show that in the absence of TgNHE1, Toxoplasma is resistant to ionophore-induced egress and extracellular death and amiloride-induced proton efflux inhibition. In addition, the mutant has increased levels of intracellular Ca(2+), which explains its decreased sensitivity to A23187. These results provide direct genetic evidence of a role for NHE1 in Ca(2+) homeostasis and important insight into how this ubiquitous pathogen senses and responds to changes in its environment.