Abstract
During invasion and egress from their host cells, Apicomplexan parasites face sharp changes in the surrounding calcium ion (Ca(2+)) concentration. Our work with Toxoplasma gondii provides evidence for Ca(2+) influx from the extracellular milieu leading to cytosolic Ca(2+) increase and enhancement of virulence traits, such as gliding motility, conoid extrusion, microneme secretion, and host cell invasion. Assays of Mn(2+) and Ba(2+) uptake do not support a canonical store-regulated Ca(2+) entry mechanism. Ca(2+) entry was blocked by the L-type Ca(2+) channel inhibitor nifedipine and stimulated by the increase in cytosolic Ca(2+) and by the specific L-type Ca(2+) channel agonist Bay K-8644. Our results demonstrate that Ca(2+) entry is critical for parasite virulence. We propose a regulated Ca(2+) entry mechanism activated by cytosolic Ca(2+) that has an enhancing effect on invasion-linked traits.
Highlights
Toxoplasma gondii is exposed to large Ca2ϩ gradients during its lytic cycle
Evidence for a Regulated Ca2ϩ Entry Pathway in T. gondii Tachyzoites—Upon egress from their host cell, T. gondii tachyzoites face a large change in extracellular Ca2ϩ concentration from nanomolar levels, when in contact with the host cytoplasm through the parasitophorous vacuole, to ϳ1.8 mM, in the extracellular milieu
To simulate conditions present upon exit of tachyzoites from the host cells, we suspended parasites obtained under intracellular conditions and loaded them with Fura-2/AM at a Ca2ϩ concentration usually present in the cytosol (110 nM; low Ca2ϩ extracellular buffer (EB); see “Experimental Procedures”)
Summary
Results: Ca2ϩ entry in T. gondii is a source of Ca2ϩ increase and is mediated by a nifedipine-sensitive pathway and not by a canonical store-operated Ca2ϩ entry (SOCE) pathway. Conclusion: Ca2ϩ entry enhances parasite virulence traits. Significance: This is the first study linking regulation of Ca2ϩ entry and virulence traits of T. gondii. During invasion and egress from their host cells, Apicomplexan parasites face sharp changes in the surrounding calcium ion (Ca2؉) concentration. Our work with Toxoplasma gondii provides evidence for Ca2؉ influx from the extracellular milieu leading to cytosolic Ca2؉ increase and enhancement of virulence traits, such as gliding motility, conoid extrusion, microneme secretion, and host cell invasion. Assays of Mn2؉ and Ba2؉ uptake do not support a canonical store-regulated Ca2؉ entry mechanism.
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