Alterations of concentrations of calcium and arachidonic acid and agglutinations of microfilaments in host cells during Toxoplasma gondii invasion.

Toxoplasma gondii (T. gondii) invasion of host cells is a complicated process of interaction between parasites and host cells. In the present study we investigated the alterations of free Ca2+ concentration ([Ca2+]i) and cytoskeletons in phagocytic and non-phagocytic host cells and arachidonic acid (AA) concentration in cells supernatant during T. gondii invasion. T. gondii invasion induced significant elevation of intracellular [Ca2+]i in phagocytic cells (J774A.1) but not in non-phagocytic cells (L929). Pre-treatment of J774A.1 cells with Phospholipase C (PLC) inhibitor (U73122), or Ca2+ chelators (EGTA, BAPTA/AM) did not block elevations of [Ca2+]i but the elevations were lower and of shorter duration than that in untreated cells. Pre-treatment of tachyzoites with Phospholipases A (PLA) inhibitors (4-BPB and AACOCF3) resulted in a similar pattern of increasing of [Ca2+]i as that in Ca2+ chelators treated cells. Agglutinations of microfilaments were observed in J774A.1 cells but not in L929 cells. No changes of microtubules were observed in either cell. Treatment of cells with cytoskeleton inhibitors (colchicines, cytochalasin-D) resulted in reduced cell infection ratios. AA concentration in J774A.1 cells supernatant reached 8.44-fold of basal concentration after T. gondii infection and those in 4-BPB or AACOCF3 pre-treated cells reached 7.70-fold and 8.09-fold of basal concentration, respectively. However, elevation of AA concentrations induced by 4-BPB or AACOCF3 treated tachyzoites were 3.02-fold and 2.65-fold of basal AA concentration. AA concentration in L929 cells supernatant reached 5.02-fold of basal concentration after T. gondii infection and those in 4-BPB or AACOCF3 pre-treated cells reached 4.75-fold and 4.78-fold of basal concentration, respectively. However, elevation of AA concentrations induced by 4-BPB or AACOCF3 treated tachyzoites were 2.06-fold and 2.43-fold of basal AA concentration. Results indicated that elevations of [Ca2+]i and AA induced by T. gondii invasion were from both host cells and parasites. T. gondii invasion activated host cell PLC and triggered the PLC-PKC signal pathway, which resulted in the flowing of extracellular Ca2+ and the releasing of intracellular Ca2+ pool. Elevated [Ca2+]i induced reorganization of host cell microfilaments. The invasion also activated secretory PLA2 (sPLA2) and cytosolic PLA2 (cPLA2) of the parasite to release AA, which increased the permeability of cell membrane.