Exosomes Secreted by Toxoplasma gondii-infected L6 cells: Their Effects on Host Cell Proliferation and Cell Cycle Changes

Abstract

Toxoplasma gondii infection induces alteration of the host cell cycle and cell proliferation. These changes are not only seen in directly invaded host cells but also in the neighboring cells. It is known that the soluble factor which mediates these changes is heat-labile and larger than 10 kDa. We tried to identify whether this alteration can be mediated by exosomes secreted by T. gondii-infected host cells.

L6 cells, a rat myoblast cell line, and RH strain of T. gondii were selected for this study. L6 cells were infected with or without T. gondii. The cellular growth patterns were identified by cell counting with trypan blue under confocal microscopy and cell cycle changes were investigated by flow cytometry. L6 cells infected with T. gondii showed decreased proliferation compared to uninfected L6 cells and revealed a tendency to stay at S or G2/M cell phase. The exosomes were isolated from the L6 cells infected with or without T. gondii infection (RH exosome and L6 exosome, respectively) by sequential ultracentrifuge. The treatment of RH exosome into L6 cells resulted in the slight enhancement of S phase at 12 hour post treatment and the attenuation of cell proliferation at 36 hour post treatment. These changes were transient and disappeared at 48 hour after the exosome treatment. This transient result could be the result of the dilution of RH exosome by the exosomes secreted from the neighboring uninfected L6 cells.

To investigate whether miRNAs contained in the exosome can induce such changes, microarray analysis was performed on the exosomal miRNAs and cellular mRNAs of the exosome-treated cells. Significantly differently expressed miRNAs were identified and their target mRNAs were predicted based on sequence matching using web-based tools. These predicted mRNAs were compared with the actually differently expressed mRNAs. As a result, 1 miRNA-mRNA pair, rno-miR-216a-5p and Hmgb1 (high mobility group box 1), revealed as candidate for molecular mediator of changes on cell proliferation.

In conclusion, our study demonstrated that the exosomes originating from T. gondii-infected cells could change the host cell proliferation and alter the host cell cycle. Hmgb1 could be the possible responsible gene inducing such changes.