Host Cell Vimentin Restrains Toxoplasma gondii Invasion and Phosphorylation of Vimentin is Partially Regulated by Interaction with TgROP18

He, Cheng; Kong, Ling; Zhou, Lijuan; Xia, Jing; Wei, Haixia; Liu, Min; Peng, Hongjuan

The dynamic exchange between the assembled and disassembled pools of vimentin in vivo is phosphorylation dependent, and phosphorylation of this protein also affects its solubility [33, 43] . To determine whether the solubility of vimentin was affected by TgROP18, vimentin solubility assays were performed using Cos7 cells infected with T. gondii RH and RH Δrop18, and the results indicated that vimentin solubility was partly regulated by TgROP18, in addition to other unknown parasitic factors ( Figure 7) . We also found that the expression levels of vimentin were regulated by the T. gondii infection; however, they were not significantly affected by T. gondii ROP18 (Figure 3D and 7F). In summary, the dynamic patterns of vimentin phosphorylation, solubility, and expression levels were regulated by T. gondii infection, while its phosphorylation level and solubility were only partly affected by TgROP18. Hence, while vimentin has important roles in T. gondii infection, only its phosphorylation and solubility, but not its expression levels, are affected by TgROP18. Maintenance of host cell actin cytoskeleton integrity is important during the T. gondii lytic cycle, including invasion and egress. For example, host cell actin cytoskeleton integrity is essential for successful T. gondii invasion [12] , since T. gondii invasion was blocked when host cell actin polymerization was inhibited using cytochalasin D [44] . There is evidence that actin organization and actin cytoskeleton integrity may be modulated by host cell vimentin [45] . Herein, in our present study we found that T. gondii invasion was affected by host cell vimentin. When vimentin was knocked out in HBMEC cells, the infection rate of T. gondii was significantly increased (Figure 4 ). The result, indicating inhibition of host cell vimentin on T. gondii invasion, is opposite to those demonstrated for Escherichia coli K1 and Japanese encephalitis virus infection [31, 46] , suggesting that vimentin regulates infection of T. gondii and other microbial pathogens by different mechanisms.