Toxoplasma gondii&nbsp;Chinese 1 Genotype Wh6 Strain Causes Mice Abnormal Cognitive Behavior through Affecting Hippocampal Neurons
Authors: Tao Qing , Cui Wen , Liu Lei , Luo Qingli , Jin Mengmeng , Zhang Famin , Zhu Jinjin , Du Jian , Yu Li , Shen Jilong , Chu Deyong 
Published: August 2020

Abstract
BackgroundChronic Toxoplasma gondii (T. gondii) infection evokes abnormal cognitive behavior of the host. Recent studies suggest that the polarization of microglia to the phenotype of classically activated macrophage (M1) and the deposition of beta-amyloid (Aβ) may be induced in brain of mice chronically infected with T. gondii. However, so far, there is no definite explanation for the relationship mechanism underlying the above between microglia polarization, Aβ deposition and T. gondii. Our previous investigations indicated that in vitro T. gondii type Ⅱ strain dense granule protein 15 (GRA15Ⅱ), one of the genotype-associated effectors of T. gondii Ⅱ strain may induce mouse macrophage to M1. While T. gondii type Ⅰ/Ⅲ strain rhoptry protein 16 (ROP16Ⅰ/Ⅲ) can drive the mouse macrophage to the phenotype of alternatively activated macrophage (M2). Unlike the archetypal strains of types Ⅰ, Ⅱ, and Ⅲ, T. gondii Chinese 1 genotype Wh6 strain (TgCtwh6) possesses both GRA15Ⅱ and ROP16Ⅰ/Ⅲ proteins, indicating the unique pathogenesis of Toxoplasma-related cognitive behavioral abnormalities.MethodsIn this study, we constructed mice model of cognitive behavioral abnormalities through chronic infection of TgCtwh6 via the oral route, and used mouse hippocampal neuronal cell line (HT22) and mouse microglial cell line (BV2) infected with TgCtwh6 in co-culture system to explore the mechanism with which TgCtwh6 infection induced mouse abnormal cognitive behavior. The immunohistochemistry, immunofluorescence, western blotting, cell culture assays, as well as an array of mouse behavior tests were adopted in the research.ResultsIn our research, the infected group showed abnormal cognitive behavior in the water maze and open field experiments in comparison with the control group. Further study showed that the number of synapses and hippocampal neurons decreased and the expression of Aβ increased in brain. In vitro, our research indicated that TgCtwh6 infection could not only directly lead to the HT22 apoptosis but also directly induce BV2 activation to M1 possibly through Notch pathway. Activated BV2 secreted pro-inflammatory factors resulting in HT22 apoptosis indirectly in transwell device. Meanwhile, our reseach demonstrated that TgCtwh6 infection caused a notable expression of β-secretase 1 (BACE1)、amyloid precursor protein (APP) and Aβ in HT22 through NF-κB signaling. Furthmore, BV2 activated by TgCtwh6 infection produced pro-inflammatory factors, such as IL-6, TNF-α and iNOS, which promoted HT22 to express APP in co-culture system. In all, our results suggested that TgCtwh6 gave rise to mouse abnormal cognitive behavior due to hippocampal neuronal apoptosis and Aβ deposition driven by indirect and indirect TgCtwh6 infection to hippocampal neuron. In this pathogenic process, microglia activation played an important role in mediating hippocampal neuronal apoptosis and Aβ deposition.ConclusionsThis study demonstrates TgCtwh6 infection can cause mice to develop AD-like symptoms and give rise to hippocampal neuronal apoptosis and Aβ deposition. Besides, microglia activation played an functional role in the pathological development.