Effects Of Motheral Exposure To Lipopolysaccharide During Late Pregnancy On Cognitive Function In Middle-Aged Offspring Mice And Its Related Mechanisms

BackgroundAlzheimer’s disease (AD) is a common and chronic progressivly neurodegenerative disease, and considered as a pathological aging. Learning and memory impairment is the core symptom. Sporadic Alzheimer disease (SAD) accounts for the vast majority of AD patients and its incidence rate increases exponentially with age. To date, the mechanism of SAD is unknown. The mainstream view is that SAD may be involved in both genetic and environmental dual mechanisms, in which an environmental factor may play a more important role. Increasing epidemiological studies have demonstrated that exposure to adverse factors in the early life (such as, the embryonic period) might induce to accelerate the decline of cognitive function and the occurrence of the age-related diseases. Infection is one of the most frequent adverse factors, and lipopolysaccharide (LPS) is an acknowledged inducer of inflammation. Some previous animal studies have showed that embryonic exposing to LPS may result in cognitive dysfunction in the offspring mice. These previous studies have comprehensively evaluated the progress of growth and change of cognitive function from postnatal to early adulthood. However, most of these studies have only observed behavioral changes until adulthood and rarely explored specificly pathophysiological mechanisms. In this study, using outbred pregnant mice exposed to LPS, we plan to observe the growth, behavioral and pathophysiological changes of aging and/or AD in the offspring mice from birth to mid-adulthood, and explore the fetal origin hypothesis of AD.ObjectiveThe aim of this study is to explore the fetal origin hypothesis of AD.MethodsAccording to the above objective, five experiments were designed. ①The18pregnant CD-1mice were randomly divide into two groups, and composed of the LPS-group and the control-group. The LPS-group mice were intraperitoneally injected LPS (50μg/kg) at days15-17after pregnancy every day. The control-group mice were intraperitoneally injected the same volume of normal saline every day. After undergoing the normal postnatal process (including pregnancy, parturition and lactation),18offspring mice (9males and9females) were randomly selected from each group at4-week-old age and were separately feed until400-day-old. We recorded the dynamic changes of body weight from4-week-old to33-week-old, then assessed behaviors using partal or all tasks or batteries as follows:species behaviors, sensorimotor behaviors, anxiety and spontaneous exploration activities, and spatial and/or non-spatial learning and memory at35-day,290-day and400-day old, respectively.②After accomplished behavioral assessment, the offspring mice were killed, then the hippocampi were separated to make the tissue microarrays. Since p-amyloid peptide (Aβ) load increased was considered to be the most prominent pathophysiology of AD, the Aβ levels in dentate gyrus (DG) and Cornu Ammonis subfield1(CA1) subregions of dorsal hippocampus (DH) were detected with immunohistochemistry method.③The temporal-pillow-top lobes were separated, homogenized and the supernatant was extracted. The levels of superoxide dismutase (SOD), malondiadehycle (MDA) and glutathione peroxidase (GSH-PX) in DG and CA1sublayers of DH were detected with ELISA method.④Before the offspring mice were killed, the blood from the eyeballs was centrifugated, the levels of free triidiohtyronine (FT3) and free thyroxine (FT4) were detected in the extracted supernatant with radioimmunoassay method.⑤The levels of Syt1, Syt4and SUMO-3were dected in hippocampal tissue microarrays with immunohistochemical method.Results①The dynamic change of body weights in postnatal during4-33weeks had no significant difference between the LPS-group and control-group offspring mice, but the male mice were more weight than the females. There was no significant difference in the anxiety behavior and spontaneous exploration activities (open-field tasks), spatial learning and memory task (six-arm radial maze, RAWM) in the35-day-old mice. There was no significant difference in species behaviors (burrowing, hoarding and nesting), anxiety behavior and spontaneous exploration activities (open-field tasks), sensorimotor behaviors (beam walking and tight rope tasks) and spatial learning and memory (RAWM) tasks in the290-day-old mice. There was no significant difference in anxiety behavior and spontaneous exploration activities (open field, elevated plus maze, black and white alley tasks) and sensorimotor behaviors (beam walking and tight rope tasks) in the400-day-old mice. However, the abilities of burrowing and hoarding in the LPS-group offspring mice were higher than those of control-group, while the nesting ability was lower than that of control-group. Compared to the control-group, the abilities of spatial learning and memory (morris water maze, MWM) and non-spatial learning and memory (novel object recognition task) in LPS-group were significantly poorer.②Compared with the control-group, the Aβ levels of LPS-group were significantly increased in the hilus (HL), molecular layer (ML) of dentate gyrus (DG), ML and original layer (OL) of Cornu Ammonis subfield1(CA1) region in DH. There are some correlations between the level of Aβ and the decline of learning and memory ability of LPS-group.③Compared with the control group, the content of MDA significantly increased, while the activities of SOD and GSH-PX significantly decreased in the LPS group. The levels of serum FT3and FT4in LPS-group were higher than those of control-group, especially the serum level of FT4. The correlation analysis showed that the levels of oxidative stress products and thyroid hormone were significantly correlated with the behavioral performance of LPS-group.④In HL, granular cell layer and ML of DG region, ML and stratum radiatum layer (RL) of CA1region, the Syt1levels of LPS-group were significantly higher than those of control-group. Comparing to the control-group, the Syt4levels in the LPS-group were significantly higher in CA1-ML, CA1-RL and CA1-OL regions. In all sublayers of DG and CA1regions of DH, the SUMO-3levels of LPS-group were similar to those of control-group. The Syt1and Syt4levels in DH were correlated with the behavioral performance in MWM of the LPS-group mice.Summary①The offspring mice, whose mothers were repeated exposure to low doses of LPS in late gestation, experienced the stage of normal development and the maturity of central nervous system. Compared to the middle age of the control-group mice, the abilities of the spatial and non-spatial learning and memory were obviously impaired in the LPS-group mice.②In the LPS group, we found that the levels of Aβ, oxidative stress products, thyroid hormone, Syt1and Syt4proteins have increased and were correlated with accelerated impairment of age-related spatial learning and memory abilities. Therefore, we speculated that embryonic exposure to low doses of LPS may be involved in cognitive decline in the middle age of offspring mice, even increased susceptivity to AD.