The role of regulatory T cells on the activation of astrocytes in the brain of high-fat diet mice following lead exposure
Introduction
Due to rapid industrialization in recent decades, human exposure to metals has greatly increased, resulting in an increased risk of cognitive impairment [1]. Several studies have revealed an association between HFD and environmental toxicants, such as heavy metals [[2], [3], [4]]. Studies also showed that the combined exposure to heavy metals and HFD promoted aging and caused memory impairment [5]. Heavy metals, especially Pb, and HFD are risk factors for cognitive impairment. Given the adverse effects of Pb exposure on human health and the expanding global burden of HFD consumption, understanding the combined effects of Pb and HFD on cognition, and gaining insights into the potential mechanisms are of prime importance.
Pb is a common environmental pollutant that can cause damage to the CNS. It can accumulate in the hippocampus of the cerebral cortex, causing neuroinflammatory response and ultimately resulting in learning and memory impairments. This process is closely related to that observed in various neurodegenerative diseases such as Alzheimer's disease [6]. Studies have shown that HFD is associated with cognitive impairment, and can cause systemic chronic inflammation which leads to a decline in the learning ability and promotes the occurrence and development of neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, and stroke [[7], [8], [9], [10]]. Both Pb and HFD are closely related to neuroinflammatory diseases. However, the neurotoxic effect of HFD following Pb exposure on the CNS has not been studied yet.
In the CNS, astrocytes are closely related to neuroinflammation and play a dual role of injury and protection under inflammatory conditions. Studies have shown that neuroinflammation could lead to the activation of astrocytes, and that neurotoxic reactive astrocytes are induced by activated microglia. Neurotoxic reactive astrocytes are destructive to synapses and could induce neuronal death following CNS injury [11]. A growing body of evidence supports the role of astrocytes in neurodegenerative diseases [12,13]. Reactive astrocytes are thought to be a pathological hallmark of many neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease [14]. Studies have revealed the involvement of astrocytes in Alzheimer's disease progression, and astrocyte activation is considered as an essential component in the development of neurodegenerative diseases [15,16]. In the brain tissue, Treg cells can inhibit the activation of astrocytes to promote the recovery of nerve function during the chronic phase of ischemic brain injury [17]. Treg cells mainly refer to CD4+ CD25+ Foxp3+ T lymphocytes, which have immunomodulatory and immunosuppressive properties. Studies have shown that in Alzheimer's disease, activating Treg cells and stabilizing Treg cells phenotypes can reduce neuroinflammation and reverse cognitive decline [18]. However, the combined effect of HFD and Pb exposure on astrocytes remains unknown. Moreover, the role of Treg cells in astrocyte activation is not yet explored. Herein, we established a mouse model of HFD combined with Pb exposure and investigated the role of Treg cells in astrocyte activation. The neurobehavioral effects of HFD and Pb was evaluated using the Morris water maze (MWM) test. Treg cells were assessed by flow cytometry and the functional evaluation of Treg cells and astrocytes was performed by western blotting and immunohistochemistry. qPCR was performed to assess the expression of neurotoxic reactive astrocyte markers in the hippocampus. The effect of cytokines secreted by Treg cells on astrocyte activation was assessed using in vitro cultured cells. Finally, to confirm the role of Treg cells in learning and memory function, pharmacological activation or inhibition of Treg cells were performed. The outcomes of our study provide a basis for further exploration to the mechanism of nerve damage caused by Pb exposure and HFD consumption.
Section snippets
Animals and treatment
Sixty specific-pathogen free male C57BL/6 mice (6–8 weeks old, 11.65 ± 1.82 g body weight) were purchased from Beijing Huafukang Biotechnology Co., Ltd. (Beijing, China, Certificate number of animals: SCXK 2014–0004). All animals were raised in a room with standard laboratory conditions (12 h light-dark cycle; temperature, 25 ± 2 °C; relative humidity, 55 ± 5%) and allowed a standard chow diet and drinking water ad libitum. All the animals were acclimatized for 1 week before starting the
Effects of HFD and Pb exposure on learning and memory function
After 12 weeks of the experiment, the spatial learning and memory performance were evaluated using the MWM test. The overall scheme for the animal study was shown in Fig. 1a. Results showed that the escape latency of HFD and Pb groups was higher than that of the control group. On the third day, the escape latency of the HFD + Pb group was significantly higher than that of the HFD and Pb groups (p < 0.05, Fig. 1b, c, d). In the spatial navigation test, the average frequency of crossing the
Discussion
Pb exposure induces synaptic dysfunction and morphological changes in the hippocampus of rats, leading to defects in spatial learning and memory [25]. Moreover, studies showed that learning and memory performance is worse in animals fed with HFD [26]. Evidence suggests that the combination of Pb exposure and HFD could exacerbate harmful effects on human health. Beier et al. reported that Pb exposure along with HFD could exacerbate the decrease in trabecular bone mass and osteoblastic function
Funding
This work was supported by the National Natural Science Foundation of China [grant number 82073598], the Natural Science Foundation of Hebei Province [grant number H2020209250], and the Science and Technology Research and Development Guidance Program of Tangshan City [grant number 19130214g].
CRediT authorship contribution statement
Lei Wu: Conceptualization, Methodology, Data curation, Visualization, Writing – original draft, Writing – review & editing. Shuang Li: Methodology, Writing – review & editing. Chao Li: Methodology, Writing – review & editing. Bin He: Data curation, Writing – review & editing. Linyi Lv: Data curation. Jia Wang: Data curation. Jierui Wang: Data curation. Weixuan Wang: Methodology, Data curation. Yanshu Zhang: Conceptualization, Methodology, Funding acquisition, Supervision, Project
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
YS Zhang designed the study. L Wu performed the experiments. YS Zhang and L Wu drafted the manuscript. S Li contributed to the analysis of data. All authors read and approved the final manuscript.
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