Discussion and conclusions
Numerous studies have shown that the central neuroinflammatory response is the main mechanism of neurodegenerative diseases, but the specific pathophysiological mechanism is still unclear. In this study, we found that disorders of the BDNF-TrkB signaling pathway and its downstream cascade in mice participated in learning and memory impairments induced by neuroinflammation, which mainly manifested as reductions in BDNF, p-TrkB, Bcl-2, p-ERK1/2, p-CaMK2, p-CREB and p-GluR1 proteins and upregulation of the expression of Bax protein in different brain regions including the mPFC, hippocampus and EC.
It is widely known that BDNF is the main neurotrophin in the brain(von Bohlen und Halbach & von Bohlen und Halbach, 2018). Many researchers have reported that BDNF has important effects on synapses including structural and functional roles in many brain regions(Bekinschtein et al., 2014; Lu et al., 2014). The different effects of BDNF on the brain are related to its diverse forms and downstream receptors that it binds to, such as pro-BDNF and mature-BDNF with the p75 neurotrophin receptor (p75NTR) and TrkB which have opposing effects on synapses(Fobian et al., 2009; Kowiański et al., 2017; Sasi, Vignoli, Canossa, & Blum, 2017). In our study, we mainly discuss the roles of mature-BDNF and TrkB receptors on learning and memory deficits induced by neuroinflammation. The expression of BDNF and p-TrkB receptors were reduced in the mPFC, hippocampus and EC regions after application of LPS in mice. This demonstrates that different ways of applying BDNF induced different changes in functions and structures(Ji et al., 2010). For example, acute up-regulation of BDNF initiated neurite elongation and spine head enlargement, however, gradual increase in BDNF potentiated dendritic branching and filopodia-like spines. In addition, transient activation of TrkB promoted synaptic transmission in the brain. However, systematic application of BDNF could be catabolized by enzymes in vivo. In this study, we used TrkB agonist 7,8-DHF to simulate physiological actions of BDNF and detected whether the activation of TrkB could alleviate cognitive impairments in LPS mice. As the results show, preventive use of 7,8-DHF effectively alleviated learning and memory dysfunction induced by LPS. At the same time, the level of p-TrkB was increased after administration of 7,8-DHF. However, the application of TrkB antagonist ANA12 completely reversed therapeutic effects of 7,8-DHF, which further indicates that the BDNF-TrkB signaling pathway disorder involved neuroinflammation associated with learning and memory impairments in mice.
Increasing evidence suggests that BDNF exerts its neuroprotective roles by suppressing excitotoxicity of NMDA receptors, promoting regeneration of synapses and inhibiting cell apoptosis(Miranda et al., 2018; Ren & Dubner, 2007; Yamada & Nabeshima, 2004). Bax is a pro-apoptotic protein which accelerates cell loss including neurons, which lead to memory disorders(Sun et al., 2021) wheread Bcl-2 is one of anti-apoptotic proteins antagonizing cell apoptosis. Neuroinflammation reduced expression of BDNF and p-TrkB proteins, further disrupting the balance of Bax and Bcl-2 generating cell apoptosis in the mPFC, hippocampus and EC regions. ERK1/2 and CREB play a vital role in learning and memory in the brain which are involved in regulating transcription factors and promoting protein synthesis(Cao et al., 2013; Zheng et al., 2020). In this study, the expression of p-ERK1/2 and p-CREB was decreased in the mPFC and hippocampus of LPS mice, while in the EC, only the level of p-CREB was downregulated and no difference was found in the expression of p-ERK1/2. In addition, CaMK2 is essential for the learning process and synaptic plasticity, and GluR1 is an important component of the postsynaptic density and controls dendrite growth(Sanderson et al., 2008; Vigil & Giese, 2018). We found that neuroinflammation reduced the expression of p-CaMK2 and p-GluR1 both in the mPFC and hippocampus, while in the EC, only the level of p-CaMK2 was decreased. These differences between the EC with the hippocampus and mPFC may be related to the mutual regulation of neural circuits.
Previous studies have reported that multiple brain regions are involved in the process of learning and memory, such as the mPFC, hippocampus and EC(Tanimizu et al., 2017),(Opitz, 2014). Researchers have demonstrated that the mPFC is the most vulnerable among them, and the hippocampus is moderate, whereas the EC is relatively less susceptible to neuroinflammation(Maiti, Muthuraju, Ilavazhagan, & Singh, 2008). But the volume change in the EC is used as the earliest indicator to evaluate preclinical cognitive deficits leading to the development of dementia(Rodrigue & Raz, 2004). The hippocampus is responsible for storing information and is a key region involved in learning and memory. The mPFC has been shown to play a vital role in the process of attention, behavioral flexibility, social and emotional behaviors, and its interactions with the hippocampus are involved in the regulation of learning and memory(Euston, Gruber, & McNaughton, 2012). The EC is also related to spatial and long-term memory(Garcia & Buffalo, 2020). Neural projections from the mPFC or EC to the hippocampus are known to be involved in the modulation of learning and memory processes(Chao, de Souza Silva, Yang, & Huston, 2020; Ladurelle et al., 2011; Vertes, 2015). Lu et al. found that the ECIIPN-CA1PVpathway was impaired with spatial learning and memory deficits in Alzheimer’s disease mice, and optogenetic activation of ECIIPN rescued ECIIPN-CA1PV pathway defects and alleviated the impairment of spatial learning and memory(X. Yang et al., 2018). In addition, electrical stimulation of the EC alleviated spatial memory deficits in Alzheimer’s disease mice infusing amyloid peptides 1-42 into the hippocampus, which suggestes that there is functional projection between the EC and hippocampus. However, it is not clear whether the mutual projection of the mPFC, hippocampus and EC regions and neuroregulation are associated with the metabolic changes in the BDNF-TrkB signaling pathway and its downstream cascade.
There are still many limitations in this study. On the one hand, we only tested protein levels of the BDNF-TrkB signaling pathway and its downstream cascade in different brain regions, and no synaptic related proteins were detected which directly indicates synaptic damages. On the other hand, whether the changes in the BDNF-TrkB signaling pathway and its downstream cascades among the mPFC, hippocampus and EC regions were regulated by the projection of nerve fibers between these regions have not been confirmed and deserves further investigation.
In summary, our research revealed that the BDNF-TrkB signaling pathway and its downstream cascade disorders participated in learning and memory impairments induced by neuroinflammation in mice (Figure 9). Intraperitoneal injection of the TrkB agonist 7,8-DHF could effectively alleviate cognitive dysfunction in LPS mice. As a result, the BDNF-TrkB signaling pathway and its downstream cascade disorders are a new viewpoint for learning and memory impairments induced by neuroinflammation, and 7,8-DHF might serve as a potential target for preventing or treating cognitive dysfunction induced by neuroinflammation in neurodegenerative diseases.