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.