The effects and potential of microglial polarization and crosstalk with other cells of the central nervous system in the treatment of Alzheimer’s disease

Yi-Ge Wu , Li-Juan Song , Li-Jun Yin Jun-Jun Yin , Qing Wang Jie-Zhong Yu, Bao-Guo Xiao, Cun-Gen Ma ,

Abstract Microglia are resident immune cells in the central nervous system.During the pathogenesis of Alzheimer’s disease, stimulatory factors continuously act on the microglia causing abnormal activation and unbalanced phenotypic changes; these events have become a significant and promising area of research.In this review, we summarize the effects of microglial polarization and crosstalk with other cells in the central nervous system in the treatment of Alzheimer’s disease.Our literature search found that phenotypic changes occur continuously in Alzheimer’s disease and that microglia exhibit extensive crosstalk with astrocytes, oligodendrocytes, neurons, and penetrated peripheral innate immune cells via specific signaling pathways and cytokines.Collectively, unlike previous efforts to modulate microglial phenotypes at a single level, targeting the phenotypes of microglia and the crosstalk with other cells in the central nervous system may be more effective in reducing inflammation in the central nervous system in Alzheimer’s disease.This would establish a theoretical basis for reducing neuronal death from central nervous system inflammation and provide an appropriate environment to promote neuronal regeneration in the treatment of Alzheimer’s disease.

Key Words: Alzheimer’s disease; amyloid; biomarker; central nervous system; cytokines; diabetes;inflammation; microglia; neuroinflammation; phagocytosis; tau

Introduction

Microglia as disease modifiers in Alzheimer’s disease

Alzheimer’s disease (AD), a degenerative disorder of the central nervous system (CNS) with increasing cognitive dysfunction and behavioral impairment as the primary clinical indications, is gaining significant attention in modern society (Dong and Chai, 2013; Tublin et al., 2019; Klegeris et al., 2021; Yang et al., 2021; Wei et al., 2021).Microglia, as intrinsic immune cells in the CNS, can activate inflammatory signaling pathways and participate in the genesis and progression of neurodegeneration, thus affecting the prognosis of patients with AD (Doust et al., 2021).Based on specific features, activated microglia can be divided into two primary groups: M1 and M2 (Franco et al., 2021).However,there are some limitations related to the M1/M2 classification (Tang, 2018).On one hand, this simple classification fails to fully express the high complexity of microglial function (Ransohoff, 2016).On the other hand, the discontinuity between phenotypic markers and functionality lacks precision when targeting M1/M2 to develop neuroinflammatory and immunomodulatory therapies(Devanney et al., 2020).Since microglial polarization is a highly dynamic process, investigating microglial alterations may result in a new era of AD prevention and treatment (Wright-Jin et al., 2019).

Polarization of microglia occurs on a continuous scale

Microglia polarize due to their variability and flexibility, developing into two distinct subpopulations with dramatically diverse phenotypes and functions,including pro-inflammatory or anti-inflammatory effects.Traditionally, these subpopulations are classified as the activated M1 phenotype (also known as the cytotoxic M1 type) and the activated M2 phenotype (also known as the cytoprotective M2 type) (Cornell et al., 2022).The signature molecules of the M1 type are major histocompatibility complex II, cyclooxygenase 2, inducible nitric oxide synthase (iNOS) and cluster of differentiation (including CD3,CD6, CD32, and CD86).The M1 phenotype can be activated by interferon-γ(INF-γ) and lipopolysaccharide (LPS) and produce tumor necrosis factor-ɑ(TNF-ɑ), interleukin-1β (IL-1β), IL-6, IL-12, nitric oxide (NO), and reactive oxygen species (ROS).NO and ROS are neurotoxic substances that promote inflammatory responses and aggravate neurological damage (Chhor et al.,2013; Sarlus and Heneka, 2017; Shi et al., 2019).The signature molecules of the M2 phenotype are CD206, insulin-like growth factor-1 (IGF-1), arginase-1,P2X4 receptor, and FIZZ1 (Yao and Zhu, 2019).The M2 phenotype can also secrete IL-4, IL-10, transforming growth factor-β (TGF-β), vascular endothelial growth factor (VEGF), brain-derived neurotrophic factor (BDNF), and other cytokines that can exert an anti-inflammatory response, engulf damaged neuronal fragments, and promote neural repair (Koh et al., 2018; Luo et al.,2020).

This simplified dichotomy is still applied in both clinical and experimental studies.However, in terms of current research advances (McQuade and Blurton-Jones, 2019), microglia in AD are known to be present in multiple different activation states and the continuum of microglial polarization can be demonstrated by describing their changes in continuity (Figure 1).Currently,the perception of pro-inflammatory M1-type microglia is often linked to chronic neuroinflammation in AD, in which the deposition of beta-amyloid(Aβ) increases with increased levels of inflammation in the CNS (Ozben and Ozben, 2020).Even the anti-inflammatory substances produced by the anti-inflammatory M2 microglia are not fully beneficial for AD (Guillot-Sestier etal., 2015) and the over-polarization of microglia toward anti-inflammatory polarization, as shown below in Figure 1 may be harmful.Therefore, in order to treat AD, it is necessary to instill a delicate balance by altering microglial polarization throughout the different stages of AD.

Figure 1｜ Polarization of microglia occurs on a continuous scale.

The theory of incorporating changes in glycolytic metabolism of the microglia in line with changes in oxidative phosphorylation (OXPHOS) into the classification of polarization may better illustrate that microglia in the AD state are in a dynamic continuum of change.Microglia that undergo glycolytic metabolism via the uptake of glucose are more likely to exhibit potential for pro-inflammatory polarization; this may be related to the fact that glycolysis promotes the release of pro-inflammatory factors via the NF-κB pathway (Preeti et al., 2021).A recent study also demonstrated that glycolysis exacerbates microglia dysfunction in AD (Pan et al., 2022).In contrast, the shift from glycolysis to OXPHOS in microglia attenuates the production of pro-inflammatory cytokines and increases the release of anti-inflammatory cytokines (Devanney et al., 2020).The dynamic changes of microglia in terms of glycolytic metabolism can also serve as a reference for phenotypic and functional changes.With this as a reference, perhaps changes in amino acid metabolism and changes in lipid metabolism could also be evaluated as potential markers of change in microglia continuity in AD (Zádori et al., 2018;Charrière et al., 2021).

Over recent years, the field of epigenetics has induced significant interest in microglia due to their high plasticity and complexity.Both the phenotype and functionality of microglia are strongly influenced by epigenetics.Therefore,exploring the specific molecular mechanisms underlying the epigenetic regulation of microglia may provide a basis for understanding the continuum of microglial changes (Martins-Ferreira et al., 2020; Wang et al., 2021).The changes of microglial phenotypic continuity in AD can be regulated via DNA methylation, histone modifications, and non-coding RNAs.The search for phenotypes related to microglia-mediated neuroinflammation could help us to identify new avenues of AD treatment, and the full use of neuroprotective epiphenomenal drugs could then inhibit the pro-inflammatory polarization of microglia (Mota et al., 2020), perhaps alleviating the chronic inflammation during AD development.The activation of receptors expressed on myeloid cells 2 (TREM2) plays a key role in the development of AD; DNA methylation in microglia is known to regulate the expression of TREM2 (Celarain et al.,2016), thus creating neuroprotective effects.Conversely, miR-34a can also act as an epigenetic regulator to attenuate TREM2 expression to induce neuroinflammation and the deposition of Aβ (Alexandrov et al., 2013).Betasite APP cleaving enzyme-1 (BACE-1) is expressed on microglia and thought to be closely related to AD; the inhibition of its expression by DNA methylation contributes to a shift of microglia phenotype toward anti-inflammatory properties, potentially facilitating the clearance of Aβ and exerting therapeutic effects on AD (Byun et al., 2011; Hampel et al., 2021).The findings of these previous studies may help us to understand the role of epigenetic modifications in the phenotypic changes of microglia, and in combination with changes in the levels of metabolic markers, may better illustrate that microglia in AD are in a continuous process of change, thus providing a potential strategy for modulating microglia phenotype and functionality to mitigate the development of AD.

Purpose of this review

Here, we intended to summarize recent literature and explore microglial changes at different stages of AD and interpret the mechanisms underlying the dialogue between microglia and other cells to provide guidance for alleviating microglia-dominated CNS inflammation and nerve regeneration.

Search Strategy

We began with an extensive search strategy to conduct a comprehensive review of the relevant literature published between 2017 and April 2022.By using specific search terms ((Microglia) AND (Alzheimer’s disease)) AND(English [Language])) in PubMed, we aimed to investigate the influence of AD microglia-mediated intercellular dialogue on the occurrence and development of AD.When reading the titles and abstracts of articles, we used a series of additional keywords (astrocytes, neurons, oligodendrocytes, and peripheral lymphocytes) with particular focus on the mechanisms underlying the dialogue between microglia and these cells and the role of secreted cytokines in AD.If there are few related articles following the strict search criteria, we adjusted the search conditions manually; for example, we relaxed the search period so as to obtain more relevant literature.In addition, to map the development of studies related to AD microglial phagocytosis, we searched the WoSCC database by limiting the subject words ((Microglia)AND (Alzheimer’s disease) AND (Phagocytosis) AND/OR (migration).Through intelligent screening and manual browsing, we analyzed studies relating to phagocytosis and microglia in AD.

Migration and Phagocytosis of Microglia in Alzheimer’s Disease

The senile plaques (SPs) generated by the continuous deposition of Aβ and the neurofibrillary tangle (NFT) formed by tau hyperphosphorylation are the most common pathogenic hallmarks of AD (Seol et al., 2020).Aggregated Aβ causes downstream reactions that result in chronic neuroinflammation, tau protein hyperphosphorylation, and the loss of synaptic and neuronal function(Scheltens et al., 2016; Rangaraju et al., 2018).

Microglia, as the main immune cells responsible for maintaining homeostasis in the CNS, continuously monitor signs of damage (such as pathogens or tissue damage) and then generate a series of responses to address the damaging factors that play an important role in immune defense, such as cellular debris removal along with synaptic maturation and construction (Li et al., 2018; Weinhard et al., 2018).Danger-associated molecular patterns(DAMPs), including beta-amyloid and hyperphosphorylated tau, continuously excite microglia during the pathogenesis of AD (Lau et al., 2021).Activated microglia subsequently constrict their cellular extension and change into round migratory cells, known as “amoeboid” cells (Das et al., 2020); these exhibit migratory and phagocytic capabilities, wrapping Aβ and degrading other substances (Huang et al., 2021), to influence the prognosis of AD.

The migratory role of microglia in AD

The formation of amyloid plaques and tau protein tangles in the brain are typical pathological features of AD and microglia usually aggregate around amyloid plaques and tau proteins due to their migrational ability in AD patients (Zhang et al., 2019).One of the major processes by which microglia transit from a resting to an active state is migration and aggregation (Yu et al.,2020).Previous studies have revealed the migratory response of microglia is caused by Aβ, as well as subsequent cytotoxic and inflammatory responses;however, the specific mechanisms underlying migration remain unknown.The migratory effects of microglia have been linked to the release of several cytokines and the activation of numerous signaling pathways (Chidambaram et al., 2022).

Microglia reside in the CNS in the physiological state and are sometimes referred to as resting microglia (Fan et al., 2017); these have small cytosomes with elongated projections and many branches on the surface and operate as gatekeepers for the CNS (Kreutzberg., 1996; Wang et al., 2020).The microglia constantly move and can sense changes in the surrounding environment and maintain homeostasis

in vivo

.However, microglia in pathological states such as AD are activated by a variety of factors and related various receptors on the surface, such as TREM2 and Toll-like receptors (TLRs); these can bind to each other with Aβ and apolipoprotein E (ApoE) and then migrate directionally to the site of injury (Jay et al., 2017; Ulrich et al., 2018).Both ionotropic P2X and metabotropic P2Y purinoceptors that are expressed on microglia play an important role in neurodegenerative and neuroinflammatory diseases,thus contributing to microglia activation and migratory movements (Inoue,2008; Salcman et al., 2021).This process of migration shows gradient changes in migration agents (adenosine diphosphate and adenosine triphosphate).The stimulation of microglia by adenosine diphosphate activates Gi-coupled P2Y receptors (e.g., P2Y12) thus leading to Akt phosphorylation by inducing protein kinase B (PKB) phosphorylation and PI3K activation (Sipe et al., 2016;Whitelaw, 2018).The PI3K-Akt signaling cascade as a regulator of microglia polarity and tropism is one of the most critical components.Similarly,fractalkine and its receptor Cx3cr1, along with IL-18 and its receptor IL-18R, are also known to be the main drivers of migratory movements in the microglia (Rezai-Zadeh et al., 2011; Zhang et al., 2018a; Ernest et al., 2021)and cause microglia migration by influencing microglia morphology.

Phagocytosis of microglia in AD

Extracellular Aβ deposition in the brain is frequently accompanied by gliosis and lipid deposition during AD pathogenesis, supporting the persistence of the inflammatory response (Domingues et al., 2017).Furthermore, microglia can recognize amyloid by receptors such as TREM2, TLRs, and complement receptor 1 CR1 (CD33), thus resulting in a distinct signaling cascade.This process drives microglia to recognize and migrate toward the lesion (Keren-Shaul et al., 2017; Ulland et al., 2018; Zhao, 2019; Zhou et al., 2020; Das and Chinnathambi, 2021).Then, microglia begin to engulf SP and other pathogenic products via phagocytosis and microphagocytosis (Teh et al.,2016; Podleśny-Drabiniok et al., 2020).Within 1–2 days of their appearance,SPs become surrounded by microglia; these microglia contain more amyloid-binding dye, thus providing direct evidence that the microglia engulf Aβ(Bolmont et al., 2008).Invagination of the cell membrane leads to vesicle formation and phagocytosis permits soluble Aβ and tau to be absorbed into the microglia.Furthermore, the cytoskeleton is specifically recognized by immune substrates and related receptors (Villani et al., 2019).After binding to intracellular substances and being fused with endosomes and lysosomes,the vesicles and their contents are destroyed by acid hydrolases and histolytic enzymes in the lysosomes; anti-inflammatory mediators are released and lipid clearance is improved (Doran et al., 2020).

Microglia can directly phagocytose AD toxic products such as Aβ and tau proteins, which appears to be advantageous in AD therapy.Recent research has revealed, however, that microglial phagocytosis is not always beneficial,and is to a certain extent related to the specific clinical stage (Anwar and Rivest, 2020; Qiao et al., 2021).Enhanced microglial phagocytosis reduces Aβ levels, slows down the formation of SP, and prevents the generation of NFT in patients with only pathophysiological changes of AD but without or only mild clinical symptoms, all of which contribute markedly to stopping the onset and delaying the clinical progression of AD (Wang et al., 2016).However, microglia have decreased capacity for phagocytosis after engulfing Aβ and tau due to the presence of fewer lysosomes (Carosi et al., 2019).Importantly, after phagocytosis, an inflammatory vesicle cascade (e.g., NLRP3) and the presence of inflammatory cytokines (e.g., TNF-α and IL-1) produced by cellular metabolism cause microglia to polarize to a pro-inflammatory direction, thus inducing the secretion of pro-inflammatory cytokines, the exacerbation of the inflammatory response, the reduction of Aβ phagocytosis and the promotion of a pathological response to tau (Ising et al., 2019; van Olst et al., 2020).Microglial phagocytosis decreases with increasing age or prolonged exposure to increasing Aβ load (Rajendran et al., 2018).

Collectively, these findings show that in the pathological process of AD,microglia phagocytosis is enhanced by Aβ activation in the early stages and generates a neuroprotective effect.However, in later stages, the microglia have a harmful effect by releasing pro-inflammatory factors and aggravating neurological damage due to their phenotypic changes.According to our retrieval strategy, we outlined the study timelines and selected the landmark articles on AD microglial phagocytosis; please refer to Figure 2.

Crosstalk between Microglia and Other Cells in Alzheimer’s Disease

Different types of glial cells in the CNS, including microglia, astrocytes, and oligodendrocytes, exert different functions and coordinate CNS homeostasis with neurons via multiple inter-cellular crosstalk mechanisms referred to as neuroimmunity (Delpech et al., 2019).Cells have the ability to undergo crosstalk with each other to magnify their functionality (Tang et al., 2019;Castellani et al., 2020; Lana et al., 2021).Thus, when disease affects one of these cells, they frequently affect other cells, either directly or indirectly(Figure 3).

Figure 2 ｜ Study timeline and landmark studies relating to the role of microglial phagocytosis in AD.

Figure 3 ｜ Crosstalk between microglia and other cells in Alzheimer’s disease.

Crosstalk between microglia and astrocytes in AD

The activation and polarization of microglia and astrocytes are often thought to exacerbate the development of AD and aging (Pan et al., 2020).Although the underlying cellular and molecular mechanisms remain unclear, multiple cytokines and signaling pathways are known to be involved in the activation of microglia and astrocytes in AD.Furthermore, the activation and polarization of microglia often contribute to the activation of astrocytes, leading to toxicity to neurons and aggravating the inflammatory environment (Fakhoury, 2018;Arranz and De Strooper, 2019; Park et al., 2021).As a result of astrocyte activation, more microglia are shifted from a resting state to an activated state.

There is crosstalk and communication between these two types of glial cells in AD (Guttikonda et al., 2021; McAlpine et al., 2021).Multiple inflammatory factors, along with Aβ, can activate primary microglia toward pro-inflammatory polarization; the converse is equally true, that is, polarized microglia communicate with astrocytes via inflammatory crosstalk andother mechanisms to activate and polarize astrocytes (Xie et al., 2020).The pathological products (such as Aβ and tau) of AD, contribute to microglial polarization, making the pro-inflammatory phenotype of microglia dominant and inducing the secretion of pro-inflammatory factors such as IL-1β, IL-6, TNF-ɑ, and NO (Tiwari et al., 2019).These not only reduce microglia phagocytosis, they also trigger astrocytes to a pro-inflammatory phenotype and increase the levels of pro-inflammatory factors (Liddelow et al., 2017;Dionisio-Santos et al., 2019).Similarly, the activation of astrocytes contributes to microglial polarization in AD.When pathological products, including Aβ,are stimulated, NF-κB-related signaling pathways in astrocytes are activated,thus leading to the release of complement C3.This activates C3 receptors on the surface of the microglia and causes pro-inflammatory polarization and impaired phagocytosis (Lian et al., 2016).In brief, inflammatory interaction between the microglia and astrocytes occurs during the evolution of AD(Rostami et al., 2021).

There is also evidence that astrocytes can efficiently inhibit the deposition of Aβ protein by the IKK2/NF-kB signaling pathway and that this is closely related with microglial polarization (Yang et al., 2021).In addition, neuroprotective astrocytes release several anti-inflammatory molecules such as TGF-β, IL-4, and IL-13, thereby enhancing microglia autophagy; this ameliorates the cognitive impairment caused by AD (Liddelow and Barres, 2017; Qin et al.,2020).Although it is debatable whether astrocytes can directly alter the Aβ produced in AD,

in vitro

studies have shown that reactive astrocytes can scavenge Aβ and minimize inflammatory damage (Liu et al., 2016; Xiang et al., 2021).The crosstalk between astrocytes and microglia may provide new concepts for the treatment of AD.

Crosstalk between microglia and oligodendrocytes in AD

Oligodendrocytes play a key role in maintaining the function and integrity of axons and myelin sheaths and are also thought to be involved in AD(Skaper et al., 2018; Selkoe DJ and Hardy, 2016).In the physiological state,oligodendrocytes are closely related to the generation of steroids; these help to protect nerves from the damage caused by degenerative diseases (Garcia-Segura and Balthazart, 2009; Chavda et al., 2016).The crosstalk between microglia and oligodendrocytes reduces neuroinflammation and prevents pro-inflammatory microglial polarization due to the neuro-anti-inflammatory effects of steroids (Chavda et al., 2022).Similarly, increasing the differentiation of oligodendrocyte upregulates the production of IGF-1 (Kabir et al., 2021);this is beneficial for treating AD.Although there have been no experimental results to support the fact that Aβ or tau may cause direct damage to oligodendrocytes (Chen et al., 2020), some researchers found that excessive Aβ or ApoE causes the loss of integrity in myelin and can induce apoptosis in oligodendrocytes (Desai et al., 2010; Garcia-Leon et al., 2020; Li et al., 2020).Previous studies have shown that oligodendrocytes may play an important role in tau accumulation in AD (Ferrer, 2018; Nasrabady et al., 2018).Singlecell sequencing techniques in the brains of AD patients have revealed that oligodendrocytes exhibit the most individual variation of the major types of brain cells (Garcia-Segura and Balthazart, 2009; Ferrer et al., 2019).In addition, it has been hypothesized that inflammation reduces the ability of oligodendrocytes to form myelin (Grubman et al., 2019); this may provide new ideas for treating AD by interfering with the crosstalk between microglia and oligodendrocytes.Since attention has been focused on the function of oligodendrocytes in maintaining the integrity of the myelin sheath in the past,many investigations have been directed toward neuroinflammation-oriented diseases such as multiple sclerosis (MS).Oligodendrocytes in AD have only been viewed as potential neuroprotective targets (Franklin and Goldman,2015); therefore, studies of microglial crosstalk have only just begun.

Crosstalk between microglia and neurons in AD

The crosstalk between microglia and astrocytes plays a vital function in the occurrence of inflammation in AD.However, the crosstalk between the microglia and neurons is also important for pathogenic and immunological responses throughout AD development (Lorenziniy et al., 2020; Simon et al.,2019) since many molecules mediate bidirectional communication between the microglia and neurons in a healthy brain.On one hand, neurons influence microglial function through neurotransmitters such as glutamate, dopamine,γ-aminobutyric acid (GABA), ADP, adenosine, and migratory factors that contribute to the maintenance of the neuroprotection provided by the microglia (Tajbakhsh et al., 2021).On the other hand, microglia-secreted cytokines such as GDNF and IGF-1 are advantageous to the physiological function of neurons and play a key role in learning and memory (Czapski and Strosznajder, 2021).

Glutamate, as the most important excitatory neurotransmitter in the CNS, is directly involved in the crosstalk between microglia and neurons.Interestingly, maintaining appropriate levels of glutamate transmission in physiological states is conducive to maintaining CNS homeostasis; too much or too little glutamate release often has serious consequences for neurotransmission, leading to the development of AD (Parkhurst et al., 2013).However, in neuroinflammatory and AD conditions, microglia tend to release excessive glutamate, leading to neuronal excitotoxicity in AD and exacerbating neurodegenerative processes (Bukke et al., 2020; Stolero and Frenkel, 2021).The cystine (Cys)/glu exchanger (Xc(-) exchanger), as well as d-serine release in response to inflammatory stimuli, directly affect the levels of glutamate in the CNS (Wang and Reddy, 2017; Lewerenz et al., 2013; Sears et al., 2021;Ploux et al., 2021).The overexpression of ionotropic glutamate receptors leads to neuronal death via excitotoxicity.Furthermore, the activation of glutamate receptors in the microglia exacerbates the inflammatory response in AD undoubtedly causing further damage to neurons (Goshi et al., 2020).

Under the conditions of persistent chronic inflammation in AD, the release of inflammatory factors such as TNF-α, IL-6, NO, and ROS increases neuronal death as the high levels of pro-inflammatory factors drive microglia toward pro-inflammatory polarization (Lull et al., 2010; Henstridge et al., 2019).The loss of homeostasis in the microglia induces the further release of proinflammatory factors and more neuronal death, thus leading to a vicious cycle(Spangenberg and Green, 2017).The crosstalk between microglia and neurons has been outlined in multiple stages over recent years by the generation of three-dimensional models of AD in the human brain.Inflammatory factors,such as the NO produced by the microglia, have been shown by modeling to be destructive to neurons via recruitment and the activation of polarization(Henstridge and Spires-Jones, 2018; Park et al., 2018).As AD progresses,neurons are damaged by the inflammatory environment induced by proinflammatory polarized microglia.The neurons themselves are also induced to exhibit various pathological features due to the accumulation of Aβ (De Strooper and Karran, 2016; Dietrich et al., 2018).The weakened neuronal function, in turn, has a significant impact on microglial polarization.The interaction between microglia and neurons resembles two ends of a balancing beam; injury to one end wreaks havoc on the other end.The activation of microglia is not a single pro- or anti-inflammatory polarization, but rather a series of changes in which cellular states differ and overlap; consequently,there may be an eventual shift to a chronic inflammatory state (Propok et al.,2013; Merlo et al., 2020).This may explain why, as AD advances, the microglia become more pro-inflammatory, and more neurons die, thus resulting in the progressive aggravation of clinical symptoms in AD patients.

Crosstalk between microglia and peripheral innate immune cells in AD

The blood-brain barrier (BBB), a continuous endothelial membrane within the brain microvasculature, maintains a highly controlled internal CNS environment that is required for optimal synaptic and neuronal activity(Kisler et al., 2017; Sweeney et al., 2018).However, imaging detection in the early stage of AD indicates BBB disruption, initially focused in certain gray and white matter sites (Montagne et al., 2016; van de Haar et al., 2016).This becomes more widespread as AD develops (van Assema et al., 2012).Once damaged, the BBB allows a flood of peripheral cells into the brain that become engaged in inflammatory and immunological responses.Because hematopoietic bone marrow cells travel to the brain during embryonic development to eventually specialize and differentiate into microglia, they are also known as resident CNS macrophages (Gate et al., 2010).The relationship between peripheral macrophages and microglia has been investigated for decades.Surprisingly, peripheral macrophages do not only penetrate the CNS for Aβ phagocytosis when the BBB is destroyed, they also phagocytose much more Aβ in the CNS than the microglia (Wisniewski et al., 1991; Krishnan et al., 2020).Rapid phenotypic transition and the antigens on the cell surface distinguish microglia from peripheral macrophages.Furthermore, peripheral macrophages are easier to reach and modify than microglia when blocked by the BBB (Finneran et al., 2019), thus suggesting that their anti-inflammatory expression could help to redirect microglial polarization.

During neuroinflammation, bone marrow-derived monocytes rapidly infiltrate the CNS in the pathological state through the BBB in a chemokine receptor 2 (CCR2)-dependent manner and then mix with the resident microglia population (Spiteri et al., 2020).Monocytes participate in the pathological process in the CNS by producing macrophage infiltration during neuroinflammation.The differential expression of chemokine (C-X3-C motif) receptor 1 (CX3CR1) and the CCR2 ligand can selectively recruit “proinflammatory” or “pro-regression” monocyte-derived cells to mediate secondary injury (Shechter et al., 2013; Jordãoi et al., 2019).As monocytes are recruited from the periphery to the CNS during the neuroinflammation associated with AD, there are significant effects on the infiltration and differentiation of macrophages in the CNS under the inflammatory state(Spiteri et al., 2020).

A mounting body of evidence implies that peripheral immune cells infiltrate the CNS via the damaged BBB to regions of disease and then release cytokines such as IL-6, IL-1, and TNF-α (Laurent et al., 2016; Dionisio-Santos et al., 2019; Wyatt-Johnson et al., 2020).T lymphocytes migrating into the brains of AD patients, for example, are specifically capable of mounting an enhanced response to Aβ (Togo et al., 2002; Wu et al., 2021).In addition,various types of Aβ-specific T cells may promote enhanced Aβ clearance,thus attenuating the inflammatory milieu in the CNS, or even reversing the reduced cognitive abilities (Cao et al., 2009; Vacinova et al., 2021).Microglia can activate T cells to exert an immunological response by phagocytosing Aβ and delivering it to T cells (Schetters et al., 2018).Furthermore, in AD, helper T lymphocytes recruited through the periphery undergo specific crosstalk with microglia to stimulate cellular responses to noxious stimuli by producing the effector cytokine IFN-γ and activating macrophages and CD8T cells.However, the specific mechanisms underlying the functionality of helper T lymphocytes remains unknown although some researchers believe that in AD, the IFN-γ produced by helper T lymphocytes helps microglia to surround Aβ plaques and accelerate clearing (Fisher et al., 2010; Browne et al., 2013;Evans et al., 2019).Nevertheless, others believe that helper T lymphocytes play an intrusive role in AD, thus causing negative consequences via common leukocyte recruitment processes (Gate et al., 2020; Rossi et al., 2021).Although there is no direct evidence that helper T lymphocytes contribute to AD progression, findings from other neuroinflammatory diseases, such as MS, show that adjuvant T lymphocytes produce both IL-17 and IFN-γ (Zhang et al., 2013; Ashtari et al., 2019), which exacerbate inflammation in the CNS.This suggests that Th1 and Th17, two types of helper T lymphocytes,may accelerate the progression of AD.While the precise role of cytotoxic Tlymphocytes in the development of AD remains unknown, the infiltration observed in AD patients may indicate that cytotoxic T cells are involved (Lueg et al., 2015; Unger et al., 2020).Thus, cytotoxic T cells and helper T lymphocytes appear to drive the development of AD and additional CNS injury through crosstalk with microglia that perpetuates chronic inflammation.

Potential for Alzheimer’s Disease Therapy:Reorienting Microglial Polarization

Although the mainstream amyloid cascade theory of AD causation was established two decades ago, the accumulation of Aβ plaques and aberrant phosphorylation of intracellular tau proteins leading to neurological impairment have long been considered essential components in AD (Cline et al., 2018).However, none of the therapeutic options that target just Aβ or tau has been successful (Cummings et al., 2018).The activation and polarization of microglia have long been thought to be a detrimental feature in AD.However, current research suggests that the early activation of microglia and the alteration of polarization may be advantageous in the disease progress.Experiments based on

in vitro

models or animal models and involving the manipulation of microglia metabolism have shown that adjusting the direction of microglial polarization has a therapeutic effect on AD.

In terms of affecting microglia metabolism, unsaturated fatty acids can bind to G protein-coupled receptor 120 on the microglia membrane, polarizing the microglia toward anti-inflammatory properties and participating in anti-inflammatory effects (Osborn and Olefsky, 2012).Because of its own doublebonded carbon chain structure, this receptor can be inserted into the cell membrane of microglia as a long-chain fatty acid, thus affecting the curvature of the cell membrane of microglia, exerting effects on receptor signaling on the membrane and increasing phagocytosis (Layé et al., 2018).The levels of anti-inflammatory factors such as IL-10 and TGF-β expressed by microglia after unsaturated fatty acid treatment were found to be remarkably elevated(Babić et al., 2020).IL-10 mediates the repair of neuronal damage in AD and has a role in maintaining immune homeostasis.TGF-β can prevent Aβfrom inducing AD.The expression of CD206 and arginase-1, phenotypic molecules of anti-inflammatory microglia, was also increased after unsaturated fatty acid treatment.Taking docosahexaenoic acid (DHA) as an example, DHA, as a typical omega-3 fatty acid, is closely related to microglial phagocytosis.DHA affects the expression of ApoE, which protects brain cells and lowers the incidence of sporadic AD (Fumagalli et al., 2018; Sala-Vila et al., 2021).As a result, unsaturated fatty acids suppress inflammatory signaling and promote the anti-inflammatory polarization of microglia (Bruce et al., 2018).

In addition, the hypoglycemic agent vincristine can effectively counteract AD caused by high levels of fat and sugar since it can reduce the inflammatory response in AD rats (Yossef et al., 2020) and prevent the pro-inflammatory polarization of microglia, thereby playing a neuroprotective and anti-apoptotic function by regulating metabolic levels and restoring normal metabolism.The integrity of microglial mitochondrial function influences the direction of activation and polarization of the microglia.In AD, microglia often suffer from mitochondrial damage, abnormal oxidative phosphorylation, and glycolytic energy supply (Fleck et al., 2019; Nolfi-Donegan et al., 2020).Enhanced glycolysis exacerbates the pro-inflammatory polarization of microglia, thus releasing inflammatory factors and neurotoxic substances to aggravate AD.From this perspective, mitochondrial division inhibitor 1 (Mdivi-1) prevents LPS-induced mitochondrial breakage, restores the oxygen consumption rate (OCR) and extracellular acidification rate (ECAR), thus resulting in the inhibition of iNOS and COX-2 expression.Ultimately, these events reduce the release of pro-inflammatory factors (Nair et al., 2019).The Rho kinase inhibitor fasudil hydrochloride may play a role in protecting mitochondrial function since it maintains the oxidative phosphorylation of microglia, reduces the expression of inflammatory factors and polarizes microglia toward an anti-inflammatory form by inhibiting the NF-kB signaling pathway.As a result,polarized microglia exert anti-inflammatory, neurophagic and neuroprotective effects (Zhang et al., 2018c).Progesterone, an endogenous steroid, plays a key role in maintaining the functional integrity of mitochondria in the CNS;this is especially important for the anti-inflammatory polarization of microglia(Gaignard et al., 2018a, b).

Traditional Chinese medicine has received considerable research interest over recent years and has been used in studies on microglial polarization in AD.Curcumin, a natural substance derived from the Chinese herb turmeric,improves microglia phagocytosis by raising the expression of the innate immunity genes

TREM2

and

TYROBP

while reducing the expression of CD33 (Teter et al., 2019).Curcumin is also an anti-inflammatory drug that enhances resistant microglia viability while inhibiting NO and the release of inflammatory cytokines by LPS-induced microglia (Tang and Taghibiglou,2019).Jujuboside A (JuA) is an anti-inflammatory, antioxidant, and neuroprotective compound derived from the Chinese herbal medicine jujube kernel.This improves cognitive impairment by increasing the expression of heat shock protein (HSP) 90 mostly via Axl/ERK-dependent signaling; it can also restore PPAR content and function in microglia, polarize microglia toward anti-inflammatory characteristics, and promote the phagocytosis of Aβ plaques (Zhang et al., 2018b).Ginsenoside is one of the main active ingredients in the Chinese medicine ginseng; its degradation product ginsenoside Compound K [20-O-β-D-glucopyranosyl-20-(S)-Protopa naxadiol,CK] can inhibit the neurotoxicity of Aβ, reduce inflammation, and delay the pro-inflammatory polarization of microglia, thus playing a neuronal protective effect.The main mechanisms involved may be related to inhibition of the release of pro-inflammatory factors, increasing the levels of anti-inflammatory factors, reducing the inflammatory response of microglia induced by Aβ,and inducing the conversion from pro-inflammatory M1 to anti-inflammatory M2 microglia (Lee et al., 2013; Ahn et al., 2021).

Discussion

Summary

Finding a promising approach for the treatment of AD has become more challenging due to the complexity and diversity of the CNS, as well as the unpredictability of the specific course of AD.Microglia, which play key roles in all phases of AD and are particularly prevalent in the neuroinflammation produced by AD, have attracted significant attention in recent studies.However, because of the phenotypic diversity of microglia after activation, it may be possible to maintain a delicate equilibrium by targeting and regulating polarization-related cytokines and metabolic pathways so that the microglia can fully exploit their phagocytic and reparative effects without causing neurotoxicity.Of course, this is simply a prospective therapeutic option for AD; the real difficulty is figuring out how to employ the polarization of microglia to intervene AD pathology at the right time as well as the impact of microglia crosstalk with other CNS cells during the disease.

Collectively, research efforts have shown that the intercellular dialogue of microglia in AD plays a key role in inflammation in the CNS.However, this is not limited to previous studies of crosstalk between microglia and specific cells.The present review focuses upon the dominant role of microglia and their crosstalk with other cells in the inflammatory environment of AD.By exploring the mechanisms and specific signaling pathways underlying the crosstalk between microglia and other cells, we provide a theoretical basis for alleviating neuroinflammation.Reducing neuroinflammation is undoubtedly of great significance for investigating nerve regeneration.By regulating the crosstalk between microglia and other cells, it may be possible to reduce the neuronal death caused by inflammatory injury, but also provide a good environment for the subsequent regeneration of new neurons.Microglia pose a challenge to this goal because of their unique continuum of changes;furthermore, not all microglia are simultaneously in an anti-inflammatory/pro-inflammatory state.Moreover, as AD is often accompanied by destruction of the BBB, it is difficult to predict the crosstalk between peripheral cells and intrinsic cells in the CNS.Whether these cells are beneficial to the treatment of AD remains controversial; this has become a stumbling block on the road to dedicated research efforts.

Limitations and outlook

There are still some limitations that need to be considered.For example, our selection of keywords may have led to some relevant literature being omitted.As a result, the crosstalk between microglia and other cells cannot be fully described in this review.However, despite these limitations, we believe that this review can provide new concepts for researchers.As Frozza et al.(2018)suggest, subtle advances in AD therapeutic strategies, even with a slight delay in onset time, can reduce the overall burden of disease.Intervening with the polarization of microglia to delay AD progression may pave the way for successful AD therapy in the future.

Author contributions:

YGW, LJS, LJY, JZY and QW contributed to research and assessment of available literature.YGW mainly wrote the manuscript.LJS, JJY,BGX, and CGM interpreted the results of previous studies.CGM finalized the manuscript.All authors approved the final version of the manuscript.

Conflicts of interest:

The authors have no conflicts of interest to declare.

Open access statement:

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