Neurobehavioral disorders
Impairment in behavioral items, such as socialization, language skills, intelligence, and limited or repetitive behavioral patterns along with movement disabilities are common traits of neurological disorders, such as autism spectrum disorders (ASD), epilepsy, subtypes of major depressive disorders (MDDs), and schizophrenia [
129]. Several hypotheses have attempted to shed light on the pathophysiology of these conditions. Especially for ASD, the central cohesion theory (CC) has been proposed, which attributes the key impairments to a specific perceptual-cognitive theme described as dampened context comprehension. In other words, this theory refers to the inability to "see the bigger picture" in ASDs [
130]. Another prevailing theory is the hypo-connectivity model. It intends that remote brain areas (e.g., the parietal and frontal lobes) show underconnectivity, while the local neural networks (e.g., in the frontal lobe) show overconnectivity. The third theory suggests that excitation–inhibition imbalances in the neural network could play a role in ASD [
131].
Researchers have recently focused on the implication of neurodevelopmental processes in the course of mental illnesses [
129,
132]. Factors predisposing individuals to cognitive disorders include defective synaptic maturation, as shown by impaired functional connectivity in the neocortex. The precise mechanism is not clear-cut, but viewing the cognitive deficits through the lens of genetic disorders has enabled the researchers to identify correlations between the presence of misfolded scaffolding proteins and autism. Also, the autopsy confirmed decelerated transcription of synaptic transmission-related genes among patients with neuropsychiatric disorders [
132].
Postmortem examinations corroborated the presence of dysmorphic microglia in the brain of patients with mental diseases. Further, an explanation for these morphological alterations in microglia is primary neuronal impairment. Other scientists believe the cause rests on the influence of environmental antigens on CNS immune replies [
133]. Another igniting vision is considering the microglia as the primary culprits in neurobehavioral disorders. As discussed, microglia are the cornerstone in the maturation process of synaptic functional connectivity boosting synaptic quantity, which is markedly attenuated in the pathophysiology of ASD and other cognitive deficit disorders [
134].
Many novel studies have focused on the roles of microglial phenotype dynamics in neurodevelopmental conditions. However, the causal relationship remains to be deciphered, whether the microglia activation precedes the neuronal deficits or occurs secondary to neuronal loss [
135]. Zhan et al. explored the influence of primary microglia damage on the development of autistic characteristics. A CX
3CR1-KO mice model was utilized in a recent study, which induces a transient deficit in microglia function, and thereby dampens synaptic elimination. Insufficient synaptic pruning during the early postnatal developmental period results in a significant load of inefficient excitatory synapses. During the first weeks of life in wild-type (WT) rodents, spontaneous excitatory postsynaptic currents (sEPSCs) exert higher amplitudes, mirroring the synaptic sprouting and boosted synaptic multiplicity [
136]. In CX
3CR1-KO rodents, researchers discovered diminished amplitudes of sEPSCs by in vivo LFP coherence and resting-state fMRI signal synchronization compared to WT mice during the same period, implying impaired synaptic multiplicity. They witnessed that insufficient synaptic elimination entails a substantial decline in functional connectivity and undermines synaptic transmission culminating in hindered sociability and amplified repetitive behaviors [
137]. These manifestations are interconnected with neurodevelopmental illnesses, such as ASDs and MDDs. Despite the clinical manifestations of hippocampal-mediated mental diseases, controversy remains about the persistency of such defects in the synaptic multiplicity during the following stages of development in CX
3CR1-KO animals [
138].
Together, impaired synaptic pruning affects synaptic numbers and precludes functional brain communications. These results support the idea of the primary role of microglia deficits in circuit-level impairments, neurobehavior, and protracted alterations in brain wiring in neuropsychiatric conditions [
138]. Last, the known genetics and environmental triggers of ASDs, such as fragile X syndrome, viral infections, medications, and air pollutants, are involved in the neuropathology of this disease by surfacing existing synaptic pruning impairments [
132,
139]. Since maintaining good functional CNS connectivity is pivotal for socialization, it should prompt an evaluation of the prominent role of disparities in synaptic elimination in the presence of a wide range of psychosocial differences and gross brain wirings [
140].
Neurodegenerative diseases
There are many neurodegenerative disorders associated with microglia. Here, we exemplify two of the most prominent examples, MS and AD.
In MS, the CD200–CD200R reactions impact the activation of the microglia. Hoek et al
. demonstrated that
CD200 null rodents form microglial aggregates rich in CD11b and CD45. The reason for microglia aggregation is attributed to an underlying neuroinflammatory and degenerative cause. They designed an experimental model of MS by myelin oligodendrocyte glycoprotein (MOG)-induced experimental allergic encephalomyelitis (EAE) in
CD200 null mice and witnessed accelerated pathological progression, as opposed to the delayed disease onset in the wild-type C57BL/6 rodents. They recorded enhanced CD68 expression in the brain of CD200 null rodents, implying an excessive microglia population [
141].
FcγR expressed on microglia mediates the multitude of their inflammatory activities by attachment of IgG immunoglobulins. This can occur both in the form of endogenous processes and therapeutics. Additionally, FcγR expression rises with normal aging, and in the neurodegenerative processes leads to proinflammatory microglia activation, damaging the neurovascular unit in neurodegenerative diseases. FcγR stimulation activates the signal regulatory protein α (SIRPα), which is produced by a range of CNS resident cells. The SIRPα and its associated ligand on myelin, CD47, in turn, impedes phagocyte engulfment potencies and myelin debris clearance. Intriguingly, Gitik et al. experimented with the CD47-KO mice model of spinal cord injury (SCI) and observed earlier and more extended microglia scavenging response toward myelin debris, leading to enhanced neuronal repair compared to CD47
+/+ mice. Similar to SCI, MS and other pathologies that involve debris aggregation might benefit from the blockade of SIRPα-mediated phagocytosis blockade. Han and colleagues recorded a reduction in CD47 mRNA in MS plaques, despite its presence in surrounding intact myelin and foamy macrophages. Also, in a CD47 null mice model of autoimmune encephalomyelitis, they found the pathology progression was affected, primarily due to impaired microglia activation upon induction with myelin antigens [
142]. Differences in brain area studied, and pathology progressiveness may implicate in the dual function of CD47 in MS.
In AD, cognitive function and behavioral abilities deteriorate. Dementia afflicts nearly 60 million individuals globally, with an incidence rate of 10 million new cases yearly. Approximately 70% of dementia cases are categorized as AD. Fibrillary Aβ aggregation (senile plaques), deposition of neurofibrillary tangles (tau), impaired synaptic plasticity, and degenerative changes are AD characteristics. Moreover, pathological processing of amyloid precursor protein (APP) contributes to senile plaques (SPs) formation, with a consequential rise in free-radicals concentration and inflammation providing a hostile environment for neurons.
Neuroinflammation has become the cornerstone of AD pathology over the past decades. In fact, chronic therapy with non-steroidal anti-inflammatory drugs (NSAIDs) diminishes the risk of AD, demonstrating the prohibitory role of anti-inflammatory medications. Concomitant microglia and astrocytes shedding proinflammatory cytokines accompanied by complement activation orchestrate brain inflammation during AD. Investigations have shown the presence of microglia and mononuclear phagocytes adjacent to SPs in autopsy and rodents with AD. Intriguingly, it has been established that even with Aβ deposition, neurons are spared from inflammatory damage until microglia activation. Microglia actively engulf Aβ fibrils and reinforce their degradation by secretion of inflammatory cytokines, thus averting the disease progression. However, it acts as a double-edged sword because the increased load of cytotoxic factors escalates the risk of local inflammation and AD aggravation. Genetic evidence highlights that the burst of the amyloidogenic Aβ may be the cornerstone for familial AD (e.g., early-onset AD) pathogenesis. Concomitant dysfunctions in APP synthesis and its modification by γ-secretase, presenilin subunits PS1/2 lays a foundation for higher propensity to accumulation in Aβ oligomers [
143]. Baik and colleagues demonstrated that in an LPS-induced mouse model of cognitive decline, LPS robustly enhances mTOR and HIF1α transcription [
144]. Activated mammalian targets of rapamycin complex 1 (mTORC1) strongly hamper autophagy through phosphorylation of p70S6K [
145].
On the other hand, impeded autophagy process and overexpression of Aβ may work in concert to amplify the susceptibility to late-onset neuronal damage and AD manifestations. The mTOR-HIF1α signaling modulates inflammatory cytokines transcription, such as IL-1β and TNF-α. This impact is affirmed by demonstrating the downregulation of IL-1β and TNF-α synthesis upon blockade of the mTOR pathway with rapamycin or metformin [
146,
147]. Controversially, Han et al
. witnessed that activated microglia in mice treated with LPS increased autophagy genes expression and blunt iNOS and IL-6 synthesis, thus increasing cell survival [
148]. Aβ administration alters the metabolism of primary microglia. A rise in extracellular acidification rate (ECAR), lactate, and a decline in mitochondrial oxygen consumption rate (OCR) indicate a shift from oxidative phosphorylation to glycolysis [
149].
Galectin-3 (Gal-3), an evolutionary conserved multifunctional protein, plays a progressively prominent role in neuroinflammatory processes in AD by orchestrating microglia [
150]. Gal-3 is a physiological ligand for TREM2, TLR4, and insulin receptor (IR), influencing a variety of microglial immune response mechanisms, such as BMP and WNT pathways in various neuropathological conditions [
151]. TLR4 is a well-known molecule of the innate immunity with versatile effects on the immune response [
11]. To further clarify, Gal-3 derives its impact on microglia through IFN-γ and the overexpression of proinflammatory factors by the JAK/STAT signaling. Gals-3–IGFR-1 interplay is considered pivotal in IGF-dependent JAK/STAT signaling and microglial growth. In addition to the triggering effect of Gal-3–TLR4 interaction on neuroinflammation, Gal-3 affects the LPS–TLR4 binding by adhesion to the LPS, thus influencing the downstream inflammatory processes. Gal-3 is needed for proper induction of TLR4 after LPS exposure [
152]. LPS-triggered microglial release of neuraminidase, leads to sialic acid destruction within the plasma membrane glycoproteins, which paves the way for phagocytosis augmentation because of the Gal-3 attachment to Mer tyrosine kinase (MerTK) [
153].
There are many studies leveraging the Gal-3’s pivotal role in AD pathophysiology [
153,
154]. Recent observational studies unraveled a substantial increase in the Gal-3’s serum and CSF level in AD cases compared to the control group [
155‐
157]. An experiment conducted by Tao et al. exhibited a concordant rise in Gal-3 and Aβ oligomerization in the frontal lobe of AD patients [
158]. Moreover, Gal-3 concentration is congruous with the cognition decline and AD’s stage. In concert with the AD brains, 5xFAD (familial Alzheimer’s disease) mice disclosed a substantially boosted expression of the Gal-3, particularly in microglia entangled with Aβ fibrils. Interestingly, Gal-3-KO 5xFAD mice exert significantly lower levels of Aβ and improved cognition [
159]. Higher concentrations of sTREM2 have been found in the plasma as well as cerebrospinal fluid (CSF) of cases with AD [
160,
161]. TREM2 and TLR-centered immune responses in microglia adjacent to Aβ plaques are significantly impeded in Gal3-KO 5xFAD mice. Microglial Gal-3 engages in fibrillary Aβ accumulation and decelerates its destruction. The stimulatory effect of the Aβ on microglial Gal-3 transcription orchestrates the inflammatory process. This occurs in concert with the previously observed Gal-3-dependent α-synuclein-stimulated microglia activation [
162,
163]. Further investigation into the potency of Gal-3 inhibition as a novel therapeutic approach to protect against Aβ toxicity is warranted.
To reduce the burden of Aβ aggregates, microglia engulf Aβ deposits. Disturbance in the transcription of the cytokines and phagocytic ligands in microglia provides a solid ground for phagocytosis impairment and Aβ accumulation [
164]. Microglia are key regulator and effector in the neuropathology of AD, however, a number of riddles regarding the reaction of microglia and AD are unanswered. Neurodegeneration has been linked to microglia-related inflammatory elements including TNF-α, IFN-γ, and IL-1β. In this condition, microglia cannot endocytose neuropathological Aβ and tau. As a result, Aβ and tau accumulation leads to inflammatory induction, culminating in a vicious cycle in AD neuroimmunopathology. Despite this, anti-inflammatory elements excreted via microglia including IL-2, IL-4, IL-10 and TGF-β, as well as the activation of certain receptors such as TREM2, help the restoration of learning and recollection dysfunctions in AD via various signaling pathways and mechanisms. Additionally, phagocytosis and autophagy of microglia mediated by several key receptors including SR-A and CD36 are responsible for the destruction of accumulated Aβ and tau in AD. Overall, even though there are many plaque-related microglia in the CNS of AD cases as well as in preclinical models of AD, microglia cannot adequately clear Aβ deposits. Interestingly, microglia could be triggered to do so by Aβ immunotherapy/vaccines attributable to anti-Aβ antibody triggering of IgG receptor (FcR)-regulated phagocytic clearing of Aβ plaques [
164,
165].
Other than injuring nerve cells' via the phagocytosis of synapses and affecting tau neuropathology, microglia interact with accumulated plaques and expiring nerve cells in a proinflammatory manner, thereby damaging nerve cells via the excretion of inflammation modulators. Aβ plaques/fibrils may function as DAMPs and trigger TLRs and the NRLP3 inflammasome [
166], leading to the microglial synthesis of TNF-α, IL-1β, and the rest of the inflammatory cytokines. In line with a pathological function for cytokine secretion, the progression of tau neuropathology in
Cx3cr1-knockout rodents was inhibited with IL-1 suppression [
167], and detrimental influences of apoE4 in the context of tau neuropathology were linked to augmented TNF-α synthesis via microglia in vitro. Gene deletion of NLRP3, caspase-1 and TLRs have been found to improve Aβ accumulation and cognitive impairments in amyloidosis rodent models, corroborating the notion that "classical" neuroinflammation advances AD neuropathogenesis.
Additionally, microglia may collaborate with astrocytes to induce nerve cell damage. Three elements secreted via induced microglia (e.g., IL-1α, TNFα, and C1q) are required and sufficient to trigger astrocytes into a neurotoxic condition named "A1," which leads to nerve cells' expiry [
168]. A1 astrocytes are present in tau transgenic rodents producing human apoE4 [
169] and in CNS tissue from cases with different neurodegenerative conditions, such as AD. Notably, A1 astrocytes demonstrate robustly activated production of complement proteins C1r, C1s, C3, and C4 [
170]. As a result, astrocytes may work together with microglia to regulate complement-reliant neural toxicity.
Inflammasome/complement/autophagy are elements of cellular breakdown mechanisms required to break down further or disfigured particles in lysosomes. Such mechanisms, maintained through a sizable enzymatic breakdown system, are disrupted during senescence and are of particular significance during AD, as demonstrated with the autophagy impairment and the augmentation of autophagosomes in cases with AD [
171]. Also, lysosomal acidification and autophagy are insulted via AD-associated PS1 mutation [
172]. Research shows that microglial Aβ phagocytosis results in neurodegeneration via activating NLRP3 and lysosomal cathepsin-B, eventually leading to maturation and secretion of IL-1β [
173]. Thus, degenerating cellular mechanisms may confer opposing roles through distinct mediation of the inflammasome. It could be defensive in neurophysiological conditions and during the early stage of neuropathology and harmful during long-lasting and late stages of illnesses [
174].