Role of microglial lysosomes in amyloid-A-beta degradation

Role of microglial lysosomes in amyloid-A-beta degradation is sponsored by National Institute on Aging (NIH). This research characterizes a process called digestive exophagy, used by phagocytic cells to digest large objects. The hypothesis is that microglia use digestive exophagy to trim and partially degrade β-amyloid, potentially limiting the expansion of β-amyloid plaques.

Microglial Dysfunction and Amyloid-Beta Pathology in Alzheimer’s Disease and HIV-Associated Neurocognitive Disorders - PMC As a library, NLM provides access to scientific literature. Inclusion in an NLM database does not imply endorsement of, or agreement with, the contents by NLM or the National Institutes of Health. .

2025 Sep 17;26(18):9069. doi: 10. 3390/ijms26189069 Microglial Dysfunction and Amyloid-Beta Pathology in Alzheimer’s Disease and HIV-Associated Neurocognitive Disorders 1 Department of Anesthesiology, College of Medicine, University of Nebraska Medical Center, Omaha, NE 68198-4455, USA; gnjoku@unmc.

edu 2 Department of Cellular and Integrative Physiology, College of Medicine, University of Nebraska Medical Center, Omaha, NE 68198-4455, USA Find articles by George Chigozie Njoku 1, 2 , Georgette Djuidje Kanmogne Georgette Djuidje Kanmogne 1 Department of Anesthesiology, College of Medicine, University of Nebraska Medical Center, Omaha, NE 68198-4455, USA; gnjoku@unmc.

edu Find articles by Georgette Djuidje Kanmogne 1 Department of Anesthesiology, College of Medicine, University of Nebraska Medical Center, Omaha, NE 68198-4455, USA; gnjoku@unmc. edu 2 Department of Cellular and Integrative Physiology, College of Medicine, University of Nebraska Medical Center, Omaha, NE 68198-4455, USA * Correspondence: gkanmogne@unmc.

edu Masashi Tanaka : Academic Editor Received 2025 Aug 6; Revised 2025 Sep 11; Accepted 2025 Sep 12; Collection date 2025 Sep. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( https://creativecommons.

org/licenses/by/4. 0/ ). PMCID: PMC12471207 PMID: 41009633 Chronic neuroinflammation and impaired protein clearance are hallmarks of neurodegenerative diseases such as Alzheimer’s disease (AD) and HIV-associated neurocognitive disorders (HAND).

Central to these processes are microglia, the brain’s resident immune cells, which normally maintain brain homeostasis by clearing amyloid-beta (Aβ) and other misfolded proteins through phagocytosis and receptor-mediated degradation. However, in both AD and HAND, microglial dysfunction promotes ongoing inflammation, impaired Aβ clearance, and progressive neuronal damage.

This review synthesizes evidence from human and animal studies showing how key microglial pattern recognition receptors, including the Triggering receptor expressed on myeloid cells 2 (TREM2), Toll-like receptors (TLRs), and scavenger receptors (SR-AI/II, CD36, SR-BI, CD163), coordinate Aβ sensing, uptake, and inflammatory responses.

We describe how HIV infection and viral proteins such as the trans-activator of transcription (Tat) and glycoprotein 120 (gp120) disrupt these pathways by altering receptor expression, lysosomal function, and microglial metabolism, creating a cycle of neurotoxicity and amyloid buildup. We further highlight current scientific gaps in elucidating how HIV affects microglial function and implications for HAND.

Keywords: microglial dysfunction, amyloid-beta clearance, Alzheimer’s disease, HAND, HIV-1 Tat, scavenger receptors, neuroinflammation, phagocytosis impairment, endolysosomal dysfunction, TREM2 The global burden of neurodegenerative diseases has risen dramatically over the past decades, with dementia alone accounting for approximately 32.

6 million disability-adjusted life years in adults aged 65 and above in 2021, while Parkinson’s disease contributed around 7. 5 million disability-adjusted life years, making them among the most disabling neurological conditions worldwide [ 1 , 2 , 3 ]. A central driver of this disease burden is chronic neuroinflammation, which has emerged as a unifying pathological hallmark across neurodegenerative disorders.

Within this context, microglia, the resident immune cells of the central nervous system (CNS), play a pivotal role in orchestrating inflammatory responses and maintaining homeostasis [ 4 ]. Derived from erythromyeloid progenitors in the embryonic yolk sac [ 5 ], microglia exhibit a branched, ramified morphology suited to their surveillance function [ 6 , 7 ].

First identified in 1919 by Spanish neuroscientist Pío del Río-Hortega as distinct from astrocytes and oligodendrocytes, microglia were originally characterized by their “resting” and “activated” states, highlighting their role in phagocytosis [ 8 ].

While initially recognized for their debris-clearing capabilities, microglia are now known to play broader roles in immune signaling, neuroprotection, myelination, and synaptic remodeling, making them essential for both brain homeostasis and defense [ 9 , 10 ].

In response to pathological stimuli such as infection, trauma, or neurodegeneration, they rapidly alter their gene expression, adopt an amoeboid shape, and secrete pro-inflammatory cytokines to mount an immune response [ 11 , 12 ].

While this acute activation is protective, sustained neuroinflammation disrupts microglial homeostasis, impairs phagocytic function, and promotes neuronal damage, contributing to the progression of neurodegenerative diseases [ 13 ].

In neurodegenerative diseases such as Alzheimer’s disease (AD) and Human Immunodeficiency Virus (HIV)-associated neurocognitive disorders (HAND), microglial dysfunction and chronic inflammation are not just passive consequences but active drivers of impaired amyloid-beta (Aβ) clearance and progressive neuronal damage [ 14 , 15 , 16 , 17 ].

In AD, defective microglial clearance of Aβ promotes plaque accumulation and sustained neuroinflammation [ 12 ]. In HAND, HIV infection drives persistent microglial activation and disrupts homeostatic functions, creating a neuroinflammatory environment that worsens neuronal injury [ 14 ]. It is still unclear whether the extent of neurodegeneration and neuronal death is directly tied to the loss of microglial homeostasis.

However, growing evidence suggests that HIV-induced neuroinflammation disrupts microglial function, contributing to impaired immune surveillance, reduced clearance of neurotoxic proteins such as Aβ, and increased neuronal vulnerability [ 14 , 15 , 16 , 17 ]. Hence, this review aims to explore the mechanisms by which microglial dysfunction contributes to impaired Aβ clearance and the implication for HAND.

We will discuss the effects of HIV and viral proteins on microglia activation and function, including its phagocytic and Aβ clearance function, and identify the key knowledge gaps that remain to be addressed through future research. 2. Microglia: History, Function, and Role in CNS Homeostasis 2.

1. Microglia Discovery and Historical Perspective The concept of immune surveillance within the brain can be traced back more than a century to the foundational work of Ilya Méchnikov, who introduced the idea of phagocytosis after observing mobile, engulfing cells in starfish larvae [ 18 ].

Years later, Nicolás Achúcarro identified similar cells in the brains of rabbits infected with the rabies virus, noting their role in clearing damaged neurons [ 19 ].

Subsequently, after refining the staining techniques initially developed by Santiago Ramón y Cajal [ 20 ], Del Río-Hortega successfully identified and characterized microglia as a distinct glial population within the CNS, specialized in response to injury and infection [ 8 , 19 , 21 ].

Del Río-Hortega’s innovations enabled detailed cellular visualization and established microglia as non-neuroectodermal, yolk-sac-derived immune sentinels of the brain. These early discoveries laid the groundwork for our understanding of microglial identity and surveillance functions, processes now recognized as central to the pathophysiology of neurodegenerative diseases [ 22 ]. 2.

2. Homeostatic Functions and Dynamic Surveillance Unlike other CNS cells derived from neuroectodermal progenitors, microglia originate from yolk sac progenitors during early embryogenesis. They constitute approximately 5–10% of the total cell population in the human brain [ 6 , 7 ] and, in addition to endothelial cells, represent the only brain-resident cell type of non-neuroectodermal origin [ 5 ].

Microglia migrate into the developing brain prior to the formation of the blood–brain barrier (BBB), colonizing the neuroepithelium where they proliferate, spread throughout the CNS, and differentiate into mature, homeostatic microglia [ 5 , 23 , 24 ]. In the adult brain, microglia are highly dynamic, constantly extending and retracting their processes to monitor the surrounding parenchyma and maintain tissue integrity [ 25 , 26 ].

They achieve this surveillance through a diverse array of pattern recognition receptors (PRRs), including Toll-like receptors (TLRs) such as TLR4 and TLR1/2, as well as co-receptors like cluster of differentiation (CD)14, CD36, and CD47, which allow microglia to sense pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) [ 25 , 26 ].

Although the exact mechanisms regulating microglia motility and morphological changes remain incompletely understood, studies suggest that purinergic receptors, ion channels, and neurotransmitters play important regulatory roles in this process [ 11 , 27 , 28 , 29 ].

Microglia also express chemokine receptors such as CX3CR1 and CXCR4, alongside integrins such as CD11b and CD11c, which contribute to their migration and enhance their ability to engage and phagocytose target cells [ 30 , 31 , 32 ]. CD11b is constitutively expressed, whereas CD11c is typically upregulated during microglial activation [ 30 , 33 , 34 ].

Common molecular markers of homeostatic microglia include hexosaminidase subunit beta, P2Y purinoceptor 12, S100 calcium-binding proteins A8 and A9, ionized calcium-binding adapter molecule 1, transmembrane protein 119, G protein-coupled receptor 34, sialic acid-binding Ig-like lectin H, the triggering receptor expressed on myeloid cells 2 (TREM2), and olfactomedin-like protein 3, which help distinguish them from other CNS cell types and from disease-associated states [ 35 , 36 ].

In addition to their immune surveillance roles, homeostatic microglia also contribute to synaptic pruning, neurotrophic support, and myelination, thereby preserving CNS structure and function under physiological conditions [ 37 , 38 , 39 , 40 ]. Altogether, these features show how microglia are uniquely suited to monitor and support brain function under normal conditions.

Loss or impairment of this microglial regulatory role can contribute to reduced clearance of harmful proteins and increased inflammation seen in neurodegenerative diseases such as AD and HAND [ 41 ]. 2. 3.

Specialized Immune Functions and Transition to Neuroinflammation In response to environmental perturbations, microglia rapidly transition from a homeostatic, ramified phenotype to an activated, amoeboid form [ 42 , 43 ].

This phenotypic switch can be triggered by PAMPs such as viral nucleic acids, bacterial deoxyribonucleic acid (DNA), and lipopolysaccharide (LPS) [ 44 , 45 ], as well as endogenous DAMPs, including heat shock proteins, high-mobility group box 1 protein, and mitochondrial DNA [ 46 ]. Recognition of these molecules by the immune system triggers microglial activation [ 47 , 48 ].

Activated microglia upregulate CD11c and secrete pro-inflammatory mediators including tumor necrosis factor-alpha (TNF-α), interleukin (IL)-1β, IL-6, nitric oxide (NO), and reactive oxygen species (ROS), leading to synaptic dysfunction, neuronal injury, and death [ 13 , 42 , 49 ].

Significantly, activated microglia also suppress anti-inflammatory transforming growth factor-beta via the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB)-mediated signaling [ 50 , 51 ], thereby disrupting immune homeostasis, promoting CNS tissue damage, and behavioral impairments [ 50 ].

Although transient microglial activation may serve a protective function during acute insults, prolonged or dysregulated activation results in chronic neuroinflammation, which is a hallmark of neurodegenerative diseases such as AD and HAND.

Notably, one key consequence of prolonged microglial activation is dysregulation of glutaminase activity, which leads to excessive production of glutamate [ 52 ]; glutamate accumulates in the extracellular space where it overstimulates N -methyl- d -aspartate receptors, particularly the extrasynaptic subtype [ 52 , 53 ], resulting in neuronal calcium (Ca 2+ ) overload and excitotoxic cascades that compromise cellular integrity [ 52 , 53 ].

Beyond excitotoxicity, Ca 2+ plays a dual pathological role in amyloidogenesis: intracellular Ca 2+ activates signaling pathways that upregulate β-site APP-cleaving enzyme 1 and γ-secretase, driving amyloidogenic APP processing and increased production of Aβ 42 peptides [ 54 ]; at the same time, Ca 2+ directly modulates amyloid fibril formation.

Experimental evidence shows that Ca 2+ binding can reshape aggregation pathways, as in the case of S100 calcium-binding protein A9 forming “worm-like” fibrils with distinct mechanical properties [ 55 ], while variations in ionic strength and protein concentration critically determine fibril polymorphism, as demonstrated for α-synuclein aggregation [ 56 ].

These processes underscore how Ca 2+ overload not only initiates Aβ production but also accelerates fibrillar growth under permissive cellular conditions. The accumulation of Aβ further amplifies neuronal dysfunction and loss, serving as a central hallmark of both AD and HAND pathogenesis [ 57 , 58 , 59 ]. 3.

Microglial Role in Aβ Clearance AD is the leading cause of dementia worldwide and significantly impacts quality of life [ 60 , 61 ]. Currently, there is no cure, and available treatments focus on alleviating symptoms, slowing disease progression, and managing the risks associated with cognitive decline [ 62 , 63 ].

Numerous hypotheses have been proposed to explain the causes of AD, which include neuroinflammation [ 64 , 65 ], mitochondrial dysfunction [ 66 , 67 , 68 , 69 ], calcium dysregulation [ 70 ], and impaired autophagy [ 71 , 72 ].

In AD, a hallmark of disease pathology is the extracellular deposition of Aβ, which is toxic to neurons [ 73 ], drives chronic inflammation [ 74 ], aggregates, and forms senile plaques that disrupt cellular function and which are linked to cognitive impairments [ 61 ].

Another hallmark of AD pathology is hyperphosphorylation of tau proteins, resulting in neurofibrillary tangles (NFTs) that are also linked to cognitive impairments [ 75 , 76 ]. NFTs are associated with the late stage of AD (LOAD), when the symptoms are generally irreversible [ 77 ], whereas Aβ plaque accumulation occurs at earlier stages of AD (EOAD), before the manifestation of cognitive deficits [ 77 ].

Notably, Aβ plaques are also detected in LOAD [ 13 , 78 ]. NFTs and Aβ aggregates cause neuronal damage, synaptic dysfunction, and cognitive decline [ 13 , 79 ]. One of the brain’s key defenders in AD is microglia [ 64 , 80 ].

Under normal conditions, Aβ is primarily produced by neurons through enzymatic cleavage of APP, while its clearance is largely mediated by microglia via mechanisms such as phagocytosis, enzymatic degradation, and transport across the BBB [ 77 , 81 , 82 , 83 ]. However, in the context of AD, this balance becomes disrupted.

As the disease progresses, microglia respond to accumulating Aβ by increased proliferation and activation [ 13 , 84 ], releasing inflammatory cytokines and ROS [ 13 , 85 , 86 ]. Instead of repairing the damages, this prolonged inflammatory state further impairs microglial function, diminishing microglia’s capacity to clear Aβ.

This leads to greater accumulation of Aβ and plaques, progression of pathology, and worsening neurocognitive decline [ 12 ]. Yet despite their increased numbers, microglia capacity to clear Aβ diminishes significantly [ 87 , 88 ]. This raises critical questions: how much Aβ can microglia effectively clear before reaching their functional limit?

At what point does their protective role shift toward a pathological one, ultimately contributing to further Aβ accumulation? These questions remain unanswered but are essential to understanding microglial dysfunction and failure of its clearance function in AD and other neurodegenerative diseases.

Supporting evidence suggests that as Aβ levels rise, microglia become overstimulated and enter a chronic inflammatory state that paradoxically impairs their phagocytic function [ 89 , 90 , 91 , 92 , 93 ]. Thus, while microglia initially act as defenders, there comes a tipping point where their response exacerbates the pathology. 3.

1. Phagocytosis and Degradation of Aβ Phagocytosis is an essential cellular process that maintains tissue homeostasis and regulates immune responses [ 94 ]. It relies on phagocytes receptors to recognize specific cells or proteins as targets for engulfment.

Beyond this receptor-mediated recognition, the complement system plays a key role by tagging (opsonizing) pathogens or protein aggregates, making them more easily identified and cleared by phagocytic cells [ 94 , 95 ].

In AD, microglial cells recognize Aβ through several receptors, including TREM2, TLRs, low-density lipoprotein receptor-related-1 (LRP1), the receptor for advanced glycation end products (RAGE), and scavenger receptors (SCARA1, CD36, CD163, and SCARB-1) [ 13 , 96 , 97 ].

Once Aβ binds to these receptors, intracellular signaling pathways are activated, triggering cytoskeletal rearrangements that enable microglia to engulf Aβ through endocytosis. After internalization, Aβ is enclosed within endocytic vesicles and transported to lysosomes, where it is degraded by proteolytic enzymes such as cathepsins and insulin-degrading enzyme [ 98 , 99 ].

This degradation process prevents the extracellular accumulation of Aβ, thereby reducing Aβ-induced inflammation and synaptic dysfunction. 3. 2.

Microglia Surface Receptors and Their Response to Aβ Overview of TREM2 in the CNS TREM2 is a transmembrane immunoglobulin-like receptor expressed predominantly on myeloid-lineage cells, including dendritic cells, tissue macrophages, and microglia, which are the principal innate immune cells in the CNS [ 40 ].

TREM2 is essential for maintaining brain homeostasis and regulating key microglial functions, including chemotaxis, survival, proliferation, and phagocytosis [ 100 ]. Although the precise repertoire of endogenous TREM2 ligands remains incompletely defined, available evidence suggests it can recognize oxidized lipids, lipoproteins, and molecular components derived from apoptotic or damaged cells [ 101 ].

Notably, the extracellular domain of TREM2 has also been shown to bind LPS, suggesting a potential role in pathogen sensing [ 100 ]. Importantly, TREM2 has been shown to binds oligomeric Aβ with high affinity, a function compromised by AD-associated mutations, such as the arginine-to-histidine substitution at position 47 (R47H)-variant [ 102 ].

Genetic studies have robustly linked the R47H mutation to a 2–4-fold increased risk of LOAD, comparable to the risk conferred by the apolipoprotein E (APOE) ε4 allele [ 103 ]. Functional studies indicate that a TREM2 deficiency impairs Aβ phagocytosis, cytokine responses, and microglial migration while also disrupting ion homeostasis, including membrane depolarization and K + currents [ 102 , 103 ].

Mechanisms and Functional Significance of TREM2 in Aβ and Tau Clearance Several lines of evidence indicate that TREM2 plays a critical role in modulating microglial interactions with Aβ and tau in AD pathology [ 11 , 100 , 104 , 105 ].

Overexpression of TREM2 reduces microglial proinflammatory signaling, whereas its deletion or knockdown enhances microglia activation and inflammation and drives a neurodegenerative microglial phenotype [ 11 , 100 ].

Beyond its effects on inflammation and tau phosphorylation, recent studies have shown that TREM2 also facilitates microglial recognition and containment of Aβ plaques by promoting microglial clustering and limiting the local spread of phosphorylated tau [ 106 ]. Mechanistically, TREM2 recognizes lipid and lipoprotein ligands, such as APOE and clusterin, as well as phospholipids exposed on damaged neurons and Aβ aggregates [ 100 , 107 ].

Upon ligand binding, TREM2 interacts with its adaptor protein, the DNAX-activating protein of 12 kDa (also known as the TYRO protein tyrosine kinase binding protein), which contains immunoreceptor tyrosine-based activation motifs.

Phosphorylation of the DNAX-activating protein of 12 kilodaltons immunoreceptor tyrosine-based activation motifs recruits spleen tyrosine kinase, triggering downstream phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) and extracellular signal-regulated kinase 1/2 (ERK1/2) pathways that drive cytoskeletal rearrangements required for efficient Aβ engulfment [ 108 ].

PI3K/AKT activation further inhibits glycogen synthase kinase-3 beta, which reduces tau hyperphosphorylation and limits the local propagation of tau pathology [ 109 , 110 ]. In parallel, TREM2 enhances microglial lipid metabolism and cholesterol efflux, boosting microglial capacity to clear lipid-rich Aβ plaques [ 111 , 112 ].

By promoting microglial clustering, TREM2 also helps form a physical barrier around plaques, containing toxic species and protecting surrounding neurons [ 11 ]. The functional significance of these mechanisms is evident in vivo. The loss of TREM2 impairs microglial recruitment and clustering, reduces phagocytic capacity, and shifts microglia toward a more pro-inflammatory, neurodegenerative state [ 113 ].

In TREM2-deficient presenilin-2 amyloid precursor protein (PS2APP) mice, microglia–plaque connections are impaired, leading to more diffuse plaque structures, elevated levels of toxic Aβ, reduced microglial proliferation, and increased neuronal dystrophy [ 114 ]. Moreover, TREM2 deficiency promotes the accumulation of pathological tau in the presence of Aβ [ 115 , 116 ].

In such cases, the amount of phosphorylated tau protein near plaques increases significantly, exacerbating tau aggregation and spreading, and ultimately contributing to cortical atrophy [ 115 , 116 ]. In contrast, in the absence of Aβ pathology, TREM2 loss does not appear to influence tau burden [ 106 , 115 ].

These findings suggest that TREM2 may mitigate tau-driven neurodegeneration by limiting Aβ-induced propagation of pathological tau, thereby slowing the progression of AD [ 106 , 115 ]. Furthermore, in vivo studies using 5XFAD transgenic mice lacking TREM2 have revealed markedly increased Aβ deposition and neurodegeneration in the hippocampus, particularly by 8. 5 months of age, whereas the cortex appears relatively unaffected [ 117 ].

This was accompanied by a significant accumulation of insoluble Aβ 40 and Aβ 42 [ 117 ]. However, at earlier stages (4 months), TREM2 deficiency did not significantly affect the Aβ burden, suggesting that the impact of TREM2 loss depends on disease progression and the specific brain region involved [ 117 ]. Supporting this idea of stage- and region-specific effects, results from other AD mouse models show similar patterns.

In APPPS1-21 mice, TREM2 deficiency led to reduced microglial clustering around plaques and different effects on Aβ levels depending on brain region and age [ 118 ]. At four months, cortical Aβ levels remained unchanged, while hippocampal Aβ levels decreased [ 118 ]; by eight months, cortical Aβ burden increased, while hippocampal levels showed no further change [ 119 ].

These findings indicate that certain brain regions, such as the hippocampus, may be more vulnerable to Aβ accumulation and neurodegeneration when TREM2 is lost, while others, like the cortex, may be less affected during the same disease stages.

Conversely, enhanced TREM2 expression achieved through transgenic overexpression of human TREM2 in 5XFAD mice reduced Aβ plaque burden and modified microglial transcriptomic signatures, supporting a dose-dependent protective role for TREM2 [ 120 ].

Notably, multiple studies have demonstrated that TREM2 plays a role in AD pathogenesis by regulating microglial phagocytosis, inflammatory response, proliferation, and lipid metabolism [ 40 , 111 , 112 , 121 , 122 , 123 , 124 ]. This highlights the crosstalk between TREM2, microglia, and AD pathology and TREM2’s critical role in coordinating microglial responses to both Aβ and tau.

TREM2 Regulates Microglial Metabolism TREM2 supports microglial metabolic fitness by regulating key pathways that control energy balance, including the mammalian target of the rapamycin (mTOR) pathway and glycolysis [ 125 ].

mTOR, particularly mTOR complex 1 (mTORC1), is a nutrient-sensing kinase that promotes glycolysis, mitochondrial biogenesis, and protein synthesis and suppresses excessive autophagy to maintain energy balance [ 126 ]. In microglia, active mTORC1 is essential for providing the energy and biosynthetic capacity needed for phagocytosis and Aβ degradation [ 127 ].

In TREM2-deficient microglia, mTORC1 activity is suppressed while adenosine monophosphate-activated protein kinase phosphorylation is elevated, indicating a shift toward an energy-deficient, catabolic state [ 128 ]. Reduced mTORC1 signaling impairs glycolytic flux and mitochondrial function, leading to lower adenosine triphosphate levels, decreased mitochondrial mass, and accumulation of autophagic vesicles [ 128 ].

Selective deletion of tuberous sclerosis complex 1 (TSC1), a negative regulator of mTORC1, in microglia reactivates mTORC1 signaling, upregulates TREM2 expression, and enhances Aβ clearance in 5XFAD mice [ 125 ]. These changes are associated with reduced synaptic spine loss and improved cognitive performance.

Notably, combined deletion of TSC1 and TREM2 abolishes these benefits, demonstrating that TREM2 acts downstream of mTORC1 in coordinating microglial Aβ clearance [ 125 ]. In addition, TSC1-deficient microglia show increased expression of lysosomal markers CD68 and lysosome-associated membrane glycoprotein 1, greater lysosomal activity, and enhanced Aβ uptake and degradation [ 125 ].

These effects are reversed by rapamycin treatment, both in vitro and in vivo. Chronic inhibition of mTORC1 with rapamycin lowers TREM2 expression, impairs microglial function, and increases Aβ plaque burden [ 125 ]. 3.

2. 2. Toll-like Receptors TLRs are a major family of the PRRs found on cell membranes that are integral to immune response, including in the CNS; they are recognized for their ability to detect both endogenous and exogenous signals of infection or cellular damage [ 129 , 130 ].

In humans and other mammals, there are 13 structurally different TLRs that recognize conserved viral, bacterial, and fungal particles expressed in various cell types, including B cells, T cells, macrophages, monocytes, dendritic cells, glial cells (microglia, oligodendrocytes, and astrocytes), and neurons [ 131 , 132 ]. These TLRs have been widely studied for their roles in regulating neuroinflammation and Aβ clearance in AD [ 130 ].

Depending on the specific TLR subtype activated, disease stage, and surrounding cellular environment, TLR signaling can have either protective or detrimental effects on AD pathology [ 130 , 133 ]. For example, some TLRs promote microglial phagocytosis and clearance of Aβ, whereas others may exacerbate chronic inflammation and contribute to neurotoxicity.

In AD, microglial TLRs are closely associated with Aβ plaques and act as key sensors of misfolded protein aggregates and damage signals [ 86 ]. TLR2 and TLR4 are consistently upregulated in microglia localized around Aβ plaques in both human postmortem tissue and transgenic mouse models, indicating their active engagement in situ [ 11 , 134 ].

TLR4 recognizes fibrillar Aβ in cooperation with co-receptors CD14 and myeloid differentiation factor 2 (MD-2), initiating robust proinflammatory signaling cascades [ 135 , 136 ]. Similarly, TLR2 colocalizes with CD14 and is enriched in plaque-associated microglia, supporting its role in innate recognition of Aβ [ 135 , 136 ].

In contrast, TLR9, typically localized in endosomes, does not show major upregulation near plaques but has been implicated in peripheral immune responses that impact AD pathology in the CNS [ 137 ]. Evidence from Human and Animal Models Data from both human AD brains and animal models reinforce the pathological relevance of TLR-mediated microglial dysfunction ( Table 1 ).

In postmortem brain tissues, TLR2 and TLR4 immunoreactivity is elevated in microglia adjacent to amyloid plaques [ 138 ], while APP23 transgenic mice showed significant transcriptional upregulation of TLR2, TLR4, TLR5, TLR7, and TLR9 in plaque-bearing regions [ 139 ].

Functional studies in knockout animals reveal divergent roles: TLR4 −/− mice exhibit impaired microglial Aβ uptake, increased amyloid plaque burden, and worsened cognitive performance [ 140 , 141 ].

In contrast, TLR2 −/− mice demonstrate enhanced microglial phagocytosis and a marked reduction in proinflammatory cytokine expression, including TNF-α, IL-1β, IL-6, and iNOS, as well as lower expression of integrin markers such as CD11a, CD11b, and CD68, suggesting that TLR2 activation skews microglia toward a proinflammatory, less phagocytic state [ 142 , 143 ].

In vitro studies have demonstrated that stimulation of TLR2 on microglia with peptidoglycan enhances the internalization of AD-associated Aβ-peptide, a process that is mediated in part by formyl peptide receptor 2 [ 144 ].

However, this effect is considered context-dependent, as it was observed under specific experimental conditions and may not fully translate to the in vivo brain microenvironment, where additional factors such as disease stage, cellular state, and interactions with other receptors can modulate microglial Aβ uptake.

Neutralizing antibodies against CD14, TLR2, or TLR4 reduce Aβ binding and ROS production in mouse microglia and human monocytes, which further confirmed the involvement of these receptors in amyloid sensing and binding [ 145 ]. TLR9, which is primarily localized in endosomes, plays a relatively minor role in CNS-resident microglia but can exert significant effects through peripheral immune activation.

Systemic administration of cytosine–phosphate–guanosine oligodeoxynucleotides (CpG ODNs), synthetic unmethylated DNA sequences that specifically activate TLR9, has been shown to boost peripheral innate immunity and promote the recruitment of monocyte-derived macrophages into the brain [ 146 ].

In transgenic AD mice, this increased infiltration of peripheral macrophages enhanced Aβ clearance, resulting in a 66% reduction in cortical amyloid burden and improved spatial memory [ 146 ]. Toll-like receptors regulating microglial Aβ clearance and inflammation in AD.

Recognizes Aβ; skews microglia to inflammatory state Interacts with CD14; activates MyD88–NF-κB pathway; ↓ phagocytosis, ↑ proinflammatory cytokines (TNF-α, IL-1β, IL-6) Co-localized with CD14+ microglia near plaques in human AD tissues TLR2 –/– mice exhibit ↓ cytokines and ↑ Aβ clearance; anti-TLR2 antibody ↓ ROS in vitro [ 142 , 143 , 145 , 147 ] Recognition of fibrillar Aβ triggers inflammation and clearance Binds Aβ via CD14–MD2 co-receptor complex; activates MyD88-dependent NF-κB and MAPK pathways ↑ TNF-α, IL-1β, ROS, iNOS Upregulated in plaque-associated microglia in AD brains TLR4 –/– mice show impaired Aβ clearance, ↑ plaque burden, cognitive decline Peripheral modulation of Aβ burden Endosomal TLR; activated by CpG ODNs, enhances peripheral monocyte activation and recruitment into the CNS; does not engage Aβ directly.

Not upregulated in microglia in AD brains Systemic CpG ODN injection ↓ cortical Aβ by 66% and improves memory in AD mice Annotations: ↓ = downregulation, ↑ = upregulation.

Abbreviations: MyD88: myeloid differentiation primary response 88; TNF-α: tumor necrosis factor-alpha; IL: interleukin; ROS: reactive oxygen species; iNOS: inducible nitric oxide synthase; NF-κB: nuclear factor kappa-light-chain-enhancer of activated B cells; MAPK: mitogen-activated protein kinase; CpG ODNs: CpG oligodeoxynucleotides; CNS: central nervous system; Aβ: amyloid-beta; TLR: toll-like receptor; AD = Alzheimer’s disease. 3. 2.

3. Scavenger Receptors (SRs) SRs are multifunctional PRRs that bind a broad array of ligands, including misfolded proteins, microbial components, and modified self-antigens [ 148 , 149 ]. In AD, microglia utilize several SRs for the recognition, uptake, and degradation of Aβ, with distinct receptors engaging in either protective clearance or triggering inflammatory signaling that can exacerbate neuroinflammation [ 150 , 151 ].

Class A and Class B SRs have received particular attention due to their well-documented roles in Aβ recognition, uptake, and clearance, as well as their impact on disease progression [ 148 ]. Their roles extend beyond Aβ binding to include the modulation of intracellular signaling cascades, glial activation states, and crosstalk with innate immune receptors such as TLRs. These roles are summarized in Table 2 .

Class A Scavenger Receptors (SR-AI/II) SR-AI is a trimer composed of three identical subunits, each containing a short intracellular region, a single transmembrane segment, and several specialized extracellular domains, including an α-helical coiled-coil domain, a collagen-like region, and a cysteine-rich domain at the C-terminus [ 152 ].

Class A SRs, including SR-AI/II (also known as SCARA1), are predominantly expressed in microglia and, to a lesser extent, astrocytes [ 149 , 153 ]. These receptors play a role in the phagocytic uptake and lysosomal degradation of Aβ, particularly fibrillar and oligomeric Aβ.

Upon binding to Aβ, SR-A mediates its internalization, likely via clathrin-independent macropinocytosis, and delivers internalized Aβ to lysosomal compartments [ 154 ]. Concurrently, SR-A also activates the PI3K/NF-κB pathway as well as MAPK pathways, including c-Jun N-terminal kinase (JNK) and p38 MAPK, to promote actin remodeling and microglial migration toward Aβ deposits [ 155 ].

SR-A activation has been shown to suppress TLR4 signaling, suggesting it may help limit excessive neuroinflammation, particularly during early stages of disease when pro-inflammatory responses are first triggered [ 156 ]. Studies using the synthetic SR-A ligand XD4 demonstrated that SR-A activation enhances microglial Aβ uptake while simultaneously dampening the secretion of TNF-α and