Mechanisms of autophagy–lysosome pathway impairment in Alzheimer's disease

Autophagosomes

Macroautophagy (hereafter referred to as autophagy) is an important catabolic mechanism, similar to the waste disposal framework, in which damaged organelles and other materials are degraded into small molecules for cell recycling and reuse. ²¹ This process ensures the continuous renewal of intracellular components, thereby maintaining cell viability. In terms of its mechanism of action, autophagy can be divided into three main types: macroautophagy, microautophagy and molecular chaperone-mediated autophagy. Macroautophagy is the main way in which most long-lived proteins and aggregation-prone toxic proteins are degraded in eukaryotic cells and is the most widely discussed form in the context of AD. Autophagy begins with the activation of the unc-51-like kinase (ULK) complex (composed of ULK1 kinase, ATG13, FIP200 and ATG101).^22–24 The complex then binds to class III phosphatidylinositol 3-kinase (PI3K) complex I (including Beclin1, VPS15, VPS34, Atg14 and UVRAG) to form a core PI3K complex. This causes the autophagic vesicle assembly site (PAS) to nucleate and fuse with cytoplasmic separation membranes to form a preautophagosome structure.^25,26 Mammalian target of rapamycin (mTOR) is a 289 kDa serine-threonine kinase that belongs to the PI3K-related kinase family. mTOR is the catalytic subunit of two major protein complexes, mTOR complex 1 (mTORC1), that regulate cell metabolism and growth, and mTOR complex 2 (mTORC2), which controls cell proliferation and survival.^27,28 mTORC1 is a critical regulator of autophagy ²⁹ . It consists of mTOR, Raptor (mTOR regulatory related protein), mLST8 (SEC13 protein lethal to mammals 8), DEPTOR (mTOR interacting protein containing DEP domain) and PRAS40 (proline-rich Akt substrate is 40 kDa).^30,31 Its regulatory mechanism in autophagy is complex. In the process of autophagy induction, mTOR integrates various intracellular signals and plays a major role in regulating autophagy by controlling downstream autophagy-related genes. Autophagy includes three main stages. ³² In the initiation phase, also known as nucleation, mTORC1 inhibits the initiation of autophagy by directly or indirectly inhibiting the ULK1 complex under nutrient-rich conditions. For example, mTORC1 directly phosphorylates ATG13 to inhibit the activity of the ULK1 complex. ³³ AMBRA1 is another key autophagy regulator. Interestingly, mTORC1 also directly phosphorylates AMBRA1, preventing it from activating ULK1 and thereby inhibiting autophagy.^29,34 However, under cell starvation or oxidative stress conditions, AMBRA1 promotes autophagosome nucleation by promoting the interaction between Beclin1 and VPS34.^35,36 In summary, ULK1 is a positive regulator of autophagy, whereas mTORC1 is a negative regulator of autophagy; therefore, the induction of autophagy requires the inhibition of mTORC1 function. AMP-activated protein kinase (AMPK) is a member of the AMPK protein kinase family and acts as an upstream regulator of mTOR during starvation. AMPK activates autophagy through three main mechanisms: First, it directly activates ULK1 to induce autophagy through the AMPK-mTOR pathway.^37,38 Second, AMPK activates autophagy by activating its negative regulator TSC2, inhibiting its subunit Raptor and inhibiting mTORC1 activity.^39,40 Third, AMPK can also phosphorylate key components of autophagy, such as VPS34 and Beclin1, to promote autophagy.^41,42 In addition, under starvation conditions, ATG13 can be dephosphorylated to form the Atg1-Atg13-Atg17 complex, which further promotes the initiation of autophagy. ⁴³

Maturation phase

The maturation of autophagosomes includes fusion with endosomes (formation of zwitterions) and, eventually, fusion with lysosomes to form autolysosomes. In autophagic lysosomes, the contents of phagocytosis are degraded, the autophagy cycle is completed, and the cells are resurrected (Figure 1).

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Figure 1.

The molecular mechanism of autophagy. Autophagy begins with the activation of the ULK kinase complex, which then binds to type III PI3K complex I to form the core PI3K complex. The complex recruits the Atg protein to nucleate the autophagic vesicle assembly site (PAS), and the free separation membrane fuses to form an autophagosome structure. mTORC1 plays a key role in regulating autophagy. Under nutrient-rich conditions, mTORC1 inhibits the initiation of autophagy by directly or indirectly inhibiting the ULK1 complex; it can also phosphorylate AMBRA1 and prevent it from activating ULK1. In contrast, AMBRA1 promotes utophagosome nucleation under cell starvation or oxidative stress conditions. Under starvation conditions, AMPK acts as an upstream regulator of mTOR. It activates autophagy through three main mechanisms: (1) ULK1 is directly activated through the AMPK-mTOR pathway; (2) it activates the negative regulator of mTORC1 and inhibits its subunits; and (3) key components of autophagy are phosphorylated. In addition, starvation-induced Atg13 dephosphorylation contributes to the formation of specific complexes that promote autophagy. During the stretching phase, the preautophagic structure expands into a double-membrane structure, engulfs cytoplasmic components, and finally closes to form a closed spherical double-membrane vesicle called an autophagosome. Most Atg proteins are subsequently dissociated and recovered from autophagosomes, while LC3-I binds to PE through a two-step ubiquitination process mediated by the enzymes Atg7 (E1-like) and Atg3 (E2-like) to form autophagy markers.

Endosomes

Endosomes are single-membrane-bound organelles in eukaryotic cells that originate from endocytosis. They are important hubs in vesicle transport and are involved in many important cellular activities, often referred to as “sorting centers” or “sorting machines”. ⁴⁹ In the sequential stage of endocytosis, endosomes are divided into early endosomes, late endosomes and recycled endosomes. ⁵⁰ Extracellular substances such as growth factors, nutrients, hormones, viruses and toxic proteins are internalized by endocytosis to form vesicles that fuse with early endosomes. Some early endosomes are fused with various organelles for sorting and transport, whereas others integrate with autophagosomes into the autophagy pathway and mature into late endosomes. The intracavitary p H of the late endosome is mainly acidified by the activity of the vacuolar ATPase (V-ATPase) on its membrane. ⁵¹ They mature by fusing with homologous endosomes to form larger vesicles; thus, they are also called multivesicular bodies (MVBs). ⁵² Late endosomes can fuse with autophagosomes to form bipolar bodies and then fuse with lysosomes to form autolysosomes. In addition, late endosomes can directly fuse with lysosomes to form endosomes, resulting in the degradation of their contents.

The main function of endosomes is to sort and transport extracellular substances that are internalized by endocytosis. Afterward, the endosomal system allocates these goods to different cell destinations; waste or harmful substances are directed to lysosomes or autophagy systems for degradation. Since both endocytosis and autophagy depend on lysosomes to degrade extracellular and intracellular “waste” and because some endosomes are eventually incorporated into autophagosomes, autophagy and the endosomal system are functionally intertwined. Their convergence constitutes an endosomal-autophagic lysosomal system. In neurons, the system is essential for clearing abnormal intracellular and extracellular proteins, and its efficiency decreases with age. Dysfunction of the lysosomal-autophagic lysosomal system occurs in the early pathological stage of AD and is considered to be a potential contributor to its pathogenesis.^53,54

Autophagy and Aβ

In the pathological process of AD, AβPP is abnormally cleaved by preβ-site proteolytic enzyme 1 (BACE1) and γ-secretase to produce the Aβ peptide. These peptides are grouped together, with Aβ oligomers exhibiting the strongest neurotoxicity. Many MVBs and autophagosomes containing Aβ oligomers have been observed in the synaptic terminals of AD mouse models and neurons of AD patients. ¹²⁸ Similarly, in neuronal cell lines overexpressing AβPP, abnormally enlarged endosomes and autophagic vesicles loaded with Aβ can be observed.^20,129

Studies have shown that novel stimuli can increase autophagy, thereby promoting synaptic plasticity in hippocampal neurons. In addition, restoring impaired autophagy in the hippocampus can rescue memory deficits to a certain extent and promote the formation of new memories. ¹³⁰ Autophagy manifests in two main forms: selective autophagy, which involves the recognition of specific cargoes that are degraded (e.g., mitochondria, lipids, and protein aggregates), and nonselective autophagy, which involves the massive transport of damaged organelles or misfolded proteins to lysosomes. ¹³¹ Autophagy maintains cell homeostasis by degrading damaged or excess intracellular components. ¹³² However, in AD, the speed of autophagic clearance lags behind the speed of Aβ production, leading to the accumulation of neurotoxic aggregates and Aβ. ¹³³ Therefore, during the progression of AD, dysfunction of the endosomal-autophagic lysosomal system may create conditions conducive to Aβ aggregation, which may exacerbate this process. Beclin1 and phosphatidylinositol 3-phosphate (PI3P) levels are significantly decreased in AD mouse models.^38,134 Beclin1 knockout mice showed impaired lysosomal acidification. Compared with those of AD mice, the levels of AβPP and autophagy markers in the brains of offspring of these mice were greater, indicating that Beclin1 is involved not only in the initiation of autophagy but also in autophagosome‒lysosome fusion. ¹³⁵ Another study revealed that knocking down Beclin1 in mice not only impaired the formation of autophagosomes but also disrupted the fusion of endosomal–lysosomal pathways.^11,136 Some studies have also suggested that a decrease in the level of PI3P in the pathological state of AD hinders the sorting and transport of AβPP in the endosome, impairs the fusion of MVBs and autophagic lysosomes, and thus promotes the deposition of senile plaques.^134,137 AD can cause dysfunction of the lysosomal–autophagic lysosomal system and promote Aβ aggregation. In turn, defects in autophagy and the endosomal system can promote the production of Aβ, thereby forming a vicious cycle and accelerating the pathological process of AD.^138,139

Autophagy and tau protein

Studies have shown that the toxic effects of Aβ are mediated by the tau protein. The degree of cognitive decline in AD patients is positively correlated with the burden of neurofibrillary tangles in neurons, highlighting the importance of tau pathology in AD. The tau proteins that cause neurofibrillary tangles in the AD brain include abnormally hyperphosphorylated tau protein (P-tau), tau protein oligomers, and paired helical filaments formed by abnormally modified tau protein aggregates. In cells, a portion of soluble tau and P-tau is cleared by autophagy. ¹⁴⁰ However, in a pathological model, morphological abnormalities were observed in endosomes, amphisomes, and autophagosomes, which are rich in P-tau. ¹⁴¹ These findings indicate that the damaged lysosome–autophagy–lysosome system hinders the clearance of abnormal tau and promotes its accumulation.

Increased mTOR activity promotes tau protein phosphorylation.^142,143 Studies have shown that mTOR interacts with glycogen synthase kinase 3 (GSK3). Knockout of GSK3 in the mouse cerebral cortex inhibits mTOR activity, ¹⁴⁴ whereas GSK3 itself affects lysosomal function by inhibiting lysosomal acidification and promoting tau phosphorylation. ¹⁴⁵ In addition, autophagy is regulated not only by mTOR but also by the endosomal system, which may be a potential target for TOR regulation. ¹⁴⁶ The hyperphosphorylation of the tau protein in AD may be caused by decreased activity or phosphorylation of certain protein phosphatases, such as protein phosphatase 2A (PP2A).^147,148 These phosphatases are involved in signal transduction regulated by endosomes, which serve as a signaling platform to receive, diversify and isolate signaling molecules to their respective downstream pathways. ¹⁴⁹ Recent studies have shown that these enzymes are also involved in the regulation of the autophagy MAPK/ERK signaling pathway. ¹⁵⁰ These findings further emphasize the close relationship between tau phosphorylation and the lysosomal-autophagic lysosomal system.

Impact of autophagy on neurons

Neurons are the basic unit of the brain. Normal cognitive function maintenance depends on proper neuronal development and a healthy and stable brain microenvironment. ¹⁵¹ The pathological process of AD is always accompanied by neuronal damage and destruction of the brain microenvironment, which is often related to microglial dysfunction.^152,153 ALP is the main intracellular pathway involved in the removal of misfolded proteins and indigestible cell waste ¹⁵⁴ and plays a vital role in maintaining neuronal activity and homeostasis of the brain microenvironment. Some lysosomal proteins, such as progranulin, ¹⁵⁵ contribute to the repair mechanism of neurons. Lysosomes are essential for maintaining neuronal function and survival. ¹⁵⁶ Given that neurons are highly polarized, functionally special cells with many metabolic needs, they are intrinsically dependent on autophagy. ¹⁵⁷

Structurally, neurons are composed of three parts: cell bodies (cell bodies), axons and dendrites. Neuronal autophagy varies by location: nonselective autophagy in distal axons, mitophagy and endoplasmic reticulum autophagy in the cell body, and dendritic autophagy. However, macroautophagy is the main form of autophagy that occurs in neurons. ¹⁵⁸ Increasing evidence has shown that neuronal autophagy defects can lead to abnormal axon development and neurodegeneration. Because axons are involved in memory function, dysfunctional neuronal autophagy is considered to be a contributing factor to the pathogenesis of AD.^159,160 Autophagy regulates axon growth and development. Ban et al. reported that inhibition of autophagy by Atg7 siRNA resulted in axon elongation, whereas activation of autophagy by rapamycin inhibited axon growth. ¹⁶¹ Moreover, the loss of the autophagy-related protein ATG7 was found to increase synaptic transmission, suggesting that autophagy can inhibit axon growth while promoting synaptic development.^{158,162–164}

Another prominent pathological feature of AD is synaptic dysfunction. The synapse is a unique structure of neurons and is the basic unit of neuronal communication. ⁸⁷ Advances in technology have produced increasing amounts of experimental data, indicating that functional autophagy is necessary for neurotransmission and synaptic plasticity, which are essential for cognitive functions such as learning and memory. ¹⁶⁵ Studies have shown that new stimuli induce autophagy by promoting synaptic plasticity in hippocampal neurons. ¹³⁹ In addition, restoring the level of autophagy in the hippocampus of elderly individuals can reverse memory deficits and promote the formation of new memories. Neuronal autophagy includes two stages, induction and nucleation, which are mediated by the ULK1 complex and class III PI3K complex. The PI3K complex recruits DFCP1 (double FYVE protein 1), the E3-like Atg12-Atg5-Atg16L1 complex and LC3-PE conjugates to promote autophagosome formation. LC3 is acylated to LC3-PE through the Atg12-Atg5-Atg16L1 complex and integrated into autophagic vesicles. The formed autophagosomes fuse with late endosomes and are retrogradely transported to the cell body by dynein. ¹⁵⁷

Lysosomes are located mainly in neuronal cell bodies and are rare in axons. ¹⁵⁶ This requires the retrograde transport of autophagosomes produced in axons back to the cell body for fusion with lysosomes and subsequent degradation. The inability of autophagosomes to be correctly transported to the cell body for lysosomal fusion may be involved in the pathogenesis of AD. Some adaptor proteins, such as c-Jun N-terminal kinase interacting protein 1 (JIP1), Huntington-associated protein 1 (HAP1) and c-Jun N-terminal kinase interacting protein 3 (JIP3), have been identified as key proteins for the retrograde transport of autophagosomes. ¹⁶⁶ Enhancing the transport capacity of axonal autophagosomes may slow the progression of AD. However, the etiology of AD is multifactorial and has not been fully elucidated. Although there is much evidence that the pathogenesis of AD is related to ALP damage, further research on neuronal autophagy may provide strategies for alleviating or even treating AD and other neurodegenerative diseases.

Role of astrocytes in the pathogenesis of AD

Astrocytes have become a hot spot in the study of the pathogenesis of AD. Although the treatment strategy is aimed mainly at Aβ aggregation, there is currently no effective treatment. An increasing number of studies have revealed the profound influence of astrocytes on the pathological trajectory of Aβ and tau aggregation. ¹⁶⁷ Genomic studies have shown that most of the more than 40 gene loci associated with late-onset AD are expressed in astrocytes. These findings highlight the important role of astrocytes in the development of neuroinflammation and neurodegeneration in AD.^168,169 Therefore, astrocytes play a pathological role in the occurrence and development of AD, suggesting that the relationship between astrocytes and AD may provide a new direction for the treatment and diagnosis of AD.

Astrocytes account for a large proportion of brain cells, approximately 20–40% of which are glial cells, and glial cells account for approximately 40% of all brain cells. ¹⁷⁰ Physiologically, astrocytes regulate a series of key processes, including the management of neurotransmitters and calcium balance, the formation and maturation of synapses, the maintenance of the blood‒brain barrier (BBB), the control of extracellular volume and ion homeostasis, and the provision of nutritional support to the brain through neurovascular units. ¹⁷¹ Under pathological conditions, astrocytes undergo diverse changes, which can be morphologically divided into three categories: (1) atrophied astrocytes; (2) pathological remodeling of astrocytes; and (3) reactive astrocyte proliferation. ¹⁷² Atrophic astrocytes are characterized by reduced cell volume, reduced processes, and weakened steady-state function. Astrocyte atrophy is seen in many central nervous system diseases, such as AD, ALS, epilepsy, and schizophrenia. ¹⁷³ The pathological remodeling of astrocytes is characterized by the presence of specific cytoplasmic inclusions called “Rosenthal fibers”, which are usually found in diseases caused by nutritional deficiencies in white matter, such as Alexander's disease. ¹⁷² Reactive astrocyte proliferation, marked by astrocyte proliferation with cell hypertrophy and thickened protrusions, is also common in a variety of central nervous system diseases, including AD. Genetic studies have shown that the overall risk of AD is largely related to glial gene expression. Notably, some AD risk genes, including clusterin/Apo J (CLU), sorting protein-related receptor 1 (SORL1), fermitin family member 2 (FERMT2) and APOE, are expressed mainly in astrocytes. During the development of AD, astrocytes undergo a series of morphological, molecular and functional changes, which indicates that astrocytes play important roles in the pathogenesis of AD. In an AD mouse model, changes in astrocytes may occur even before the appearance of Aβ. Studies have shown that induced pluripotent stem cell (iPSC)-derived astrocytes in AD patients also atrophy in vitro. ¹⁷⁴ Furthermore, transcriptomic analysis of reactive astrocytes isolated from AD model mice revealed strong induction of the expression of inflammatory genes such as Cst7, CCl4, Il1b, Clec7a and Tyrobp, indicating that astrocytes are involved in the neuroinflammatory process of AD. ¹⁷⁵

When microglia are stimulated, the emergence of A1-reactive astrocytes marks the beginning of AD neuronal death. Microglia induce these toxic A1 astrocytes by secreting interleukin-1α (IL-1α), tumor necrosis factor-α (TNF-α) and complement component 1q (C1q). ¹⁶⁹ A1-reactive astrocytes further upregulate complement cascade genes, including complement component 3 (C3). Approximately 60% of astrocytes in the prefrontal cortex of AD mice express C3 and release an unknown neurotoxin that induces neuronal and oligodendrocyte death. In addition, A1-reactive astrocytes in mice lose their normal ability to support synaptic formation and function, engulf synaptic and myelin debris, and promote neuronal survival. ¹⁶⁹ Neuroinflammation induces neurotoxic A1-reactive astrocytes, whereas ischemia induces protective A2-type astrocytes. These A2-type astrocytes upregulate the expression of neurotrophic genes such as cardiotrophin-like cytokine factor 1 (CLCF1), transglutaminase 1 (TGM1), pentraxin 3 (PTX3), calcium-binding protein A10 (s100a10 gene) and sphingosine kinase 1 (SPHK1). A2 astrocytes further secrete neurotrophic factors, promote neuronal survival and growth, and participate in synaptic repair. ¹⁷⁶ Morphological analysis of reactive astrocytes revealed that in AD patients and mouse models, astrocytes around Aβ plaques exhibit phagocytic activity and can engulf malnourished neurites. ¹⁷⁷ In addition, Aβ-activated astrocytes secrete transforming growth factor β, promote the uptake of Aβ by glial cells, and protect neurons from its toxic effects. ¹⁷⁸

There is a complex relationship between astrocytes and Aβ. In an AD mouse model, Aβ promotes the activation of the NF-κB pathway by astrocytes, leading to the release of C3. Aβ-activated astrocytes also exhibit the A1 phenotype. ¹⁷⁹ C3 binds to neurons through C3aR receptors and destroys dendritic morphology and function. The interaction between C3 and glial cells also affects the phagocytosis of Aβ. Both pathways may be involved in the pathogenesis of AD. ¹⁸⁰ Additionally, reactive astrocytes upregulate the expression of AβPP and BACE1, which are involved in the production of Aβ in the AD brain. ¹⁸¹ On the other hand, astrocytes also participate in Aβ clearance through the secretion of factors such as ApoE, apolipoprotein J (Apo J, aggrecan), α1-antichymotrypsin, α2-macroglobulin, etc., and promote the transport of Aβ across the BBB through receptors such as low-density lipoprotein receptor-related protein-1and very low-density lipoprotein receptor. ¹⁸² Although astrocytes express a variety of Aβ-degrading enzymes, quantitative studies on the overall effect of astrocytes on Aβ levels in the brain are lacking. These complex interactions need to be further explored in the context of the pathological state of AD (Figure 2).

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Figure 2.

A1-reactive astrocytes are involved in the pathogenesis of AD. The emergence of A1-reactive astrocytes marks the beginning of AD neuronal death. Induced by factors secreted by microglia, A1 astrocytes upregulate complement cascade genes (e.g., C3), release an unknown neurotoxin, kill neurons and oligodendrocytes, and lose the ability to support normal neuronal function. In contrast, ischemia induces the formation of protective A2 astrocytes. These A2 astrocytes upregulate the expression of neurotrophic genes, secrete neurotrophic factors, and participate in synaptic repair. Astrocytes have a complex relationship with Aβ. In an AD mouse model, Aβ promotes the activation of the NF-κB pathway by astrocytes, resulting in C3 release and the A1 phenotype. C3 destroys neuronal function and impairs Aβ phagocytosis, thereby promoting the pathogenesis of AD. Reactive astrocytes upregulate AβPP and BACE1, which are involved in the production of Aβ. In turn, they also promote the clearance and transport of Aβ through the secretion of specific proteins.

Microglial dysfunction in AD pathogenesis

In the brain, microglia belong to the mononuclear phagocyte system, accounting for 10–15% of all brain cells, and are the main sentinels for immune defense against pathogen invasion. ¹⁸³ Under normal circumstances, microglial activation is essential for tissue repair and maintenance of homeostasis in the brain through the engulfment of harmful substances and the secretion of anti-inflammatory factors. However, abnormal activation of these cells increases the levels of inflammatory mediators, impairs immune clearance mechanisms, and triggers neurological disorders. ¹⁸⁴

Microglia are divided into two phenotypes, “resting M0” and “activated M1/M2”, according to their activation status. They can rapidly change their shape and function in response to changes in the brain's microenvironment. By secreting neurotransmitters (such as GABA, acetylcholine, and norepinephrine) that bind to microglial cell membrane receptors, neurons help maintain the resting M0 state, allowing microglia to continuously monitor the surrounding brain environment. Under the stimulation of various factors, microglia are transformed into activated M1 or M2 types, which are characterized by larger cell bodies and thicker protrusions. Different activation phenotypes have different immune functions in the nervous system. Stimulation with ligands such as lipopolysaccharide, interferon-γ, Aβ, and α-synuclein polarizes microglia to the classically activated M1 phenotype. M1 microglia secrete inflammatory mediators such as TNF-α, nitric oxide, and reactive oxygen species (ROS) to damage nerve cells. In contrast, interleukin (IL) 4/13 induced microglia to alternatively activate M2 polarization. M2 microglia mainly promote the production of arginase-1 (Arg-1) and chitinase-3-like proteins (Chil31 and YKL-40), thereby exerting anti-inflammatory effects, promoting tissue repair, and contributing to extracellular matrix remodeling. In addition, IL-10 and transforming growth factor-β can promote the transition to an inactivated M2 phenotype, which is aimed mainly at inhibiting the production of proinflammatory cytokines and reducing acute inflammation, thereby facilitating inflammation resolution and wound healing.^{183,185–187}

In AD, abnormal accumulation of Aβ and tau proteins leads to chronic activation of microglia, causing dysfunction and an imbalance of M1/M2 subtypes. This leads to the release of a large number of inflammatory mediators and accelerates the phosphorylation of tau. Increased levels of ROS and reactive nitrogen species also drive microglia to polarize to the M1 phenotype, further increasing the secretion of proinflammatory cytokines and other inflammatory factors and exacerbating neuronal damage. ¹⁸⁸ Additionally, tau overexpression induces synaptic dysfunction, increases the phagocytic burden of microglia, and promotes the formation of neurofibrillary tangles. ¹⁸⁹ The absence of CX3C chemokine receptor 1 (CX3CR1) on microglia leads to increased IL-1β levels, increased neuronal AβPP expression, and increased Aβ deposition. ¹⁹⁰ In AD, IL-1β can cause neuronal calcium overload, neuronal loss or death, and increased neurofibrillary tangles. ¹⁹¹ A study by Shi et al. ¹⁹² revealed that the elimination or control of microglial activation can block the phosphorylation of the tau protein, suggesting that the AD risk gene APOE may regulate tau pathology by affecting the polarization of M1/M2 microglia. Neurons form abundant synaptic connections to generate signals during development. ¹⁹³ Microglia perceive dynamic changes in the extracellular environment through the processes and secretion of nerve growth factor, basic fibroblast growth factor, brain-derived neurotrophic factor, and other factors to maintain cell survival, promote the phagocytosis of apoptotic neurons, and prune synapses.^183,194 In a variety of neurodegenerative diseases, the balance between M1 and M2 microglial polarization is disrupted, resulting in decreased or excessive phagocytosis, which is a common pathological feature.

Microglia release different signals according to their surface complement receptors, which can promote or inhibit phagocytosis. In the AD brain, Aβ deposition induces abundant C1q production, resulting in increased levels of downstream C3. C3 signaling is transmitted to microglia and binds to complement receptor 3, triggering excessive phagocytosis of synapses by microglia and eventually leading to synaptic loss and neuronal death. ¹⁹⁵ Cluster of differentiation 47 (CD47) binds to signal regulatory protein α (SIRPα) on microglia to inhibit synaptic phagocytosis. ¹⁹⁶ However, the absence of SIRPα in microglia impairs the recognition of CD47, thereby increasing synaptic phagocytosis and exacerbating cognitive impairment. ¹⁹⁷ In an AD mouse model, knockdown of CX3CR1 can reduce the excessive phagocytosis of synapses by microglia and reduce synaptic loss. ¹⁹⁸ Therefore, regulating the phagocytic activity of microglia is essential for maintaining synaptic homeostasis.

Therefore, excessive microglial activation significantly exacerbates cognitive dysfunction in patients with AD. Therefore, therapeutic strategies may include inhibiting the activation of microglia by upregulating the expression of anti-inflammatory cytokines and downregulating the expression of proinflammatory cytokines, regulating inflammation of the systemic immune system and the central nervous system, inhibiting the excessive activation of microglia, inducing M2 polarization and the secretion of anti-inflammatory substances, enhancing antioxidant capacity, regulating mitophagy, and reducing brain Aβ deposition and neurofibrillary tangles, thereby slowing the progression of AD.

Oligodendrocytes

Oligodendrocytes, together with astrocytes, microglia and neurons, form crucial collections in the central nervous system of vertebrates. Oligodendrocytes play a crucial role in maintaining homeostasis in the brain, and their unique function is the myelination of neuronal axons.^199,200 However, increasing evidence has shown that oligodendrocytes, in addition to their role as the shell of neuronal axons, play an indispensable metabolic support role in maintaining the metabolic homeostasis of neuronal axons. ²⁰¹ Because axon formation requires more energy, these processes are highly dependent on the metabolites provided by oligodendrocytes through myelin channels. Notably, highly lipid-rich myelin can provide key lipid and energy substrates for axons to maintain synaptic survival under early metabolic stress induced by cerebral ischemia or AD. ²⁰²

In the process of myelination, oligodendrocyte progenitor cells (OPCs) proliferate and migrate and ultimately differentiate into mature oligodendrocytes, forming myelin around axons. They play key roles in myelination and remyelination under pathological conditions.^203,204 Oligodendrocytes not only facilitate the rapid transmission of signals but also provide metabolic support for axons. ²⁰⁵ ALP is the core hub for maintaining oligodendrocyte function and myelin homeostasis. Glial cells undergoing myelin sheath formation are highly dependent on autophagy to continuously remove aging-damaged myelin debris and lipid peroxides. ²⁰⁶ Under physiological conditions, OPCs must be reprogrammed by precise autophagic metabolism to successfully differentiate into mature oligodendrocytes and undergo myelination. ²⁰⁷ Recent mononuclear RNA sequencing studies revealed that in the pathological microenvironment of AD, ALP damage can lead to a large accumulation of undegraded oxidized lipids and Aβ, which promotes the transformation of healthy oligodendrocytes to disease-related oligodendrocytes.^208,209 When the ALP is damaged, undegradable toxic proteins and abnormal lipids accumulate in oligodendrocytes, which not only directly hinders the differentiation of OPCs and the regeneration of the white matter myelin sheath but also cuts off the metabolic supply of oligodendrocytes to axons.^210,211 Therefore, the occurrence of AD, multiple sclerosis, ALS, and other neurodegenerative diseases is closely related to the dysfunction of oligodendrocytes. ²⁰⁹ Recent studies have shown that Aβ deposition is associated with myelin dysfunction and demyelination in AD mouse models. Myelin sheath injury can aggravate Aβ deposition, affect AβPP metabolism, and impair the ability of microglia to remove Aβ aggregates. ²¹² Crucially, ALP defects in oligodendrocytes form a vicious cycle with Aβ pathology. On the one hand, Aβ toxicity and local neuroinflammation directly attack oligodendrocytes and induce demyelination. ²¹³ On the other hand, recent breakthrough studies have confirmed that myelin dysfunction due to impaired ALP itself directly drives the accelerated deposition of Aβ, further worsening cognitive decline.^212,214

Myelin basic protein (MBP) is a protein that is rich in basic amino acids and is essential for maintaining the structural and functional stability of the myelin sheath in the central nervous system. However, Chen et al. reported that the level of MBP protein in the hippocampus of APP/PS1 mice was decreased and that the MBP-positive areas in the cortex and hippocampus of AD patients were significantly reduced, indicating that there is a potential correlation between myelin loss and cognitive impairment. ²¹⁵ The differentiation of OPCs seems to be affected by muscarinic receptor 1 (M1R). The downregulation of M1R expression can promote OPC differentiation into mature oligodendrocytes, thereby improving myelination. ²¹⁶ Wang et al. further confirmed that enhanced myelination and regeneration can improve memory function and reduce synaptic loss in the hippocampus of aged mice. ²¹⁷ Nevertheless, the specific pathways and mechanisms through which myelin dysfunction leads to neurodegenerative diseases are still unclear.

A recent study revealed that microglial dysfunction affects normal myelin development. ²¹⁸ Neurodegenerative diseases are not caused by a single factor but involve a series of cascade reactions. Neurodegenerative diseases are not caused by a single factor but involve a series of cascade reactions.

Although the complex relationship between oligodendrocytes and AD is still unclear, targeted restoration of autophagic flux in oligodendrocytes not only protects white matter structure but also rests metabolic support of damaged axons. For example, enhanced myelin renewal has been shown to reverse cognitive dysfunction in AD mouse models. ²¹⁵ These findings provide a new pathological basis for the intervention of AD from the perspective of oligodendrocyte metabolism.^207,211,214 However, the molecular mechanism of ALP damage in oligodendrocytes and its direct contribution to AD are still relatively unclear, and future research needs to fill this gap. To elucidate the causal relationship between the two and elucidate its potential mechanism, follow-up studies can be carried out on the basis of the following two key dimensions: (1) Construction of a cell-specific autophagy loss model: Cre tool mice driven by oligodendrocyte-specific promoters (such as MBP or Plp1) were crossed with conditional knockout mice of key autophagy regulatory genes (such as Atg7 or Tfeb) and introduced into the AD genetic background (such as APP/PS1 or 5xFAD). This model can specifically block lysosomal degradation flow in oligodendrocytes to directly evaluate the specific mechanism through which ALP failure in oligodendrocytes affects the maintenance of myelin integrity and the effect on surrounding Aβ plaques. (2) Multidimensional omics combined with in vitro simulation verification: A coculture model was established by using oligodendrocytes derived from human iPSCs and neurons, and spatial transcriptomics technology was used to analyze the metabolic abnormalities between oligodendrocytes and neurons in the AD environment. This method helps to identify how oligodendrocyte lineages in different states evolve and participate in disease progression in the context of ALP damage. ²¹⁹ By analyzing spatial location-related gene expression characteristics, we can further explore whether oligodendrocytes release proinflammatory mediators or metabolic toxic products through secretome remodeling under cell stress caused by impaired ALP function, thereby inducing axonal degeneration and synaptic dysfunction through paracrine effects. ²²⁰ This in-depth mechanistic analysis provides key evidence for the role of oligodendrocytes in the pathological evolution of AD.

Finally, we summarized the cell type-specific changes and effects of ALP dysfunction in different brain cells in patients with AD (Table 2), providing a systematic framework for understanding the multiple roles of ALP in the pathogenesis of AD.

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Table 2.

Characteristics of ALP and the influence of main brain cell types in patients with AD.

Cell types	Functional characteristics of ALP	Core manifestations in AD	Effects on Aβ/tau pathology	References
Neurons	Inhibits axonal development	Synaptic dysfunction	Inhibits the clearance of Aβ and tau	221,222
A1-type astrocytes	Impairs synaptogenesis	Triggers neuroinflammation	Impairs the clearance capacity of misfolded proteins	223
A2-type astrocytes	Phagocytoses dystrophic neurons and Aβ	Enhances Aβ clearance capacity	The main contributor to the phagocytosis of Aβ and tau	15,224,225
Microglia	Promotes phagocytosis of apoptotic neurons and synaptic pruning	Excessive synaptic pruning	Accelerates tau phosphorylation	188,226,227
Oligodendrocytes		Myelin dysfunction and demyelination	Aggravates Aβ deposition	211,212,228