1 Introduction

Autophagy represents a mechanism by which eukaryotic organisms maintain homeostasis within their intracellular environment. This process involves the degradation of misfolded proteins, necrotic organelles, foreign pathogens and other cellular components. The general process of autophagy involves the separation of damaged organelles from healthy organelles to form autophagosomes, which subsequently fuse with lysosomes to form autophagic lysosomes. These autophagic lysosomes degrade proteins through the action of lysosomal enzymes (Kaushik et al., 2021). In normal circumstances, the level of autophagy within the body is relatively low. However, in response to external stimuli such as starvation, hypoxia and other forms of stress, the number of autophagosomes increases significantly, thereby elevating the overall level of autophagy within the body. Autophagy is classified into three main categories: macroautophagy, microautophagy and chaperone-mediated autophagy. Of these, macroautophagy is the most prevalent form of autophagy. Consequently, macroautophagy is also defined as autophagy in a narrow sense. Macroautophagy is primarily facilitated by double-membrane organelles, including mitochondria, the endoplasmic reticulum, the Golgi apparatus, and the plasma membrane. Mitochondria are of significant importance in the context of energy metabolism within eukaryotic cells. In addition to this, they play a regulatory role in processes such as cell development, the cell cycle and cell death.

The health of mitochondria is of critical importance for neuronal cell function. In recent years, mitophagy has emerged as a mechanism that plays a crucial role in maintaining mitochondrial quality. In order to maintain mitochondrial numbers and physiological functions, the body initiates appropriate mitochondrial biogenesis, damage repair, or autophagic clearance mechanisms under stress, called mitochondrial quality control (MQC) (Song M. et al., 2022). Mitophagy is currently acknowledged as the primary mechanism of MQC in neurons. In the absence of any damage, mitochondria are able to fuse with neighboring intact mitochondria, thereby restoring their functionality (Chan, 2006). Nevertheless, when the extent of mitochondrial damage surpasses the capacity for repair, it results in an alteration in the membrane potential of the outer mitochondrial membrane (OMM), thereby initiating mitophagy. Mitophagy can be broadly categorized into three main types, depending on the physiological condition of the cell: steady-state mitophagy, programmed mitophagy and stress-induced mitophagy (Ke, 2020; Montava-Garriga and Ganley, 2020). The three distinct types of mitophagy serve disparate functions within the cell. Homeostatic mitophagy, which typically occurs under physiological conditions, is responsible for the removal of damaged or aged mitochondria, thereby ensuring the maintenance of mitochondrial quantity and quality. Programmed mitophagy usually refers to the selective degradation of specific mitochondria, a process mostly associated with cell differentiation, development, or other specific physiological processes. Stress-induced mitophagy usually occurs when cells are exposed to external stressors, such as oxidative stress, energy deprivation, or DNA damage. This process prevents cellular damage and apoptosis by facilitating the removal of damaged mitochondria, thereby enabling cells to adapt to stressful conditions. Mitophagy is a process by which damaged or malfunctioning mitochondria are removed from cells. This process is essential for maintaining mitochondrial structural integrity, energy metabolism, regulating the cell cycle and proliferation, preventing oxidative stress, and preventing cell death (Hu et al., 2020). We focus on the phenomenon of stress-induced mitosis. In the event of mitochondrial damage, a reduction in membrane potential is observed, which is followed by the activation of mitophagy genes or factors. This is accompanied by the formation of autophagic vesicles, which wrap around damaged mitochondria to form autophagosomes. These subsequently bind to lysosomes, forming autophagolysosomes, whereby the mitochondria are broken down into amino acids and lipids. Mitophagy is a highly sophisticated process that involves multiple protein complexes and signaling pathways. A detailed description of these will be provided subsequently.

Oxidative stress can be defined as the disruption of the equilibrium between ROS and antioxidant systems within the body. This leads to an accumulation of ROS, which in turn causes damage to proteins, cell membranes, and DNA, while also promoting the development of inflammation and accelerating the process of cellular ageing. Common ROS are Superoxide Anion Radical (O2−), Hydroxyl Radical (·OH), Hydrogen Peroxide (H2O2), Nitric Oxide Radical (NO·), Lipid Peroxyl Radical (Lipid Peroxyl Radical, LOO·), Peroxynitrite Radical (Peroxynitrite, ONOO−), and others. The presence of one or more unpaired electrons renders these molecules highly unstable, both structurally and in terms of their reactivity with other molecules. ROS can be generated by many pathways, such as the electron transport chain during mitochondrial oxidative phosphorylation, inflammatory reactions, and UV or radiation exposure, with the electron transport chain during oxidative phosphorylation being the main source of ROS (Lee et al., 2012).

The following section will examine the process of ROS production in mitochondrial oxidative phosphorylation. (1) Glycolysis: within the cytoplasm, glucose is metabolized into two pyruvate molecules, resulting in the generation of small quantities of ATP and reduced nicotinamide adenine dinucleotide (NADH). (2) Pyruvate oxidation and the citric acid cycle: Pyruvate produced by glycolysis is transported to the mitochondria to be converted into acetyl coenzyme A. Acetyl coenzyme A combines with oxaloacetate to form citric acid, which then undergoes a series of chemical reactions that ultimately produce oxaloacetate and NADH, reduced flavin adenine dinucleotide (FADH2) and a small amount of ATP. (3) Electron Chain Transfer: NADH and FADH2 transfer electrons to the electron transfer chain in the inner mitochondrial membrane, where protons (H+) are pumped from the mitochondrial matrix into the mitochondrial membrane gap in the presence of complexes I, II, III, and IV, as well as auxiliary molecules such as coenzyme Q and cytochrome c (Cyt c), which creates a gradient of protons on both sides of the membrane. (4) Chemically permeable coupling: protons are returned to the substrate by ATP synthase, a process that enables ATP synthase to add phosphate groups to adenosine diphosphate (ADP) to produce ATP. 5. Ultimately, O2 combines with electrons and protons to form H2O. Throughout this process, the mitochondria can produce approximately 30–32 ATP for every glucose molecule broken down. In typical circumstances, electron transfer is highly efficient. However, a minor leakage of electrons occurs during the transfer processes involving complexes I and III. These leaked electrons reduce oxygen to superoxide anion (O2−), which is often rapidly converted by superoxide dismutase (SOD) to H2O2, which is relatively stable and can cross the cell membrane. Under certain conditions, H2O2 can further react to form more reactive ROS (e.g., ·OH). ROS cause lipid peroxidation of unsaturated fatty acids in cell membranes, ultimately resulting in the formation of malondialdehyde (MDA), which impairs the function and structure of cell membranes (Mukherjee et al., 2019). In addition, ROS can cause DNA damage, protein oxidation, and induce inflammation and apoptosis (Suski et al., 2012; Wang Y. et al., 2019). In addition to ROS production during electron chain transfer, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, xanthine oxidase, and dysfunctional endothelial nitric oxide synthase (eNOS) can also produce ROS (Vassort and Turan, 2010). In order to prevent the occurrence of oxidative stress, a variety of antioxidants and enzymes are present in the body to prevent oxidative damage, such as superoxide dismutase (SOD), glutathione (GSH), vitamins C and vitamins E, and so on. Among them, GSH is one of the important reducing agents in the brain, which is oxidized by ROS and converted to glutathione disulfide (GSSG), which is subsequently reduced to GSH by glutathione reductase (Couto et al., 2016). Oxidative stress occurs when the redox imbalance in the body produces ROS that exceed the scavenging capacity of antioxidants. The brain accounts for approximately 20% of the total energy consumed by the entire body, thereby necessitating a higher demand for energy and oxygen. Consequently, the mitochondria within the brain are more active than those found in other tissues. This, coupled with the fact that the brain contains large amounts of unsaturated fatty acids and an imperfect oxidizing enzyme system (e.g., superoxide dismutase, glutathione peroxidase), results in the brain being more susceptible to oxidative stress than other tissues and organs.

Mitophagy and oxidative stress are both intimately associated with mitochondria, and the two are mutually interactive. Collectively, they are implicated in the regulation of cellular function and the maintenance of internal cellular homeostasis. When mitochondria are damaged, the mitophagy program is initiated to prevent the damaged mitochondria from causing damage to the organism; damaged mitochondria are unable to maintain ATP and Ca2+ metabolism, leading to a lack of energy and the leakage of electrons, which accelerates the generation of ROS, one of the key factors in the activation of autophagy (Albers and Beal, 2000; Finkel and Holbrook, 2000). Mitochondria are the main site of ROS generation and the main target of ROS. Oxidative stress has been demonstrated to result in impairment of mitochondrial function, including a reduction in mitochondrial membrane potential and damage to mitochondrial membrane integrity. A reduction in mitochondrial membrane potential or damage to the mitochondria can induce mitophagy, which is the process by which damaged mitochondria are removed from the cell, thereby reducing ROS production. A reduction in mitophagy levels, resulting from the inability to remove damaged mitochondria in a timely and effective manner, will lead to an exacerbation of ROS production and accumulation. This, in turn, will cause further damage to the cells, potentially leading to cell death. Mitophagy and oxidative stress are two complex biological processes that are intertwined in multiple signaling pathways. Such as PI3K/AKT/mTOR pathway, AMPK pathway, Nrf2/Keap1 pathway, p53, FOXO, etc. The PI3K/AKT/mTOR signaling pathway is a critical regulator of cellular processes, including growth, metabolism and survival. Mammalian target of rapamycin protein (mTOR) is an important nutrient and energy sensor that plays a role in both oxidative stress and mitophagy. mTOR decreased activity promotes mitophagy while enhancing cellular tolerance to oxidative stress. Adenosine 5-monophosphate (AMP)-activated protein kinase (AMPK) is a key monitor of cellular energy status. AMPK is activated in response to decreased ATP levels, promoting ATP production and inhibiting ATP depletion. p53 is a tumor suppressor protein that plays an important role in DNA damage response and apoptosis. p53 is also involved in the regulation of oxidative stress and mitophagy. p53 in its active form is able to induce mitophagy, and prevents the occurrence of oxidative stress by affecting the expression of antioxidant genes. The active form of p53 induces mitophagy and prevents oxidative stress by affecting the expression of antioxidant genes. The FOXO transcription factor family plays an important role in cellular antioxidant defense and lifespan regulation. Stimulated by ROS, FOXO transcription factors are able to activate a variety of antioxidant genes, including those related to mitophagy. These pathways coordinate the processes of oxidative stress and mitophagy through different mechanisms to maintain normal cellular function and the stability of the intracellular environment.

There is growing evidence that mitophagy and oxidative stress have important roles in the development of neurological diseases. Cerebral ischemia/reperfusion injury (CI/RI), vascular dementia (VaD), and Alzheimer’s disease (AD) are the most common neurological disorders in the clinic. In CI/RI, mitophagy and oxidative stress play important roles. The sudden increase in O2 levels after ischemia and hypoxia-reperfusion impairs mitochondrial function and generates a large amount of ROS. ROS initiate mitophagy through the PINK1/Parkin signaling pathway, PI3K/AKT/mTOR signaling, and the FUNDC1 protein to clear damaged mitochondria, thus alleviating oxidative stress and maintaining intracellular environmental homeostasis. In VaD, damage to brain tissue caused by various factors, including insufficient cerebral perfusion, small vessel lesions, and neuroinflammation, can result in oxidative stress, which in turn leads to an excessive accumulation of ROS and further damage to mitochondria. Following the onset of oxidative stress, the mitophagy process is initiated, which results in the removal of damaged mitochondria and a reduction in ROS production. The production of ROS in AD can be accelerated by three main factors: the excessive accumulation of Aβ, excessive phosphorylation of tau protein and inflammation. The BBB is an important functional barrier in the brain and plays a crucial role in maintaining homeostasis of the brain’s internal environment. Among them, tight junction proteins are important proteins for maintaining BBB permeability. Altered BBB permeability is closely related to the development of neurological diseases (CI/RI, VaD, AD, etc.). ROS have the potential to directly damage the endothelial cell membrane of the BBB, leading to cellular damage. Additionally, ROS can disrupt the structure and function of tight junction proteins, including ZO-1, occludin, and claudin, which may result in an increased permeability of the BBB. In conclusion, mitophagy and oxidative stress are of great importance in the physiopathological processes of the organism, and may represent a novel avenue for the treatment of neurological disorders. CI/RI, VaD, and AD, as three common neurological disorders, have a serious impact on the quality of life and the safety of human life. In conclusion, mitophagy and oxidative stress affect the homeostasis of the brain’s internal environment, and a clearer understanding of the cell death mechanisms in neurological disorders, leading to the development of more promising therapeutic agents is imperative and imminent.

2 Mechanisms of mitophagy and oxidative stress 2.1 Mechanisms of mitophagy

Mitophagy represents a principal mechanism of MQC, enabling the reduction of cellular damage. A growing corpus of evidence indicates that multiple signaling pathways or factors may be involved in the regulation of mitophagy in the context of the development of neurological diseases. PINK1 has a role in participating in MQC and preventing apoptosis. Parkin is a downstream molecule of PINK1, and the PINK1/Parkin signaling pathway is an important pathway for regulating mitophagy. FUNDC1, an autophagy receptor located in the OMM, regulates mitochondrial fusion/fission and induces mitophagy under hypoxic conditions. Autophagy-related genes (Atg) are important specific genes in the process of cellular autophagy. Mitophagy begins with the activation of the Atg/ULK1 complex, which then activates PI3K, which further activates the PI3K/AKT/mTOR signaling pathway and participates in the regulation of mitophagy. UNC-51-like kinase 1 (ULK1) is an important component of autophagic vesicles and a major regulator of the initiation step of autophagy. The main regulator of autophagy initiation is ULK1. Beclin1 is a key protein in ULK1-activated autophagy. Under normal conditions, the mTOR pathway inhibits Beclin1, thereby reducing mitophagy. On the contrary, under nutrient deficiency or stress conditions, pathways such as AMPK activate Beclin1 and promote mitophagy. There are numerous mechanisms regulating mitochondria, and the above signaling pathways or molecules are only some of them. The aim of this review is to provide a systematic account of the process of regulating mitophagy by the above signaling pathways or molecules, as well as their roles in the occurrence and development of neurological disorders. This will serve to establish a theoretical basis and inspire the development of therapeutic strategies for this class of diseases. (Figure 1).

Figure 1. Diagram of mitophagy mechanism.

2.1.1 PINK1/Parkin signaling pathway

The PINK1/Parkin pathway is one of the most important pathways in the mitophagy pathway and most of the mitophagy is mediated by this pathway (Painter et al., 2020). PINK1 is a serine/threonine ubiquitin kinase located primarily in the inner mitochondrial membrane, and its downstream molecule, Parkin, is an E3 ubiquitin ligase (Zhao et al., 2024). Normally, PINK1 is rapidly cleaved and degraded by mitochondrial processing protease (MPP) and proteolytic PINK1 protein [pesenilin-associated rhomboid-like (PARL)], and is therefore extremely low in mitochondria (Geisler et al., 2010; Poole et al., 2010). Under stress, the mitochondrial membrane potential is depolarized and related proteases such as MPP and PARL are inhibited. Originally low levels of PINK1 begin to accumulate and become phosphorylated at the Ser228 and 402 sites, thereby activating PINK1 kinase (Sekine and Youle, 2018). Subsequently, cytoplasmic Parkin translocates to the damaged mitochondrial outer membrane. Mediated by PINK1, Parkin’s Ser 65 site is phosphorylated and initiates its E3 ubiquitin ligase activity, resulting in ubiquitin phosphorylation of mitochondrial outer membrane protein junctions (Harper et al., 2018). Phosphorylated ubiquitin has a high affinity for Parkin, which leads to Parkin recruitment, and newly recruited Parkin will further ubiquitinate mitochondrial outer membrane proteins, thus forming a strong positive feedback loop that amplifies ubiquitin signaling and promotes the aggregation of autophagy receptor 62 (isolation vesicle 1) at the same time (Matsuda and Yamano, 2020). P62 contains an LC3 binding domain (LIR), which helps transport damaged mitochondria to autophagosomes by binding to microtubule-associated protein 1 light chain 3 (LC3) on the autophagosome membrane via LIR. In addition, P62 has a specific ubiquitin-binding domain, UbA (ubiquitin-associated binding domain), a structure that allows P62 to directly recognize and bind to ubiquitinated proteins (Danieli and Martens, 2018). Ubiquitin (Ub) is a small protein that attaches to other proteins by ubiquitination. The process of ubiquitination involves three enzymes: activating enzyme (E1), conjugating enzyme (E2), and ligase (E3), which work in concert to attach ubiquitin molecules to target proteins. During mitophagy, the ubiquitinated protein or organelle is recognized by the p62 autophagy receptor. In conclusion, PINK1 accumulates in mitochondria and phosphorylates at Ser228 and 402 sites, followed by the transfer of Parkin to the OMM and phosphorylates at Ser 65, initiating its E3 ubiquitin ligase activity and phosphorylating the Ub of the mitochondrial outer membrane protein linkage. Then P62 starts to recruit and binds to LC3-II, which sends the damaged mitochondria to autophagosomes and is phagocytosed by autophagosomes. Mature autophagosomes bind to lysosomes to form autophagolytic lysosomes, and eventually the damaged mitochondria are degraded (Lazarou et al., 2015).

2.1.2 FUNDC1 protein

FUNDC1, an autophagy receptor protein located in the OMM and containing a transmembrane LIR structural domain, regulates mitochondrial fusion/fission (DRP1 and OPA1) and induces mitophagy under hypoxic conditions (Chen et al., 2016). In the normal state, FUNDC1 is stably present in the OMM and remains highly phosphorylated (Ser13 and Tyr18 sites phosphorylated) (Greene et al., 2012). FUNDC1 activity is regulated by a variety of signaling pathways. Bcl-2 is primarily involved in apoptosis, but it also plays an important role in the regulation of mitophagy (Dikic and Elazar, 2018). Bcl-2 interacts with phosphoglycerate mutase/protein phosphatase (PGAM5), a member of the phosphoglycerate mutase family, and prevents dephosphorylation of the LIR structural domain of FUNDC1, thereby inhibiting FUNDC1-mediated mitophagy (Wu H. et al., 2014). However, under hypoxia, PGAM5 dephosphorylates the LIR structural domain of FUNDC1 and binds to LC3, thereby inducing mitophagy and helping the cell to remove damaged mitochondria and maintain cellular stability (Wu W. et al., 2014; Chen et al., 2016). LC3 is a key protein in the autophagy process, which is involved in the formation and maturation of autophagosomes. In mitophagy, the interaction between LC3 and FUNDC1 is particularly important, and the LIR domain of FUNDC1 directly binds to LC3 to promote autophagosome formation, encapsulation and disassembly of damaged mitochondria (Wu W. et al., 2014). In addition, AMPK also regulates mitophagy. Activated AMPK under normal conditions inhibits mitophagy by activating AKT, which promotes the phosphorylation of FUNDC1. Under energy-deficient conditions, activated AMPK dephosphorylates FUNDC1, thereby activating mitophagy. In conclusion, FUNDC1, Bcl-2, and LC3 interact with each other to form a complex and sophisticated regulatory system. Dephosphorylated FUNDC1 is able to bind to LC3, thus activating mitophagy. At the same time, Bcl-2 in turn can affect the activity of FUNDC1, thereby regulating the process of mitophagy. This ensures that mitophagy is able to regulate mitochondrial mass in a timely and accurate manner under physiological and pathological conditions, which is crucial for the brain with high energy demands.

2.1.3 PI3K/AKT/mTOR signaling pathway

Autophagy related gene (Atg) is an important gene in the autophagy process, and UNC-51-like kinase1 (ULK1) is the only serine/threonine protein kinase among the 38 known autophagy related proteins, which is an indispensable component of autophagic vesicles. The Atg/ULK1 complex is the major molecule that initiates mitophagy. The Atg/ULK1 complex consists of the interaction of ULK1, family interacting protein of 200 kDa (FIP200), Atg13, and Atg101. The PI3K/AKT/mTOR signaling pathway is an important pathway in the regulation of mitophagy. Phosphoinositide 3-kinase (PI3K) consists of different catalytic subunits, each with a different function. The α-catalytic subunit of PI3K inhibits autophagy while the β-catalytic subunit promotes autophagy (Kma and Baruah, 2022). When ROS are over-accumulated, the β-catalytic subunit of PI3K is activated to produce inositol triphosphate diphosphate (PIP3), and accumulation of PIP3 leads to the recruitment and activation of AKT (also known as protein kinase B) (Wang et al., 2020). Activation of AKT inhibits the downstream mTOR. mTOR is a complex of 2 protein kinases, mTOR1 and mTOR2, that is sensitive to cellular nutrient levels and plays a role in the regulation of cell growth and survival through autophagy. mTOR is a key factor in the inhibition of mitophagy as a key factor in the inhibition of mitophagy. When mTORC1 activity is reduced, it promotes the phosphorylation of ULK1 and Atg13, which negatively regulates the ULK1 complex and promotes the formation of autophagic vesicles. With the activation of the ULK1 complex, Atg9 vesicles can only be recruited to the site of autophagosome formation, thus initiating autophagy. It was shown that cognitive deficits in VaD rats could be improved through the PI3K/Akt/mTOR signaling pathway, which may be related to the increase in the level of mitophagy and the inhibition of neuronal apoptosis, which attenuated neuronal damage and mitochondrial dysfunction (Zheng et al., 2021).

2.1.4 FOXOs gene

FOXOs belong to subgroup O of the forkhead family of proteins, which play important roles in the regulation of cell autophagy, mitochondrial function, proliferation and differentiation, oxidative stress, etc. The FOXOs family contains four members, FOXO1, FOXO3, FOXO4 and FOXO6. Among them, FOXO1 is the only member of the forkhead family of transcription factors with a forkhead structure, and its activity is regulated by PI3K/AKT/mTOR and AMPK signaling pathways (Brunet et al., 1999). The PI3K/AKT/mTOR signaling pathway is associated with cell growth and proliferation. Under physiological conditions, the PI3K/AKT/mTOR signaling pathway is highly active, and activated AKT phosphorylates the Ser25 site of FOXO1, leading to its translocation from the nucleus to the cytoplasm, thus inhibiting its transcriptional activity. Under stress or energy deprivation, AMPK was activated and the above pathway was inhibited. FOXO1 entered the nucleus to activate the expression of LC3 gene and increased the level of mitophagy, thus enabling the cell to maintain energy homeostasis under energy deprivation.

2.1.5 AMPK signaling pathway

AMPK (adenylate-activated protein kinase) regulates mitophagy by directly phosphorylating ULK1 or indirectly acting on the ULK1 complex (Egan et al., 2011). Under hypoxia, lack of energy, or elevated ROS, the intracellular AMP/ATP ratio is elevated, and AMP binds directly to AMPK, thereby activating AMPK (Herzig and Shaw, 2018; Matzinger et al., 2020). AMPK can directly phosphorylate specific amino acid residues (e.g., Ser317 and Ser777) of the ULK1 protein, thereby enhancing ULK1 activity and initiating autophagy. ULK1 forms a complex with several key proteins, such as ATG13, FIP200 (RB1CC1), and ATG101, which together regulate the initiation of autophagy. AMPK can enhance the interaction between ATG13 and ULK1 by phosphorylating ATG13, thereby promoting the stability and function of the autophagy complex (Herzig and Shaw, 2018; Lin and Hardie, 2018). In addition, AMPK activates ULK1 by inhibiting signaling associated with mTORC1. mTORC1 normally inhibits ULK1 under nutrient-sufficient conditions, thereby inhibiting autophagy. When AMPK is activated, it can derepress ULK1 by inhibiting mTORC1 or phosphorylating proteins that interact with mTORC1. At low energy or activated AMPK can promote FUNDC1 dephosphorylation, which activates mitophagy. Studies have shown that activation of AMPK increases FUNDC1 expression (Cai et al., 2021). In contrast, under normal conditions, activated AMPK inhibits mitophagy by activating AKT and promoting phosphorylation of FUNDC1. Metformin is an adenosine monophosphate-activated protein kinase (AMPK) agonist that crosses the BBB and activates AMPK to exert neuroprotective effects (Kersten et al., 2022; Guo et al., 2023). A recent study has shown that metformin is neuroprotective by activating the AMPK/ULK1/PINK1/Parkin signaling pathway and regulating the levels of apoptosis and mitophagy in a hyperglycemic constructed CI/RI animal model and a cellular model of hyperglycemic incubation oxygen–glucose deprivation/reperfusion (OGD/R) (Guo et al., 2023).

2.1.6 Others

Mitochondrial DNA (mtDNA) is a special kind of DNA that exists in the mitochondria of cells. mtDNA is usually a circular double-stranded, unlike the linear DNA in the nucleus. Due to the insufficient repair system of mtDNA and the lack of protective histones, mtDNA is easily damaged when oxidative stress occurs (Kazak et al., 2012). Damaged mtDNA can lead to dysfunction of the mitochondrial respiratory chain, resulting in elevated levels of ROS, which will further damage mtDNA, it was found that ATAD3B contains a LIR motif, which binds to LC3 and promotes mitophagy in a manner that is independent of the PINK1 signaling pathway, resulting in the clearance of the damaged mtDNA (Shu et al., 2021). Under normal conditions, ATAD3B is heterologously coupled to ATAD3A, thereby promoting the targeting of the C-terminal region of ATAD3B to the mitochondrial membrane gap, whereas OS-induced mtDNA damage reduces the heterogeneous heterodimerization of ATAD3B-ATAD3A, leading to the exposure of the C-terminal end of ATAD3B on the mitochondrial outer membrane, which in turn recruits LC3 to initiate mitophagy (Shu et al., 2021).

2.2 Mechanisms of oxidative stress

Increasing evidence suggests that multiple signaling pathways or factors can be involved in the regulation of neurological diseases during the development of oxidative stress. The Nrf2/Keap1 pathway plays an important role in maintaining redox homeostasis and cellular defense mechanisms. Under oxidative stress, Nrf2 dissociates from its repressor Keap1 and activates the expression of antioxidant response genes. ROS can directly activate the JAK2/STAT3 pathway, which can lead to the expression of antioxidant genes, such as superoxide dismutase (SOD) and glutathione peroxidase (GPx), and thus help to scavenge ROS and alleviate the damage caused by oxidative stress. Under hypoxia, lack of energy, or increased ROS, the intracellular AMP/ATP ratio is elevated, and AMP binds directly to AMPK, thereby activating AMPK. activated AMPK phosphorylates Nrf2 and Keap1, which alters their conformation, thereby decreasing the binding capacity of the two, and promotes the displacement of Nrf2 to the nucleus and its binding to the anti-oxidative stress element (ARE) that activates the expression of a series of antioxidant genes. In addition, AMPK can maintain the quality of mitochondria by promoting mitophagy and new mitochondria production, thus reducing the generation of ROS from the root cause. The activation of MAPK/JNK signaling pathway can phosphorylate a variety of transcription factors and promote the expression of antioxidant genes, thus exerting antioxidant effects. In conclusion, there are numerous mechanisms that regulate OS, and the above signaling pathways or molecules are only some of them. The purpose of this review is to present a systematic account of the process of OS regulation by the above signalling pathways and their roles in the development of neurological disorders. The aim is to provide a theoretical basis and inspiration for the development of therapeutic strategies for this class of diseases. (Figure 2).

Figure 2. Oxidative stress Schematic diagram of the mechanism.

2.2.1 Nrf2/Keap1 signaling pathway

Nuclear factor erythroid 2-related factor 2 (Nrf2) belongs to the leucine zipper family and includes different functional domains such as Neh1 to Neh7 (Kaspar et al., 2009). Nrf2 is a transcription factor that maintains cellular redox homeostasis and is mainly responsible for regulating the expression of a series of antioxidant response genes to prevent cellular damage caused by ROS. When not activated by ROS, Nrf2 mainly exists in the cytoplasm and binds to Kelch-like ECH-associated protein 1 (Keap1) to form a relatively stable complex, and Keap1 restricts the activity of Nrf2 by promoting ubiquitination and subsequent proteasomal degradation of Nrf2 (Sykiotis and Bohmann, 2008). In the case of oxidative stress occurring with excessive accumulation of ROS, this leads to a decrease in the binding capacity of Nrf2, which is therefore no longer ubiquitinated and degraded but accumulates in the cytoplasm (Li et al., 2018). Keap1 dissociates from Nrf2, followed by Nrf2 nuclear translocation into the nucleus and binding to the antioxidant response element (ARE), which activates the expression of a series of antioxidant and cytoprotective genes, such as Heme oxygenase-1 (HO-1), GST, and GSH, thereby protecting the organism from ROS (He et al., 2020; Chen, 2021). HO-1 has anti-oxidative stress and maintenance of homeostasis in the intracellular environment, and its expression is increased under conditions of oxidative stress, inflammation, heavy metal exposure, and hyperthermia. HO-1 catalyzes the breakdown of hemoglobin into CO, Fe2+, and bilirubin. This process removes both heme, which is harmful to the body, and produces bilirubin, which has antioxidant properties, and CO, which is anti-inflammatory and vasodilatory, thus helping to reduce oxidative stress and inflammatory responses and preventing cellular damage. When the level of oxidative stress is reduced, the newly synthesized Keap1 promotes ubiquitination and degradation of Nrf2, and the activity of Nrf2 is re-inhibited, returning the organism to its normal state.

2.2.2 JAK2/STAT signaling pathway

Janus Kinase 2 (JAK2) is a tyrosine kinase belonging to the Janus family that has only one catalytic structural domain and no Src homology 2 (SH2) structural domain. Therefore, JAK2 can not only phosphorylate to bind to other cytokine receptors, but also phosphorylate multiple signaling molecules containing SH2 structural domains (Zhong et al., 2021). Signal Transducer and Activator of Transcription3 (STAT3) is a downstream molecule of JAK2 and belongs to the STAT family of transcription factors. STAT3 consists of an N-terminal structural domain, coiled-coil domain, DNA-binding domain, linker domain, SH2 (Src homology region 2) domain and C-terminal activation domain. Domain, DNA binding domain, linker domain, SH2 (Src homology region 2) domain and C-terminal activation domain. When cytokines (e.g., erythropoietin, interferon, interleukin, etc.) bind to its receptor, they cause a conformational change in the receptor, leading to activation of JAK2 phosphorylation. Activated JAK2 promotes the phosphorylation of STAT3, which undergoes a conformational change to form a dimer and translocate to the nucleus, thereby activating the transcription of antioxidant genes. ROS can directly activate the JAK2/STAT3 pathway, which leads to the expression of antioxidant genes, such as superoxide dismutase (SOD) and glutathione peroxidase (GPx), to help scavenge ROS and reduce the damage caused by oxidative stress.

2.2.3 AMPK signaling pathway

AMPK is a serine/threonine protein kinase complex consisting of an alpha catalytic subunit and two regulatory subunits, beta and gamma (Chen et al., 2018). Under hypoxia, lack of energy, or elevated ROS, the intracellular AMP/ATP ratio is elevated, and AMP binds directly to AMPK, thereby activating AMPK (Herzig and Shaw, 2018). Activated AMPK phosphorylates Nrf2 and Keap1, changing their conformation, thereby reducing their binding capacity, promoting the displacement of Nrf2 to the nucleus and binding to the anti-oxidative stress element (ARE), and activating the expression of a series of antioxidant genes. In addition, AMPK maintains mitochondrial quality by promoting mitophagy and neo-mitochondrial production, which reduces ROS production at the source (Wang et al., 2022). Curcumin promotes Parkin-mediated mitophagy and ameliorates mitochondrial damage through the AMPK/TFEB signaling pathway (Cao et al., 2020). Activation of the AMPK/Nrf2/HO-1 pathway reduces ROS and MDA levels as well as increases SOD and GPx activity and ameliorates CI/RI-induced oxidative stress (Huo et al., 2023). In addition to its antioxidant role by interfering with the Nrf2/Keap1 signaling pathway, AMPK can also play a role by activating FOXO and SIRT1. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is another one of the sources of ROS, which produces O2− by catalyzing the oxidation reaction of NADPH. Activated AMPK activates FOXO and SIRT1, which in turn promotes the expression of SOD and glutathione peroxidase, inhibits the activity of NADPH oxidase, and improves cellular antioxidant levels (Tang et al., 2014; Duan et al., 2017; Tang et al., 2021).

FOXO (Forkhead Box O) is a transcription factor that promotes the expression of antioxidant and longevity genes, of which FOXO1, FOXO3, FOXO4, and FOXO6 are involved in the expression of antioxidant genes. AMPK is activated under low energy and hypoxia. Activated AMPK can activate FOXO through phosphorylation, which enhances the transcriptional activity of FOXO and translocates to the nucleus. Promote the expression of antioxidant genes to generate SOD, GPx, glutathione S-transferase (GST), and glutathione synthetase (GCL), which can lighten the cellular damage by oxidative stress (Guo et al., 2017). In addition, AMPK can indirectly regulate the activity of FOXO family proteins through mTOR. As shown previously, mTOR promotes protein synthesis and cell growth mainly through mTORC1 (mTOR complex 1). Under stress, activated AMPK can phosphorylate and activate the activity of TSC2 (Tuberous Sclerosis Complex 2), a component of mTORC1, thus reducing energy consumption. Whereas mTORC1 promotes the activity of AKT kinase, activated AKT phosphorylates FOXO at Ser256, Ser319 and Thr24 sites, which leads to the transfer of FOXO from the nucleus to the cytoplasm, where it binds to phospho-specific proteins and is thus confined to the cytoplasm (Li et al., 2017; Omorou et al., 2023). Since FOXO is required for its initiation of transcription in the nucleus, when FOXO is phosphorylated by AKT it is unable to transcribe antioxidant genes. While AMPK inhibits mTORC1, AKT activity is also inhibited. When AKT activity is low, phosphorylation and extra-nuclear translocation of FOXO protein is reduced, and therefore, FOXO protein can remain in the nucleus to activate the expression of antioxidant genes (Deng et al., 2023).

SIRT1 (Sirtuin 1) and SIRT13 belong to the same sirtuin family, NAD+-dependent deacetylases with regulatory and anti-aging effects on oxidative stress (Singh and Ubaid, 2020). AMPK increases intracellular NAD+ levels by promoting metabolic pathways such as fatty acid oxidation and glycolysis. NAD+ is an essential substrate for SIRT1, and increasing NAD+ levels enhances the deacetylation activity of SIRT1. SIRT1 deacetylates a variety of transcription factors, such as FOXO proteins, p53, and NF-κB. FOXO proteins and the p53 transcription factor have a role in the regulation of oxidative stress. After FOXO acetylation, its ability to bind to DNA is reduced, thereby decreasing its transcriptional activity. FOXO has a role in regulating oxidative stress, and the acetylation of FOXO reduces its ability to bind to DNA, thereby decreasing its transcriptional activity. Activated SIRT1 deacetylates FOXO and enhances the transcriptional activity of FOXO, enabling it to bind to DNA more efficiently and activate its downstream target genes of SOD, catalase, and GPx to improve the cellular antioxidant defense capacity (She et al., 2018). In addition, SIRT1 participates in the regulation of oxidative stress by modulating the activity of Tumor Protein 53 (p53) through deacetylation. p53 is a tumor suppressor protein that is widely present in human cells, and its activity can be modulated by acetylation and deacetylation, which enhances the transcriptional activity of p53 and contributes to the activation of apoptosis and cell cycle arrest (Wu et al., 2022). On the other hand, SIRT1 regulates the activity of p53 through deacetylation, thereby modulating the expression of antioxidant-related genes such as glutathione peroxidase 1 (GPX1), SOD2, TP53-induced glycolysis and Apoptosis regulator (TIGAR), and aldehyde dehydrogenase 4 (ALDH4), and reduces the cellular damage caused by ROS (Liu et al., 2020). In summary, AMPK plays an antioxidant role by activating the Nrf2/HO-1 signaling pathway and regulating FOXO and SIRT1 factors to promote the expression of antioxidant genes. AMPK also maintains the quality of mitochondria by promoting mitophagy and neo-mitochondrial production, which reduces ROS production from the root. Mitochondrial deacetylase Sirtuin-3 (SIRT3) has an antioxidant effect. Cellular experiments have demonstrated that Aβ decreases the level of SIRT3 in neurons, whereas a lack of SIRT3 leads to increased levels of oxidative stress, neuronal hyperexcitability, and a decrease in survival, and that the use of SIRT3 agonists reduces the level of ROS and inhibits neuronal hyperexcitability (Ying et al., 2022). Also, animal experiments showed that inhibition of SIRT3 resulted in elevated neuronal excitability, whereas decreasing ROS levels reversed the neuronal hyperactivity caused by the lack of SIRT3, and these findings suggest that SIRT3 regulates the level of oxidative stress in AD neurons (Ying et al., 2022).

2.2.4 MAPK/JNK signaling pathway

MAPK kinase kinase (MAPKKK), MAPK kinase (MAPK kinase, MAPKK) is the upstream molecule of the MAPK signaling pathway. JNK belongs to the MAP kinase (mitogen-activated protein kinase) family of kinases, which plays an important role in a variety of physiological and pathological processes, such as cell cycle, reproduction, apoptosis and cellular stress. It plays an important role in various physiological and pathological processes including cell cycle, reproduction, apoptosis and cellular stress, especially in the process of oxidative stress (Johnson and Lapadat, 2002). There are three isoforms of the JNK family, including JNK1, JNK2, and JNK3. When oxidative stress occurs, ROS can activate JNK by affecting upstream kinases of JNK, such as MAPKKK and MAPKK. ASK1 (Apoptosis Signal-regulating Kinase 1) belongs to an isoform of MAPKKK, also known as MAPKKK5, and can be activated by ROS, tumor necrosis factor-alpha (TNF-alpha), and lipopolysaccharide, among others. Trx is an ASK1-binding protein, and inhibits the activity of ASK1 through binding to it (Matsuzawa and Ichijo, 2008). Under oxidative stress, the sulfhydryl groups in the two cysteine residues contained in Trx are oxidized to form a disulfide bond, which separates Trx from ASK1 and induces the aggregation and activation of ASK1, which in turn activates downstream JNK (Matsuzawa and Ichijo, 2008; Katagiri et al., 2010). JNK activation phosphorylates a variety of transcription factors and promotes the expression of antioxidant genes, thereby exerting an antioxidant effect. Activation of JNK/SAPK in response to oxidative stress mediates the activation of BACE, which in turn leads to elevated Aβ levels (Tamagno et al., 2005, 2006).

3 The role of mitophagy and oxidative stress in neurological disorders 3.1 The role of mitophagy in neurologic diseases

Mitophagy represents a crucial process in MCQ, whereby the removal of damaged mitochondria is facilitated. This process plays a pivotal role in maintaining intracellular homeostasis and enabling cells to respond effectively to environmental stresses. The relationship between mitophagy and neurological disorders (e.g., CI/RI, VAD, AD) is currently the subject of extensive research. A number of studies have indicated that mitophagy may be a significant factor in the development and progression of neurological disorders. For example, CI/RI has been demonstrated to result in oxidative stress and neuroinflammation, which in turn cause damage to cells. Mitophagy can maintain intracellular environmental homeostasis by removing damaged mitochondria and reducing ROS production. More and more studies have shown that mitophagy plays a complex role in the development and progression of VaD and AD, which may be related to factors such as inflammation, oxidative stress, vascular injury, and abnormal vascular function. However, the relationship between mitophagy and neurological disorders, especially CI/RI, has not been studied deeply enough, and in-depth studies are still needed to reveal the specific mechanisms. We conducted a systematic review of the role of mitophagy in the development of three neurological diseases: CI/RI, VaD, and AD. We then provided strategies for intervening in neurological diseases from the mitophagy pathway (Table 1 and Figure 3).

Table 1. Experimental Study on Correlation between CI/RI, VaD and AD.

Figure 3. Relationship of mitophagy, OS with CI/RI, VaD and AD.

3.1.1 Mitophagy and CI/RI

CI/RI is a form of brain tissue damage that occurs as a result of the recirculation of blood in the brain following a period of cerebral ischemia and hypoxia (Salameh and Dhein, 2015; Liu et al., 2023). This leads to a range of cognitive impairments, including reduced learning and memory abilities. The Circle of Willis is a ring of arteries located at the base of the brain, consisting of the anterior cerebral arteries, middle cerebral arteries, posterior cerebral arteries, basilar arteries, and communicating arteries, which equalize and distribute blood flow from the internal carotid arteries and vertebral arteries (Mastorakos and McGavern, 2019). The anterior, middle, and posterior cerebral arteries and their branches form abundant cortical arteries and arterioles on the surface of the brain. Cortical artery branches penetrate deep into the brain parenchyma, giving rise to smaller arterioles, and the perivascular space has become an important site for the removal of unwanted proteins and peptides (e.g., Aβ) (Gorelick et al., 2011; Laman and Weller, 2013). The vascular structure of the brain, especially its “outside-in” blood supply, places the deeper part of the brain at the junction of the two main blood supply areas - the watershed areas - which are most susceptible to ischemic injury in the event of a drop in blood pressure or a decrease in blood flow.

There are 8.6 billion neurons and 25–30 billion glial cells in the brain, making the brain the organ that consumes the most oxygen (Huang et al., 2023). Compared with muscle and liver, the brain has relatively insufficient energy reserves, which largely depends on blood supply, and is therefore extremely sensitive to ischemia. When ischemia and hypoxia occur, ATP synthesis in the brain is reduced, neurons are depolarized, excitatory amino acids are released from the presynaptic membrane, excitatory amino acids are reduced from the postsynaptic membrane, and Ca2+ is overloaded, which ultimately results in the gradual death of neurons and produces structural damage to the brain tissue. During reperfusion, the above damage is further aggravated, resulting in mitochondrial damage and the production of large amounts of inflammatory factors and ROS (Li et al., 2015). In conclusion, the series of reactions mentioned above (ROS, Ca2+ overload and excitatory amino acid damage, etc.) can damage the structure of mitochondria, cause mitochondrial dysfunction and activate intracellular mitophagy. In the acute phase of ischemia, mitochondrial damage was severe but no obvious mitophagy was detected; however, with the prolongation of ischemia over a certain period of time, the degree of mitophagy increased (Orellana-Urzúa et al., 2020). In the early stage of cerebral ischemia–reperfusion injury, damaged mitochondria can be cleared in time by mitophagy, which reduces the production of ROS and thus exerts neuroprotective effects. Under normal conditions, mitophagy helps to maintain the energy metabolism and barrier function of BBB endothelial cells. However, after the occurrence of CI/RI the level of inflammatory factors in the brain increased, and BBB permeability increased (Choi et al., 2014). Microglia are the main immune cells in the nervous system. Microglia are activated in the early stage of CI/RI and play the roles of phagocytosis of damaged and dead cells, release of inflammatory factors and cytokines. Inflammatory factors such as TNF-α, IL-1β, and IL-6 bind to the target cells through specific receptors and activate the downstream inflammatory signaling pathways (e.g., the NF-κB signaling pathway), which further promote inflammatory factors and chemokine expression. Inflammatory factors such as TNF-α, IL-1β and IL-6 bind to target cells through specific receptors and activate downstream inflammatory signaling pathways (e.g., NF-κB signaling pathway), further promoting the expression of inflammatory factors and chemokines, exacerbating inflammatory responses, inducing apoptosis and necrosis, and resulting in brain tissue damage. However, sustained high levels of inflammatory factors induce or exacerbate oxidative stress and mitochondrial dysfunction, inhibit mitophagy, and cause greater tissue damage. In the case of CI/RI, inflammatory factors or ROS cause neuronal death and impairment of the structural integrity of the BBB, leading to easier entry of blood components and inflammatory factors into the brain tissue and exacerbating brain damage (Zhong et al., 2022). In conclusion, in CI/RI, neuroinflammation, mitophagy, and oxidative stress are not isolated from each other; there is a complex interplay between them, and the complex mechanism between them determines the degree of brain tissue damage.

Studies have shown that Jiawei Sijunzi Tang can significantly improve the neurological function, reduce mNSS score, maintain mitochondrial homeostasis, increase the mitochondrial membrane potential level and Ca2+-Mg2+-ATPase activity, and reduce the O2− content in the CI/RI model rats through the PINK1/Parkin signaling pathway (SONG et al., 2023). When the PINK1/Parkin signaling pathway is activated, it promotes mitochondrial autophagy, inhibits the activation of NLRP3 inflammatory vesicles and reduces the level of inflammation (Fan et al., 2021). Both in vivo and in vitro experiments demonstrated that Ligustilide (Chuanxiong Lactone) promotes mitophagy via PINK1/Parkin, thereby ameliorating neuronal injury in CI/RI. Therefore, neuronal injury can be ameliorated by targeting intervention in the PINK1/Parkin signaling pathway to promote mitophagy (Mao et al., 2022). Therefore, neuronal injury can be ameliorated by targeting intervention in the PINK1/Parkin signaling pathway to promote mitophagy. Electroacupuncture (EA) is a method of preventing and treating diseases by using a combination of acupuncture and electrical stimulation, which has the effect of increasing synaptic plasticity and improving the level of neurotrophic factors, after the needle is inserted into the acupoint to obtain Qi (Lai et al., 2019). EA was found to enhance Pink1/Parkin-mediated mitochondrial autophagic clearance, reduce accumulation of damaged mitochondria, and ameliorate nitro/oxidative stress-induced functional impairment of brain CI/RI mitochondria (Wang et al., 2019). Tissue-type plasminogen activator (tPA), the only FDA-approved clinical thrombolytic agent, exerts neuroprotection by activating AMPK to increase FUNDC1 expression, thereby inhibiting apoptosis and improving mitochondrial function (Cai et al., 2021). Polyrhachis vicina (Roger) increased the level of mitophagy and promoted angiogenesis through activation of the SIRT3-mediated Pink1/Parkin signaling pathway, significantly reduced neurological scores and infarct area, alleviated cortical neuron apoptosis, and increased hippocampal Nissl density in CI/RI rats (Wei et al., 2023).

3.1.2 Mitophagy and VaD

VaD is a cognitive impairment caused by various cerebrovascular diseases and is one of the most common forms of dementia, accounting for 20% of all cases. As life expectancy increases and the incidence of cardiovascular diseases continues to rise, it is likely that the prevalence of VaD will continue to grow in the coming decades (Wu et al., 2016). The primary clinical manifestations of vascular dementia (VaD) include cognitive impairment and motor abnormalities, which have a significant adverse impact on the quality of life of the patients (Gorelick et al., 2011; O’Brien and Thomas, 2015). There are many causes of VaD, such as insufficient cerebral perfusion, oxidative stress, neuroinflammation, vascular lesions in the brain (atherosclerosis, endothelial dysfunction, and cerebral small-vessel disease, etc.), alterations in the permeability of the blood–brain barrier (BBB), and cholinergic damage (Sharp et al., 2009; Iadecola, 2013; Liu et al., 2017). However, reduced cerebral blood supply due to ischemic or hemorrhagic brain lesions, for example, thus leaving the brain in a state of chronic cerebral hypoperfusion (CCH) is the most common cause of VaD (Wang et al., 2010; Iadecola, 2013). Decreased cerebral blood flow (CBF) can lead to changes in spatial memory (Sigfridsson et al., 2020), CCH can cause some damage to brain tissue, such as lacunar infarcts, brain atrophy, white matter damage, microhemorrhages, and microinfarcts, which can lead to brain dysfunction and cognitive impairment (Alosco et al., 2013). Study shows reduced serum levels of Parkin protein in patients with VaD and AD (Castellazzi et al., 2019). Rapamycin (RAPA) improved cognitive deficits and attenuated neuronal damage and mitochondrial dysfunction in VaD rats by increasing the level of mitophagy and inhibiting neuronal apoptosis via the PI3K/Akt/mTOR signaling pathway (Zheng et al., 2021). ZiShenHuoXue Formula (ZSHXF) is composed of He Shouwu, Chinese wolfberry, mulberry, Schisandra chinensis, Salviae Miltiorrhizae, Puerariae Root, Yizi Ren, Acorus calamus, Ulmus, Preparation of Yuanzhi, Scorpion, Hawthorn, and it is an empirical formula for the treatment of VaD, which was formed under the guidance of the master of national medicine Liu Zuyi, and has exact clinical efficacy (Zhou et al., 2004; Wu et al., 2015). It was also shown that ZSHXD could attenuate neuronal damage through PINK1/Parkin-mediated mitophagy and exert neuroprotective effects in VaD rats (Zhao et al., 2024).

3.1.3 Mitophagy and AD

AD is a neurodegenerative disease that mostly occurs in the elderly population, and its main clinical manifestations are memory loss, cognitive impairment, language impairment and accompanied by depression, apathy and other emotions, which make it difficult to take care of oneself in daily life, and bring a heavy burden to patients and their families. Aging is one of the most important factors in the development of AD. With the aging of the world’s population, the incidence of AD is on the rise year by year, and it is expected that by 2050, there will be as many as 152 million AD patients in the world, and the cost of treating AD will be as high as 1.1 trillion U.S. dollars by then (Orobets and Karamyshev, 2023; Yang et al., 2023b). The main pathologic manifestations of AD are amyloid-β (Aβ) formation of senile plaques, tau protein formation of neurofibrillary tangle (NTF), neuronal damage and synaptic dysfunction (Yeo et al., 2014). Numerous studies have shown that neuroinflammation is another important factor exacerbating AD. Microglia (MG) are important immune cells in the central nervous system, which can recognize and remove pathogens, damaged cells to maintain the role of brain microenvironmental homeostasis. In the early stages of AD, MGs have a role in recognizing and clearing Aβ deposits. However, as AD progresses, MGs are in an over-activated state, releasing pro-inflammatory cytokines and exacerbating the pathologic process of AD. Thus, MG plays a complex role in the pathophysiology of AD, both as a defense mechanism in the early stages of the disease and as a facilitator in the late stages of the disease. Higher levels of inflammatory factors inhibit mitophagy, and in AD, low levels of PINK1 and Parkin lead to impaired mitophagy (Pradeepkiran and Reddy, 2020; Tran and Reddy, 2020; Morton et al., 2021). Increased levels of mitophagy were found to reduce Aβ deposition and NTF in human neuronal cells and improve memory deficits in AD model mice (Fang et al., 2019). Synaptophysin, a membrane glycoprotein mainly found in the synaptic vesicle membrane of neurons, has a role in transmitting neural signals and regulating synaptic