Targeted Removal of Damaged Mitochondria in Different Parts of the Spinal Disc During Degeneration: Processes and Treatment Approaches

The Ub-dependent pathways rely on extensive ubiquitination of damaged mitochondrial surface proteins to promote mitophagy. In the field of Ub-dependent mitophagy, the Parkin-dependent pathway, mediated by the kinase PINK1 and the E3 ubiquitin ligase Parkin, is the most extensively studied mechanism (Clark et al., 2006; Ashrafi and Schwarz, 2013). PINK1, a highly conserved mitochondrial protein encoded by the PARK6 gene, is involved in regulating various cellular physiological processes, particularly crucial for the mitochondrial function (Wang N. et al., 2020). In healthy mitochondria, PINK1 is continuously imported into the IMM and degraded, maintaining low expression levels (Narendra et al., 2010; Jin et al., 2010; Yamano and Youle, 2013). However, when mitochondrial membrane potential (MMP) is compromised, leading to mitochondrial dysfunction, the import channel for PINK1 into the IMM is blocked, causing its accumulation at the translocase complex on the outer mitochondrial membrane (OMM) of the damaged mitochondria [45, 46]. At this location, PINK1 undergoes dimerization, triggering autophosphorylation and activation (Gan et al., 2022; Rasool et al., 2022). The activated PINK1 subsequently phosphorylates serine 65 of Ub and the Ub-like domain of Parkin, resulting in the further categorization of Ub-dependent mitophagy into Parkin-dependent and Parkin-independent pathways (Uoselis et al., 2023). In the Parkin-dependent pathway, Parkin, encoded by the PARK2 gene, is responsible for conjugating Ub-molecules to substrates (Riley et al., 2013). Autophagy receptor proteins, such as optineurin (OPTN) and nuclear dot protein 52 (NDP52), play a pivotal role in the PINK1/Parkin pathway (Lazarou et al., 2015; Heo et al., 2015; Wong and Holzbaur, 2014). These autophagy receptor proteins bind to ubiquitinated substrates and associate with ATG8 family members, facilitating the capture of damaged mitochondria by autophagosomes through the Atg8ylation machinery (Atg8yl-Mach). The Atg8yl-Mach is composed of the ATG12-ATG5-ATG16L1 complex, which anchors ATG8 to the autophagosome membrane by promoting its conjugation to phosphatidylethanolamine, thereby promoting the formation and expansion of autophagosomes on the surface of damaged mitochondria. Members of the ATG8 family are divided into the microtubule-associated protein one light chain 3 (LC3) and GABA receptor-associated protein (GABARAP) subgroups. During the downstream process of mitophagy initiation, ATG8 family members play a crucial role in the fusion of autophagosomes with lysosomes for the degradation of mitochondrial substrates, a process primarily driven by the GABARAP subgroup (Nguyen et al., 2016; Vaites et al., 2017).

In addition to the classical PINK1/Parkin pathway, there exist other Ub-dependent pathways that are independent of Parkin. In Parkin-independent mitophagy, activated PINK1 can directly recruit autophagy receptor proteins to mitochondria by phosphorylation of Ub at the serine 65 site. Next, the Ub chains generated on OMM substrates serve as recruitment platforms for Ub-binding autophagy receptor proteins, including sequestosome one/p62 (SQSTM1/p62), neighbor of BRCA1 (NBR1), OPTN, NDP52, and Tax1-binding protein 1 (TAX1BP1). These receptor proteins function by initiating the formation of autophagosomes (Lazarou et al., 2015; Richter et al., 2016). The aforementioned OMM autophagy-related proteins contain both LC3-interacting region (LIR) and Ub-binding domain (UBD), thereby mediating the binding of LC3 to Ub chains on targeted mitochondria. As a result, these proteins anchor ubiquitinated mitochondria to autophagosomes (Fan et al., 2021).

Ub chains are not the only factor recruiting autophagy receptor proteins. The autophagy receptor proteins on the OMM inherently possess LIR. These autophagy receptors can directly bind to LC3 without ubiquitination, thus initiating mitophagy. In mammals, such receptors primarily include NIX receptor, BCL2-interacting protein 3 (BNIP3) receptor, and FUN14 domain-containing protein 1 (FUNDC1) receptor, among others (Lu et al., 2023) (Figure 2).

NIX and BNIP3 share 56% homology and both contain the BCL2 homology 3 (BH3) domain, which allows them to directly bind to LC3 through their BH3 domains and induce mitophagy (Novak et al., 2010). FUNDC1 can interact with LC3 to induce Parkin-independent mitophagy under hypoxic conditions (Liu et al., 2012).

Non-canonical autophagy, independent of autophagosome formation, represents an endosomal-dependent mitophagy pathway activated under specific stress conditions. Specifically, upon mitochondrial DNA damage, mitochondrial nucleoids are eliminated via the endosome-mitophagy pathway. MitoEVs that bud from mitochondrial networks have been implicated as a means of delivering mitochondrial components to lysosomes (Konig et al., 2021; Iorio et al., 2024; Soubannier et al., 2012), thereby compensating for deficiencies in canonical mitophagy (Towers et al., 2021). In addition to mitoEVs-mediated mitophagy, mitochondrial herniation leads to the exposure and ubiquitination of the IMM, initiating the induction of an apoptotic mitophagy sequestration pathway. IMM-mitophagy has the potential to capture herniating mitochondria, theoretically preventing mtDNA release into the cytosol at an earlier stage of the process before any transcriptional response can be triggered. Using proximity proteomics, researchers have also identified the protein required for the clearance of mutated mitochondrial nucleoids from the mitochondrial matrix. Among these, ATAD3 and SAMM50 regulate both the architecture of mitochondrial cristae and nucleoid interactions. SAMM50 cooperates with the retromer complex protein VPS35 to sequester mitochondrial DNA within endosomes, thereby preventing excessive immune response (Sen et al., 2023).

The complex multi-tissue structure of the intervertebral disc allows it to absorb and distribute mechanical stresses during physical activities (Konig et al., 2021). Specifically, the NP is primarily composed of NP cells and extracellular matrix (ECM), with the ECM of the NP consisting of type II collagen and proteoglycans (Risbud et al., 2015). NP cells maintain the biomechanical homeostasis of the NP by synthesizing and secreting ECM (Vamvakas et al., 2017; Lin J. et al., 2019; Silagi et al., 2018). Nutrients and metabolites enter and exit the disc through diffusion within the dense ECM (Wise et al., 2020). Evidence suggests that IVDD initially occurs in the NP region of the disc (Guerrero et al., 2021), and research on the mechanisms and therapeutic strategies of IVDD has predominantly focused on NP cells (Xin et al., 2022; Wu et al., 2022; Zhang et al., 2021; Sun et al., 2022). Currently, the etiology of RCD induced by mitochondrial dysfunction in NP cells involves multiple factors, including inflammation, oxidative stress, nutrient deficiency, compression, and hyperlipidemia (Figure 3).

Firstly, inflammation is considered a significant pathogenic factor. Pro-inflammatory cytokines, such as interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6), collectively accelerate the progression of IVDD by promoting ECM degradation, chemokine production, immune cell recruitment, and phenotypic changes in disc cells (Xu et al., 2017; Zhang et al., 2018; Risbud and Shapiro, 2014; Peng et al., 2022). The relationship between inflammation induction and mitophagy has garnered attention from Zhang and his team (Zhang et al., 2018). They made pioneering observations that Parkin expression is not only elevated in degenerated human NP but also increased in rat NP stimulated with TNF-α. Meanwhile, TNF-α stimulates NP cells to produce more reactive oxygen species (ROS), subsequently activating autophagy and apoptosis processes. During this process, despite increased expression levels of LC3 and Beclin-1, p62 levels also rise in NP cells, indicating impaired autophagy flux. Dysfunction in Parkin-dependent mitophagy has been confirmed as a pivotal cause. Further research has found that the NF-κB signaling pathway is a potential mechanism through which pro-inflammatory cytokines exert their effects. Studies by Zhao et al. (2020) and Yu et al. (2021) demonstrated that the inflammation induced by TNF-α can affect mitochondrial function in NP cells through the NF-κB pathway, which in turn triggers a series of secondary phenotypic changes, including exacerbated inflammation, oxidative stress, and pyroptosis. Among these, infiltration and activation of immune cells further amplify the inflammatory cascade, leading to aggravated inflammation (Risbud and Shapiro, 2014). Cytokines induce oxidative stress by increasing ROS accumulation (Yang et al., 2023). Furthermore, pyroptosis is dependent on inflammasome activation and is accompanied by the massive release of inflammatory cytokines. Evidence suggests that activated NLRP3 inflammasomes aggregate around mitochondria, and their potential detrimental effects in IVDD have attracted widespread attention (Zhou et al., 2011; Xia et al., 2019). NLRP3 inflammasome induces pyroptosis and release inflammatory cytokines in NP cells, a process that promotes the secretion of metalloproteinases and leads to NP degradation (Song et al., 2017; A et al., 2020), thereby accelerating the pathological progression of IVDD (Tang et al., 2021). Research by Peng et al. (Peng et al., 2022) found that in a lipopolysaccharide (LPS) induced inflammation model, activation of the NLRP3 inflammasome promotes ROS production and inhibits mitophagy flux. This leads to pyroptosis and apoptosis of NP cells, resulting in accelerated degeneration of the intervertebral disc NP.

Secondly, mitochondria are both the primary source of cellular ROS and highly susceptible to oxidative stress damage, leading to dysfunction (Hm et al., 2015). Impaired mitophagy will result in mitochondrial dysfunction and ROS accumulation (Zhang et al., 2018; Lin Q. et al., 2019). ROS accumulation, in turn, leads to exaggerated inflammation, disordered metabolism, and enhanced apoptosis in cells (Zhou et al., 2011). Among the molecules triggered by mitochondrial ROS, the NLRP3 inflammasome has been extensively studied for its detrimental role in IVDD (Zhou et al., 2011; Xia et al., 2019). Activation of the NLRP3 inflammasome can elevate the production of IL-1β, which facilitates the secretion of metalloproteinases and subsequently causes degradation of NP tissue (Song et al., 2017; A et al., 2020). Furthermore, NLRP3 is linked to the mitochondrial apoptosis pathway, programmed cell death, and apoptosis through several mechanisms in NP cells (Wn et al., 2019). In Wang et al.‘s (Y et al., 2018) study, treatment of human NP cells with H₂O₂ led to impaired mitophagy, manifesting as ROS accumulation, decreased ECM synthesis, and accelerated senescence. These alterations collectively contributed to the degeneration of NP cells. Additionally, tert-butyl hydroperoxide (TBHP) is also widely used to simulate oxidative stress environments. Various scholars (Xie et al., 2019; Lin et al., 2020; Wang et al., 2018; Wn et al., 2019; Chen et al., 2019; Chen Y. et al., 2020) have explored the role of mitophagy in TBHP-induced oxidative stress using different experimental models, finding that TBHP treatment results in impaired mitophagy in NP cells of the intervertebral disc. This includes both the Ub-dependent classical pathway, characterized by decreased expression levels of PINK1 and Parkin proteins, reduced LC3 II/I ratio, and decreased MMP and ATP levels, as well as BNIP3-mediated Ub-independent mitophagy (Wang et al., 2018). BNIP3, initially identified as a pro-apoptotic protein, features an atypical BH3 domain localized to the OMM. Induction of BNIP3 triggers the translocation and activation of BCL2-antagonist/killer 1 (BAK1) and Bcl-2 Associated X Protein (BAX) to mitochondria, leading to increased mitochondrial membrane permeability, subsequent release of cytochrome C from mitochondria to the cytosol, and ultimately initiating the caspase cascade of apoptosis. Emerging research has found that BNIP3 also serves as a key receptor for mitophagy, playing a role in promoting cell survival (Madhu et al., 2020). The bidirectional functions of BNIP3 in apoptosis and mitophagy suggest that it may be a critical regulator of cell fate (Madhu et al., 2023). The aforementioned changes in mitophagy will further affect processes such as apoptosis, senescence, ROS generation, and ECM degradation, ultimately influencing the degeneration of the NP of the intervertebral disc.

In addition to inflammation and oxidative stress, nutrient deficiency and abnormal mechanical loads are also key risk factors for IVDD. Wang et al. (2020b) found that nutrient deficiency and aging can downregulate FOXO3, leading to mitochondrial dysfunction and inhibited mitophagy, resulting in increased NP cell apoptosis and ECM degradation. Abnormal compressive forces, tensile forces, and increased matrix stiffness all exert detrimental effects on disc cells (Wang D. et al., 2022; Xiao et al., 2022; Wang et al., 2021). Further research has shown that excessive mechanical stress applied to NP cells can also lead to oxidative stress and mitochondrial dysfunction (Kang et al., 2020a; Hu et al., 2022). Kang et al. (Kang et al., 2020a) found that mechanical compression can cause mitochondrial dysfunction in NP cells of the intervertebral disc, with increased ROS production, exacerbated mitochondrial dysfunction, and increased apoptosis, thereby promoting NP cell degeneration. Mitochondrial dysfunction can further increase ROS production, leading to a vicious cycle between mitochondrial dysfunction and ROS accumulation, causing sustained oxidative damage (Chen et al., 2018).

Combining the above evidence, mitochondrial damage leads to pathological phenotypes in NP cells, including apoptosis, oxidative stress, senescence, ECM degradation and synthesis, excessive autophagy, exacerbated inflammation, and pyroptosis (Table 1).

As the outer structure of the intervertebral disc, the primary physiological function of the AF lies in its ability to effectively encapsulate the NP, preventing its herniation through its unique hydraulic sealing properties, and evenly distributing the various pressures acting on the disc (Moore, 2006). The AF comprises two distinct components: the inner AF, which is adjacent to the NP and consists of chondrocytes with an ECM primarily composed of type II collagen; and the outer AF, which is mainly composed of fibroblast-like cells with an ECM primarily composed of type I collagen. This lamellar structure provides the intervertebral disc with high flexibility and adaptability across multiple planes of motion, ensuring the stability and normal function of the disc structure (Smith et al., 2011; Roughley, 2004). The outer AF receives nutritional support from capillaries within the surrounding soft tissues, while the rest of the AF exchanges nutrients and metabolic waste through a capillary network at the CEP via diffusion (Wise et al., 2020) (Figure 3). Due to the unique structure and physiological function of the AF, the pathological repair process following AF injury exhibits distinct characteristics (Bailey et al., 2013). Clinically, both acute trauma and chronic degeneration of the AF can exacerbate IVDD. Studies have demonstrated that AF injury contributes to disc instability and disrupts the intradiscal microenvironment. Meanwhile, AF injuries often persist due to insufficient endogenous repair capacity [105]. Histologically, AF scar healing is predominantly characterized by disorganized type III collagen deposition, with sparse type I collagen bundles observed in the outer layer. The key subsequent effects include reduced resistance to small molecule permeation, decreased tensile strength, diminished disc height, NP fibrosis, and CEP subchondral ossification (Kuivaniemi and Tromp, 2019). These findings underscore the necessity of elucidating AF repair mechanisms and their systemic impact on disc homeostasis.

Oxidative stress and abnormal lipid metabolism are currently recognized as significant factors inducing IVDD through mitophagy-mediated AF damage. Xu et al. (Konig et al., 2021) demonstrated that TBHP-induced oxidative stress can lead to mitochondrial dysfunction accompanied by downregulation of mitophagy levels. When mitophagy function is impaired, oxidative stress and apoptosis levels significantly increase, accelerating the occurrence and progression of IVDD. Further research revealed that SIRT2, upstream of this pathway, is a key target that influences the expression levels of peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), Parkin, and LC3 II, thereby regulating mitophagy. Wu et al. (Wu et al., 2021) found that oxidized low-density lipoprotein (oxLDL) can promote mitochondrial fission, further exacerbating mitochondrial dysfunction and increasing AF cell apoptosis, thereby accelerating the pathological process of IVDD.

In brief, oxidative stress and abnormal lipid metabolism are crucial factors inducing IVDD through mitophagy-mediated AF damage. The pathological phenotypes of AF cells resulting from mitochondrial damage include apoptosis and oxidative stress (Table 1).

The CEP consists of a biological tissue layer rich in type II collagen and chondrocytes, situated between the intervertebral disc and adjacent vertebral body. It is similar to other articular tissues in the body, and it exhibits the highest cellular density among all structures of the intervertebral disc (Kirnaz et al., 2022). The CEP serves as the primary pathway for nutrient delivery from vertebral body capillaries to the disc, as well as for the excretion of waste products from the disc. Degeneration of the CEP can hinder the nutrition and waste exchange of the intervertebral disc, leading to the disruption of its homeostasis and the initiation of IVDD (Wong et al., 2019). Kang et al. (Kang et al., 2020b) investigated the regulatory role of oxidative stress on mitophagy mechanisms in CEP cells. Their study induced an oxidative stress state with H₂O₂, resulting in mitochondrial dysfunction manifested as decreased MMP, reduced ATP synthesis, increased ROS levels, and opening of the mitochondrial permeability transition pore (mPTP). Concurrently, mitophagy was inhibited, further exacerbating cellular apoptosis and ultimately accelerating the progression of IVDD (Figure 3).

In summary, impaired mitophagy function leads to a series of pathological phenotypes in CEP cells, including enhanced apoptosis and oxidative stress. Additionally, this mechanism has been extensively studied in other articular cartilage tissues structurally similar to CEP cells and has been shown to be closely related to the pathological processes of these tissues (Sun et al., 2021) (Table 1).

Mitophagy, as an important mechanism for cellular self-renewal and homeostasis maintenance, exhibits significant correlations with various forms of RCD (Sun et al., 2018; Sun et al., 2019). It is important to note that mitophagy is a double-edged sword. Moderate mitophagy can protect cells from various external stimuli, whereas excessive mitophagy can also accelerate cellular apoptosis and the progression of IVDD (Kang et al., 2020a; Xu W-N. et al., 2019).

Numerous natural products exhibit therapeutic potential for improving IVDD by modulating mitophagy levels. Hydrogen sulfide (H2S), along with nitric oxide and carbon monoxide, is regarded as one of three endogenously produced gaseous signaling molecules. These molecules possess diverse biological functions, including anti-inflammatory and anti-apoptotic effects, and exert impacts on multiple key mechanisms and pathways both in vivo and in vitro (Hu et al., 2007; Hu et al., 2009). Recent studies have demonstrated that H₂S effectively improves mitochondrial function by closing the mPTP, enhancing MMP, and ATP levels, thereby reducing cellular apoptosis and showing therapeutic potential for IVDD (Xu et al., 2017). Salidroside, a phenylpropanoid glycoside extracted from Rhodiola, and Polydatin, a resveratrol glycoside extracted from the rhizomes of Polygonum cuspidatum, can both activate mitophagy through a Parkin-dependent pathway, upregulate Parkin protein expression, promote the reduction of ROS accumulation, and effectively inhibit cellular apoptosis, thereby ameliorating mitochondrial damage and apoptosis in NP and CEP cells, respectively (Zhang et al., 2018; Kang et al., 2020b). Notably, Polydatin can also simultaneously activate the Nrf2 pathway, upregulating Nrf2 protein expression and its nuclear translocation, further improving mitochondrial dysfunction (Kang et al., 2020b). Urolithin A, a metabolite of ellagitannins and ellagic acid abundant in pomegranates, strawberries, and other nuts (Cerdá et al., 2005), can specifically induce mitophagy both in vivo and in vitro (Ryu et al., 2016; Fang et al., 2019). Mechanistic studies have shown that Urolithin A inhibits NP cell apoptosis by activating mitophagy through the AMPK pathway, thereby slowing down the progression of IVDD (Lin et al., 2020). Honokiol, a natural flavonoid compound derived from the roots and bark of Magnolia officinalis, also exerts therapeutic effects by activating the AMPK pathway. Honokiol demonstrates multiple pharmacological effects such as antioxidant, anti-lipid peroxidation, anti-inflammatory, and neuroprotective activities, showing therapeutic potential in cartilage protection and IVDD (Chen et al., 2014; Chen et al., 2015). To explore the mechanism of Honokiol-induced SIRT3 expression enhancement, Wang et al. (Wang et al., 2018) examined the activation of two major energy sensor molecules, namely, AMPK and PGC-1α. By activating the AMPK pathway, upregulating PGC-1α and SIRT3, regulating mitochondrial dynamics, and enhancing mitophagy flux through the autophagy receptor BNIP3, Honokiol protects NP cells from oxidative stress damage, reversing the processes of aging and apoptosis (Wang et al., 2018). Furthermore, natural isothiocyanate compounds such as Sulforaphane, found in cruciferous vegetables, especially broccoli, have been shown to improve mitochondrial dysfunction, reduce mitochondrial morphological abnormalities, and enhance mitochondrial dynamics. The specific mechanisms involve upregulating PGC-1α expression and AMPK phosphorylation, reducing ROS accumulation, delaying aging, inhibiting apoptosis, and reducing ECM degradation (Xu X. et al., 2019). Mangiferin exhibits potent free radical scavenging activity, with mango trees serving as its primary and readily accessible source (Zhao et al., 2017). It possesses multiple pharmacological potentials, including antioxidant, anti-inflammatory, anti-diabetic, anti-hyperlipidemic, and anti-atherosclerotic properties, with mechanisms involving the counteraction of oxidative stress and mitochondrial dysfunction (Alberdi et al., 2018; Li et al., 2019). Opa1, Drp1, and TFAM are biomarkers of mitochondrial dynamics. Mangiferin not only downregulates Drp1 expression but also upregulates Opa1 and TFAM levels, thereby reducing inflammation, ECM degradation, oxidative stress, and apoptosis (Yu et al., 2021). Selenium can also reduce oxidative stress and cellular apoptosis by regulating mitochondrial dynamics and the expression of autophagy-related proteins (Wang P. et al., 2022) (Table 2).

Beyond natural products, certain hormones have also been identified as possessing the potential to regulate mitophagy. For instance, Melatonin, an endogenous molecule released by the pineal gland, has been proven to effectively delay oxidative stress, inflammatory responses, and apoptosis in osteoarthritis models (Pei et al., 2009; Liu et al., 2013; Lim et al., 2012), while also enhancing mitophagy levels in various tissues such as the brain and liver (Lin et al., 2016; Kang et al., 2016). Chen et al. (Chen et al., 2019) found that Melatonin can promote mitophagy by upregulating Parkin protein expression and the LC3 II/I ratio, thereby improving oxidative stress-induced mitochondrial dysfunction and apoptosis, and exhibiting potential therapeutic effects on IVDD. Cortistatin, a cyclic neuropeptide, is an appealing therapeutic candidate in the treatment of degenerative and inflammatory diseases (Gonzalez-Rey et al., 2007; Duran-Prado et al., 2013; Gruber et al., 2014), including its role in mitigating TNF-α-induced chondrocyte inflammation to counteract articular cartilage degeneration in osteoarthritis (Zhao et al., 2019). Zhao et al. (Zhao et al., 2020) discovered that Cortistatin inhibits apoptosis by suppressing the NF-κB pathway and regulating mitochondrial dynamics, thereby reducing ROS accumulation and NLRP3 inflammasome activation. Specifically, in this study, Cortistatin, by activating the AMPK/PGC-1α pathway, upregulated the expression levels of fusion-related markers Opa1, Mfn1, and Mfn2, while simultaneously downregulating the expression of fission marker Drp1. Inhibition of proteins involved in mitochondrial fission also demonstrated a positive therapeutic effect on IVDD (Wu et al., 2021) (Table 2).

Targeted strategies aimed at mitochondrial function also constitute effective avenues for regulating mitophagy and ameliorating IVDD. Studies have confirmed that oxidative products are significantly increased in IVDD, and inhibiting the excessive production of ROS while promoting their clearance has been demonstrated to effectively delay the progression of IVDD [136–138] (Kang et al., 2020a; Suzuki et al., 2015). Among these, activating the Nrf2 antioxidant defense system emerges as a potent therapeutic strategy for IVDD. Nrf2, a crucial redox-sensitive transcription factor, regulates the antioxidant system by activating the expression of cytoprotective genes in response to oxidative stress (Xiang et al., 2022). For instance, antioxidants such as MitoQ and Mito-TEMPO significantly mitigate oxidative stress and mitochondrial dysfunction by activating the Nrf2 pathway and upregulating PINK1/Parkin-mediated mitophagy (Kang et al., 2020a; Kang et al., 2020b).

With the advancement of gene editing technology, an increasing number of studies have begun to explore their potential in regulating mitophagy and improving IVDD. For example, knocking down PINK1 expression using sh-PINK1 can affect the mitophagy process, leading to accelerated aging and increased ROS accumulation (Y et al., 2018). Additionally, circERCC2 promotes PINK1/Parkin-mediated mitophagy by downregulating miR-182-5p and upregulating SIRT1, thereby reducing NP cell apoptosis, senescence, and ECM degradation (Xie et al., 2019). Additionally, overexpression of genes such as Mfn2 and FOXO3 has also shown positive effects on mitophagy and IVDD treatment (Chen Y. et al., 2020; Wang et al., 2020b).

During the development of IVDD, various risk factors have been found to induce mitochondrial damage by inhibiting mitophagy, ultimately leading to RCD of disc cells. Therefore, enhancing mitophagy is considered a potential therapeutic approach for alleviating IVDD (Wang et al., 2020c; Lan et al., 2022). However, not all activation of mitophagy is positively correlated with halting the progression of IVDD. Excessive mitochondrial fission and autophagy can directly contribute to the occurrence and development of IVDD (Lin et al., 2023). Prolonged duration of mechanical loading has been reported to result in excessive removal of mitochondria by mitophagy, thus exacerbating NP cell senescence, and inhibiting mitophagy can have a positive effect (Huang et al., 2020). Further research has found that overexpression of the NDUFA4L2 gene exerts a positive therapeutic effect on IVDD by inhibiting excessive mitophagy induced through the Parkin-dependent pathway (Wn et al., 2019). NLRX1, as the only Nod-like receptor located in mitochondria, plays a crucial role in sensing mitochondrial damage and regulating mitochondrial function (Zhang et al., 2019; Killackey et al., 2022; Killackey et al., 2023). Song et al. (Song et al., 2024) revealed that mitophagy is activated in both NLRX1-overexpressing and NLRX1-deficient NP cells exposed to oxidative stress. However, distinct biological outcomes were observed. When NLRX1 was overexpressed, pharmacological intervention targeting the NLRX1-SLC39A7 pathway showed great potential for promoting disc regeneration. Conversely, NLRX1 deficiency promoted PINK1/Parkin-mediated mitophagy, inducing excessive mitophagy and accelerating the progression of IVDD. Therefore, from a mechanistic perspective, the zinc transporter SLC39A7, a novel NLRX1-interacting protein, has been identified and proven to regulate mitochondrial dynamics and promote beneficial and synchronized mitophagy (Song et al., 2024) (Table 2).

Proteins such as HSP70 also mitigate oxidative stress and apoptosis by modulating mitochondrial dynamics and the expression of autophagy-related proteins (Hu et al., 2022). Furthermore, proteins including A20 and SIRT2 have been identified as possessing potential to regulate mitophagy (Xu W-N. et al., 2019; Peng et al., 2022). In the study by Peng et al. (Peng et al., 2022), following LPS treatment, the mitochondrial fission protein Drp1 translocated from the cytoplasm to mitochondria, while the expression of Mfn1 significantly decreased. A20 mitigated the LPS-induced changes and promoted the normalization of mitochondrial morphology. A20 significantly reduced the NLRP3 aggregation around mitochondria induced by LPS. Analysis revealed that A20 protected NP cells from LPS-induced mPTP collapse and massive ROS production. These findings suggest that A20 may exert protective effects by facilitating the elimination of ROS through mitophagy.