1. Introduction
It is estimated that approximately 84% of individuals experience low back pain at some point in their lives [
1]. Recent research has revealed a strong link between low back pain and intervertebral disc degeneration (IVDD), with approximately 23% of people with low back pain suffering from IVDD [
2]. IVDD is a common degenerative disease of the skeletal system that primarily affects middle-aged and elderly adults. It is a leading cause of chronic low back pain, affecting approximately 40% of the global population [
3]. Additionally, IVDD greatly reduces the quality of life for many patients and imposes a significant economic burden because of its strong association with chronic disability. Currently, clinical treatments for IVDD mainly include conservative management, pharmacological therapy, and surgical intervention. Conservative care includes physiotherapy, exercise, acupuncture, and massage, which are safe but yield limited symptomatic relief [
4]. Oral analgesics are commonly prescribed to alleviate pain among IVDD patients and are often combined with physiotherapy or exercise [
5]. However, prolonged drug use is usually followed by side effects, including gastrointestinal issues, renal dysfunction, and cardiovascular risks [
6,
7]. Surgical intervention can relieve compression on protruding medullary nerve roots, and patients recover quickly to a normal state. However, long-term surgical outcomes may result in unfavorable side effects, such as chronic or recurring low back pain [
8], and may even lead to the degeneration of adjacent discs [
9,
10], without reversing or halting IVDD progression by repairing the disc microenvironment or improving the pathological changes in the intervertebral disc (IVD). Therefore, there is an urgent need for more effective therapeutic strategies that can delay IVDD progression or promote the regeneration of degenerated IVDs.
2. IVD structure and pathological basis of IVDD
The IVD consists of three main parts: the nucleus pulposus, annulus fibrosus, and cartilaginous endplate. Situated between adjacent vertebrae, the IVD acts as a cushion, bearing mechanical loads and enhancing spinal flexibility. The nucleus pulposus is a gel-like substance located at the center of the IVD and is primarily composed of water, collagen, and proteoglycans. Its main function is to absorb and distribute mechanical pressure across the spine. The annulus fibrosus surrounds the nucleus pulposus and is a robust structure composed of multiple layers of collagen fibers that provide strength and limit movement. The annulus fibrosus is primarily composed of a well-organized network of type I collagen (Col I) and type II collagen (Col II), which effectively stabilizes the nucleus pulposus [
11]. The cartilaginous endplate is a thin layer of cartilage tissue located at the top and bottom of the IVD, anchoring the disc to the adjacent vertebral bodies.
Primary pathological changes in IVDs include inflammation, mechanical stress, oxidative stress, and cell apoptosis. Prolonged exposure of IVD cells to these adverse conditions leads to a gradual decline in disc function and structural integrity. With accumulative external stress, the activities of both nucleus pulposus cells (NPCs) and annulus fibrosus cells progressively diminish. Once the annulus fibrosus ruptures, the nucleus pulposus is exposed to the immune system, triggering the recruitment of inflammatory mediators such as interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), as well as immune cells, including T cells, macrophages, and natural killer cells. These immune responses ultimately promote extracellular matrix (ECM) degradation, contributing to the progression of IVDD [
[12],
[13],
[14],
[15]]. Furthermore, the microenvironment of IVDs is characterized by a limited nutrient supply, hypoxia, hypertonicity, and low pH. These factors collectively reduce cellular activity and severely impair the intrinsic regenerative capacity of the disc, hindering the restoration of disc structure and function following tissue injury and thereby accelerating the progression of IVDD [
16,
17].
3. IVDD is mainly associated with NPC senescence
3.1. ECM degradation in NPCs promotes IVDD
NPCs are the primary cell type in the nucleus pulposus and are responsible for synthesizing and maintaining the ECM, including proteoglycans and Col II [
18]. These ECM components are essential for preserving disc integrity by resisting mechanical pressure during physical activity. Proteoglycans, particularly aggrecan, contribute to the highwater content of discs, allowing them to absorb and distribute compressive forces, whereas Col II provides tensile strength and elasticity, preventing disc deformation under mechanical stress [
19]. The decline in nucleus pulposus function is considered the fundamental pathological basis of IVDD, characterized by elevated catabolism regulated by ECM proteases (such as matrix metalloproteinases (MMPs) and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)) and reduced anabolism [
20]. The decreased secretion of Col II deteriorates the structural integrity of the nucleus pulposus and weakens its loading capacity. Consequently, external pressure exacerbates the tearing of the annulus fibrosus. The IVD is avascular and relies on the cartilaginous endplate for nutrient diffusion and waste removal. Aging or sustained mechanical stress can damage the endplate, impairing nutrient supply and accelerating disc degeneration. Furthermore, the lack of neurovascular structures surrounding NPCs severely limits the self-repair capacity of the IVD [
21,
22].
3.2. A feedback loop exists between NPC senescence and ECM degradation
Cellular senescence is defined as an irreversible state of cell cycle arrest characterized by the permanent cessation of cell division. This process can be broadly categorized into replicative senescence and stress-induced senescence [
23]. Replicative senescence is driven primarily by the accumulation of DNA damage over successive cell divisions, whereas stress-induced senescence is triggered by various external stimuli, such as reactive oxygen species (ROS) and mitochondrial dysfunction.
A hallmark of cellular senescence is the development of the senescence-associated secretory phenotype (SASP), which is characterized by the upregulation of proinflammatory factors such as IL-1β, IL-6, and TNF-α. These inflammatory mediators promote ECM degradation through regulating the activities of MMPs and ADAMTS in NPCs [
[24],
[25],
[26]]. Multiple molecular signaling pathways are involved in regulating cellular senescence, among which the tumor protein p53–cyclin-dependent kinase inhibitor 1A (p21
CIP1A/Waf1)–retinoblastoma protein (p53–p21–Rb) and cyclin-dependent kinase inhibitor 2A (p16
INK4A)–retinoblastoma protein (p16–Rb) pathways are the most well characterized. Both pathways are activated during disc degeneration and play pivotal roles in inducing NPC senescence [
27].
There is a strong correlation between alterations in ECM components and NPC senescence [
28]. The ECM within nucleus pulposus tissue facilitates cellular functions that are essential for preserving overall disc health and preventing degeneration [
29]. With aging or under degenerative conditions, the degradation of ECM components results in a loss of hydration, diminished mechanical properties, and increased tissue stiffness, all of which in turn accelerate the senescence of NPCs [
30]. With senescence, NPCs lose the ability to synthesize ECM components. Moreover, senescent NPCs shift their secretory profile toward a SASP phenotype, producing proinflammatory cytokines and matrix-degrading enzymes, further exacerbating ECM breakdown. The interplay between ECM degradation and NPC senescence forms a vicious cycle that drives the progression of IVDD. Therefore, interrupting this cycle represents a promising therapeutic strategy for IVDD. The anatomical structure of the IVD and the pathological basis of IVDD are illustrated in
Fig. 1(a).
3.3. Alleviating NPC senescence mitigates the progression of IVDD
Recently, extensive studies have focused on delaying NPC senescence to slow the progression of IVDD. An
in vitro study has demonstrated that pyrroloquinoline quinone (PQQ) activates the nuclear factor erythroid-2 related factor 2 (Nrf2)–antioxidant response element (ARE) signaling pathway by dissociating the Kelch-like ECH-associated protein 1 (Keap1)–Nrf2 complex, thereby promoting the nuclear translocation of Nrf2. This activation helps mitigate IVDD progression by preserving cellular and matrix integrity [
31]. Similarly, morroniside has been shown to protect against NPC senescence by inhibiting ROS production and suppressing the ROS–Hippo–p53 pathway [
32]. Furthermore, NPC senescence can be delayed by inhibiting the Hippo Yes-associated protein (Yap)/transcriptional coactivator with PDZ-binding motif (Taz) pathway via selenophosphate synthetase 1 (SEPHS1) or by modulating the interaction between monoacylglycerol lipase (MAGL) and stimulator of interferon genes (STING), both of which help restore the balance of ECM metabolism [
33,
34]. Both
in vitro and
in vivo experiments have further demonstrated that anisodamine improves apoptosis, modulates senescence-associated gene expression, and enhances ECM synthesis in IL-1β-induced human and rat NPCs by regulating the IL-6/Janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) pathway [
35]. The mechanisms of interaction among signaling pathways regulating cellular senescence are illustrated in
Fig. 1(b).
4. Molecular mechanism of NPC senescence
4.1. Imbalanced redox homeostasis is linked to NPC senescence
The disturbed equilibrium between the pro-oxidant and antioxidant systems is considered a major contributor to IVDD. ROS molecules include superoxide anion radicals (O
2–), hydroxyl radicals (ċOH), hydrogen peroxide (H
2O
2), nitric oxide (NO), and nitrogen dioxide (NO
2). Moreover, the human body has ROS-scavenging systems, including enzymes such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx), to maintain redox homeostasis [
36].
When external factors lead to excessive ROS accumulation or an impaired antioxidant defense system, intracellular ROS levels rise beyond physiological thresholds, causing oxidative modifications of macromolecules such as proteins, nucleic acids, membrane lipids, and carbohydrates, ultimately damaging the function of normal cells and contributing to cellular senescence [
[37],
[38],
[39]]. Owing to prolonged exposure to a hypoxic environment, NPCs generate large amounts of free radicals, resulting in cellular damage and apoptosis. Numerous studies have established a strong positive correlation between oxidative stress and NPC senescence, especially oxidative stress-induced senescence, which is recognized as a critical risk factor for IVDD [
40,
41]. In addition, exposure of NPCs to high oxygen tension triggers DNA damage, which activates the p53–p21–Rb and p16–Rb pathways via extracellular regulated kinase (ERK) signaling, ultimately driving NPC senescence [
42]. Recently, the ROS-responsive release platform thiol-coated magnetic particle (TMP)@alginate (Alg)-phenylboronic acid (PBA)/polyvinyl alcohol (PVA), which specifically targets mitochondria, was shown to effectively scavenge ROS and minimize ECM degradation, thus slowing the progression of IVDD [
43].
4.2. Highly activated inflammatory factors and signaling pathways are correlated with NPC senescence
IVDD is often accompanied by an inflammatory response that disrupts the anabolic–catabolic equilibrium of the ECM in NPCs. Different stimuli, including mechanical stress and injury, prompt disc cells to release proinflammatory factors (such as IL-1 and TNF-α), which increase the expression of MMPs, further degrading the ECM and destroying the disc structure [
44]. Additionally, most studies suggest that inflammatory factors contribute to the initiation of cellular senescence by promoting the secretion of SASP-related factors. For example, TNF-α has been shown to significantly accelerate premature senescence in NPCs by activating the nuclear factor-κB (NF-κB) pathway and inducing excessive ROS production [
45,
46]. Bovine NPCs treated with TNF-α exhibit senescent characteristics, as evidenced by increased levels of senescence-associated β-galactosidase and p16, along with decreased expression of NPC markers such as aggrecan, Col II, and SRY-box transcription factor 9 (Sox9). Furthermore, TNF-α-induced senescent NPCs affect neighboring healthy cells through a paracrine mechanism, thereby contributing to the accumulation of senescent cells in aged IVDs [
47]. Interfering with lumican effectively mitigates the TNF-α-induced inflammatory response and senescence in human NPCs by inhibiting the apoptosis signal-regulating kinase 1 (ASK1)/p38 signaling pathway [
48]. Similarly, human follistatin-like 1 was found to increase the expression of SASP- and β-galactosidase (SA-β-gal)-positive cells while downregulating ECM components in NPCs, disrupting ECM homeostasis [
49]. Moreover, previous studies have shown that GATA binding protein 4 (GATA4), arginase II, and exosomes derived from M1 macrophages activate the NF-κB signaling pathway and expedite NPC senescence [
[50],
[51],
[52]]. Conversely, sirtuin 1 (SIRT1) overexpression was found to suppress the NF-κB-mediated inflammatory response and improve NPC senescence in vitamin D receptor (VDR)-deficient IVDD mice [
53].
4.3. Abnormal mechanical loading is related to NPC senescence
Physiological mechanical stress on IVDs promotes anabolic processes, whereas prolonged exposure to nonphysiological stresses, such as poor posture or heavy lifting, may trigger catabolic responses in NPCs and accelerate disc degeneration [
54]. Mechanical stress enhances the expression of proinflammatory cytokines and catabolic enzymes, leading to degradation of the ECM, disruption of the collagen structure, reduction of disc height, and potential tearing or rupture of the annulus fibrosus. These pathological changes can result in nucleus pulposus herniation or disc bulging, which compresses nerve roots and causes pain and neurological symptoms [
55]. Moreover, available evidence suggests that mechanical loading-related stress induces NPC senescence by increasing the release of ROS from mitochondria [
56]. Inhibition of the p38 mitogen-activated protein kinase (MAPK) pathway has been shown to significantly attenuate ROS levels and mechanical overloading-induced NPC senescence [
57]. Furthermore, continuous mechanical compression can accelerate NPC senescence by promoting excessive mitochondrial degradation through mitophagy. Mechanical compression activates the mitophagy-related PINK1–Parkin RBR E3 ubiquitin protein ligase (PARKIN) pathways, leading to increased β-galactosidase activity and increased expression of proinflammatory factors, whereas inhibition of the PINK1/PARKIN pathway reduces oxidative stress and delays cellular senescence [
58].
Additionally, a mechanoreceptor primarily expressed on the cell membrane, PIEZO1, converts mechanical signals into electrical signals during IVDD, activating downstream signaling pathways that regulate the biological behavior of annulus fibrosus cells [
59]. Upon mechanical loading, the activation of PIEZO1 initiates the NF-κB signaling pathway, which subsequently induces the expression of periostin. Periostin further enhances NF-κB activity, forming a self-amplifying loop that exacerbates NPC senescence and promotes disc degeneration [
60].
4.4. DNA damage is relevant to NPC senescence
DNA damage and cellular senescence are closely interconnected processes in cellular biology. DNA damage rarely occurs alone; it can be triggered or exacerbated by various factors, including oxidative stress, ultraviolet (UV) radiation, and replication errors. Damaged DNA activates the p53–p21 and p16
INK4A pathways, leading to cell cycle arrest and senescence [
61,
62]. N-acetyl proline–glycine–proline (N-Ac-PGP) induces NPC senescence by promoting ROS production and DNA damage, further resulting in the activation of senescence-associated pathways (p53–p21–Rb and p16–Rb). Additionally, N-Ac-PGP also represses the expression of antioxidant genes while upregulating the expression of matrix-degrading and proinflammatory genes in NPCs [
63]. The cylic GMP-AMP synthase (cGAS)/STING pathway regulates the inflammatory response in various cells and plays a central role in monitoring DNA damage. Abnormal genomic DNA damage has been shown to promote inflammation-associated senescence of NPCs via activation of the cGAS/STING axis [
64]. Notably, metformin treatment has been reported to reduce DNA damage through inactivating the cGAS/STING pathway, thereby contributing to the suppression of cellular senescence [
65].
4.5. The acidic microenvironment of the IVD induces NPC senescence
Owing to the hypoxic microenvironment of IVDs, NPCs rely primarily on anaerobic glycolysis for energy production, generating large amounts of lactic acid [
66]. Under normal physiological conditions, the production, transport, and clearance of lactic acid are maintained in dynamic equilibrium, which helps to regulate the intracellular pH and preserve IVD homeostasis. However, when this balance is disrupted, elevated lactic acid levels become a key mediator of metabolic stress, promoting NPC senescence [
67].
The local acidic environment in the nucleus pulposus stimulates increased expression of acid-sensing ion channels (ASIC1 and ASIC3) in nucleus pulposus mesenchymal stem cells (MSCs), leading to increased calcium ion influx, triggering the activation of downstream signaling pathways such as the p53 and p21 pathways, and inducing cell cycle arrest and the expression of senescence markers [
68]. Excessive lactic acid also induces NPC senescence by interacting with the protein kinase B (Akt) pathway, further modulating downstream cascades, including the Akt/p21/p27/cyclin D1 and Akt/ Nrf2/heme oxygenase-1 (HO-1) cascades [
67].
Different factors contribute to NPC senescence, and these factors are often interrelated. For example, mechanical overload and DNA damage are closely associated with inflammation and oxidative stress. Additionally, the acidic environment of IVDs also induces NPC senescence via the activation of inflammation-related pathways. These findings further highlight that oxidative stress and inflammation are central mechanisms underlying physiological aging and are linked to degenerative diseases [
69].
4.6. An imbalance in autophagy is associated with NPC senescence
Various risk factors exist in the intracellular and extracellular environments of NPCs and contribute to cellular senescence. However, autophagy is an intracellular degradation and recycling process in which damaged proteins and malfunctioning organelles are enclosed and delivered to lysosomes for breakdown, thereby maintaining cellular homeostasis and delaying senescence. Dysregulation of autophagy has been implicated in numerous diseases, including neurodegenerative disorders, cancer, and metabolic syndromes [
70].
Under normal conditions, STING is degraded via autophagy in disc cells. However, in senescent NPCs, impaired degradation of STING leads to its accumulation and overexpression, triggering chronic inflammation and cellular senescence [
71]. Similarly, reduced human antigen R (HuR) levels in diabetic nucleus pulposus tissues and high glucose-treated NPCs are associated with cellular senescence, whereas the activation of autophagy by stabilizing autophagy-related 7 (Atg7) attenuates NPC senescence [
72]. Defects in chaperone-mediated autophagy (CMA) have also been linked to NPC senescence. The abnormal accumulation of phospholipase C gamma 1 (PLCG1) due to CMA inhibition leads to calcium overload, triggering NPC senescence [
73].
Mitophagy, a special form of autophagy, specifically removes damaged or dysfunctional mitochondria, preventing excessive ROS production. Activating mitophagy has demonstrated therapeutic potential in protecting NPCs from oxidative damage [
74]. However, excessive mitophagy can lead to NPC death and accelerate cellular senescence [
58,
75]. NOD-like receptor X1 (NLRX1) interacts with the zinc transporter SLC39A7 to form a complex that regulates mitochondrial Zn
2+ trafficking, coordinating mitochondrial dynamics and mitophagy. Loss of NLRX1 leads to mitochondrial collapse and excessive mitophagy via the compensatory PINK1/PARKIN pathway, thereby promoting NPC senescence [
76].
Emerging evidence suggests that modulating the balance of autophagy can effectively slow NPC senescence. For example, delphinidin (Delp) treatment reduces the levels of senescence markers such as p53 and p21 by restoring autophagy in oxidative stress-induced human NPCs. However, autophagy inhibition blocked the beneficial effects of Delp on senescence and ECM homeostasis [
77]. Similarly, apigenin treatment reduces apoptosis, senescence, and ECM degradation in tert-butyl hydroperoxide (TBHP)-induced NPCs and restores disrupted autophagic flux by promoting the nuclear translocation of transcription factor EB (TFEB) via the AMPK/mTOR pathway, counteracting lysosomal dysfunction [
78,
79]. Furthermore, TANK-binding kinase 1 (TBK1) overexpression attenuates senescence and apoptosis and promotes NPC survival by increasing autophagy, ultimately protecting against IVDD [
80]. The molecular mechanisms of NPC senescence are shown in
Fig. 2.
5. Antiaging therapy for IVDD
The ultimate goal of IVDD treatment is to restore the metabolic balance of the ECM, as disruptions in this balance can induce NPC senescence, which in turn exacerbates metabolic dysfunction in the ECM. Traditional IVDD treatments, including drug therapies and surgical interventions, often fail to restore the biological function of the disc. Presently, targeting NPC senescence has emerged as one of the most effective approaches for IVDD treatment [
[81],
[82],
[83]]. Various antiaging strategies have demonstrated efficacy, including manipulating the expression of specific RNAs, discovering active antisenescent ingredients, utilizing extracellular vehicles (EVs), and developing antisenescent biomaterials.
5.1. Blocking NPC senescence via different forms of RNA
Both coding RNAs and noncoding RNAs participate in regulating the synthesis and degradation of the ECM in NPCs. Modulating the expression levels of these RNAs improves disc cell function and cell survival, representing an important approach for treating IVDD.
5.1.1. Messenger RNAs (mRNAs)
Numerous genes have been shown to alleviate IVDD by suppressing NPC senescence. For example, polo-like kinase 1 (PLK1) regulates the cell cycle, affecting cell proliferation and senescence. Inhibition of PLK1 kinase activity in normal NPCs triggers senescence by increasing p53 expression [
84]. Cannabinoid receptor type 2 (CB2R) has also been associated with antisenescent activities and is expressed at lower levels in IVDD. Activating CB2R markedly decreases the number of SA-β-gal-positive NPCs and SASP-related factor expression and increases ECM expression [
85]. Moreover, mitochondria and the endoplasmic reticulum are essential for maintaining NPC homeostasis. Disruption of the mitochondria-associated endoplasmic reticulum membrane (MAM) occurs in TBHP-induced NPCs, accompanied by mitochondrial Zn
2+ overload and NPC senescence closely related to synaptojanin 2 binding protein (SYNJ2BP) loss. SYNJ2BP overexpression can promote mitochondrial Zn
2+ homeostasis, alleviating NPC senescence [
86]. Additionally, T-box transcription factor T (Tbxt) is decreased in degenerated NPCs, and silencing
Tbxt aggravates NPC senescence, whereas its upregulation mitigates this effect [
87]. Similarly, phosphatidylinositol-4-phosphate 5-kinase type I gamma (Pip5k1γ) was proven to be downregulated in NPCs from aged mice and IVDD patients. Deletion of
Pip5k1γ impairs NPC anabolism by inhibiting the activation of the AMPK pathway, thus accelerating NPC senescence [
88].
Conversely, the expression of certain genes induces NPC senescence. Oxidized low-density lipoprotein receptor 1 (OLR1) disrupts autophagy in senescent NPCs by blocking the autophagic degradation of GATA4, increasing SASP-related factor expression. Therefore, silencing
OLR1 restores autophagic activity and mitigates GATA4-induced senescence [
89]. The upregulated expression of periostin (POSTN) promotes senescence and disrupts ECM metabolism in NPCs, correlating with IVDD severity. Inhibiting POSTN can efficiently delay IVDD progression [
90]. Inositol-requiring enzyme 1 (IRE1) is a critical enzyme that regulates endoplasmic reticulum stress. Inhibiting IRE1 improves H
2O
2-induced NPC senescence and ameliorates IVDD [
91]. Furthermore, sirtuin 6 (SIRT6), known for its antisenescent effects on various pathological conditions, such as arterial calcification and osteoarthritis [
92,
93], has been shown to suppress NPC senescence by inducing autophagy [
94].
5.1.2. Noncoding RNAs
Compared with coding RNAs (mRNAs), noncoding RNAs such as microRNAs (miRNAs), long noncoding RNAs (lncRNAs), and circular RNAs (circRNAs) regulate gene expression without affecting the cell nucleus [
95]. Therefore, noncoding RNA-based therapies have garnered considerable attention in IVDD. miR-623 is downregulated in IVDD tissue, and its overexpression inhibits apoptosis and senescence in NPCs by suppressing C–X–C motif chemokine ligand 12 (CXCL12), thus increasing ECM secretion and reducing the levels of inflammatory factors [
96].
Furthermore, circRNAs are promising therapeutic targets for the treatment of IVDD because they sponge miRNAs to regulate the expression of mRNAs. circ
SPG21, whose expression is decreased in degenerated NPCs, is closely linked to the imbalance between anabolism and catabolism, ultimately contributing to cellular senescence. circ
SPG21 sponges miR-1197, relieving its inhibition of ATPase Na
+/K
+ transporting subunit beta 3 (ATP1B3), which suppresses NPC senescence [
97]. circ
ATXN1 is overaccumulated in senescent NPCs, and the mislocalization of progerin in IVD cells accelerates the aging process. Silencing circ
ATXN1 corrects progerin mislocalization in the cytoplasm, alleviating NPC senescence [
98]. lncRNAs exert their regulatory functions primarily through posttranscriptional mechanisms, often by acting as competing endogenous RNAs (ceRNAs) or miRNA sponges. Transient receptor potential canonical 7 antisense RNA 1 (TRPC7-AS1) is downregulated in degenerated nucleus pulposus tissues and senescent NPCs and plays a protective role by binding directly to miR-4769-5p, thereby reducing the degradation of hepsin mRNA. The overexpression of TRPC7-AS1 reverses NPC senescence and restores ECM homeostasis [
99].
5.1.3. N6-methyladenosine (m6A) modification of nucleotides
m6A methylation, the most common RNA modification in eukaryotic cells, participates in regulating autophagy‐related genes (ATGs) and AMPK [
100]. Recently, m6A modification of ATGs in NPCs has been proposed as a therapeutic method for IVDD [
101]. The levels of m6A modification are regulated by methyltransferases, such as methyltransferase-like 3 (METTL3) and METTL14, as well as by demethylases, such as AlkB homolog 5 (ALKBH5). Lysine methyltransferase 2A (KMT2A) promotes NPC senescence and IVDD progression by increasing the expression of METTL3, which mediates the m6A modification of autophagy related 4A cysteine peptidase (ATG4a), reducing autophagic activity in NPCs. Silencing
KMT2A and
METTL3 effectively suppresses the SASP phenotype and alleviates NPC senescence [
102]. Moreover, previous studies have shown that miR-34a-5p is positively correlated with cellular senescence [
41,
103] and that METTL14‐mediated m6A modification of miR-34a-5p promotes TNF-α-induced senescence in human NPCs [
104]. Additionally, the m6A modification of DNA methyltransferase 3 beta (
DNMT3B) mRNA is mediated by alkane monooxygenase (AlkB) homolog 5 (ALKBH5). Elevated ALKBH5 activity in IVDD increases DNMT3B levels and accelerates NPC senescence [
105]. The methylation levels of the lncRNA NORAD modified by Wilms’ tumor 1-associating protein (WTAP) are increased in senescent NPCs, suggesting that interfering with NORAD m6A modification is a potential therapeutic approach to inhibit NPC senescence [
106].
5.2. Selectively clearing senescent cells with senolytic agents
Senotherapeutics encompass two different antisenescent methods: senomorphics and senolytics. These approaches target senescent cells through various molecular mechanisms to alleviate IVDD. Senomorphics are reported to inhibit the secretion of SASP-related factors by improving the extracellular microenvironment, while senolytics eliminate senescent disc cells by directly inducing apoptosis.
Several senolytic drugs have been investigated for IVDD treatment and have shown promising therapeutic effects. Quercetin alleviates IVDD in rats by inhibiting oxidative stress-induced senescence in nucleus pulposus MSCs and NPCs [
41,
107,
108]. The combination of dasatinib and quercetin reduces the expression of senescence markers and SASP-related factors in NPCs, rescuing the senescent phenotypes caused by site-1 protease (S1P) deficiency [
109,
110]. In addition to targeting NPC senescence, dasatinib and quercetin also inhibit the senescence of endothelial cells in the microvasculature of the bony endplate, thereby increasing nutrient transport to the IVD [
111]. Both curcumin and its metabolite o-vanillin exhibit senolytic activities, including clearing senescent human IVD cells, decreasing SASP-related factors, and increasing ECM synthesis [
112]. Moreover, o-vanillin has been proven to be more effective than RG7112 (a compound approved for safety by the US Food and Drug Administration (FDA)) in anti-inflammatory and antisenescent effects [
113]. The combination of RG7112 and o-vanillin is more potent than RG7112 or o-vanillin alone in reducing inflammatory factors and pain mediators in senescent human NPCs [
114]. In addition, ABT263 (Navitoclax) encapsulated in poly(lactic-
co-glycolic acid) (PLGA) nanoparticles (NPs) (PLGA–ABT) has shown senolytic effects. A single intradiscal injection of PLGA–ABT reduces the release of proinflammatory cytokines, decreases matrix-degrading enzyme expression, and clears senescent NPCs [
115].
5.3. Protecting NPCs from senescence via natural components
Numerous natural bioactive molecules derived from Chinese medicinal herbs have been shown to have antisenescent effects in IVDD. For example, dehydrocostus lactone [
116], eupatilin [
117], and baicalein [
118] inhibit NPC senescence by modulating the activity of the MAPK and NF-κB pathways. Genkwanin [
119] and isorhapontigenin [
120] mitigate NPC senescence mainly via the phosphoinositide 3-kinase (PI3K)/Akt pathway. In addition, the JAK/STAT3 pathway is also regulated by anisodamine to attenuate NPC senescence [
35]. Psoralen alleviates inflammation-induced NPC senescence [
121]. Moreover, morroniside and polydatin suppress NPC senescence by reducing oxidative stress [
32,
122], and apigenin-induced autophagy via the AMPK/mTOR pathway alleviates NPC senescence [
78]. Kaempferol, an active compound from the Chinese medicine Du Zhong, blocks NPC senescence via its anti-inflammatory and antioxidative properties [
123]. A self-healing hydrogel that contains multiple components from
Spatholobi caulis (SC) exhibits anti-senescence activity toward NPCs by reducing inflammation and oxidative stress and activating autophagy [
124].
In addition to herbal compounds, certain natural products from vegetables and fruits have also shown antisenescent effects on NPCs. The combination of quercetin and dasatinib further enhances the antisenescent effects of the individual components on NPCs. Maslinic acid, a natural compound in olive plants, ameliorates NPC senescence by increasing ECM levels and downregulating MMP and ADAMTS levels [
125]. Food-derived active components such as PQQ [
31], Delp [
77], sulforaphane (SFN) [
126], and ascorbic acid [
127] inhibit NPC senescence via antioxidant mechanisms. Other natural metabolites, such as taurine [
128], α-ketoglutaric acid (α-KG) [
129], and glutamine [
130], which are either obtained from the diet or endogenously biosynthesized in the body, rescue NPC function via antioxidation, anti-inflammatory effects, and the activation of autophagy.
5.4. Improving NPC senescence via synthetic compounds
Some senolytic drugs, such as RG7112, o-vanillin, curcumin, and ABT263, cannot be obtained from food or synthesized in the body and must be synthesized artificially. Moreover, many chemically synthesized drugs that were originally used to treat other diseases, such as rosuvastatin for lowering cholesterol [
131], flurbiprofen for analgesia [
132], bardoxolone methyl for treating chronic kidney disease and diabetic kidney disease [
133], and metformin for treating type 2 diabetes [
65], have been shown to have protective effects against NPC senescence.
N-acetylcysteine (NAC), a widely used medication and dietary supplement for respiratory disorders, has also been shown to alleviate ROS-induced NPC senescence [
134,
135]. Furthermore, the small-molecule drug 20-deoxy-enol (20-DOI) enhances lysosomal activity and promotes autophagic flux via TFEB nuclear translocation, preventing oxidative stress-induced NPC senescence [
136]. Antisenescent drugs from different sources are summarized in
Table 1 [
31,
32,
35,
41,
65,
77,
78,
[107],
[108],
[109],
[110],
112,
113,
[115],
[116],
[117],
[118],
[119],
[120],
[121],
[122],
[123],
[124],
[125],
[126],
[127],
[128],
[129],
[130],
[131],
[132],
[133],
[134],
[135],
[136]].
5.5. Interfering with NPC senescence via growth factors
Growth factors are another type of bioactive molecule that play a crucial role in IVDD. The activation of insulin-like growth factor (IGF) signaling at an appropriate level is critical for NPC viability, promotes ECM synthesis, inhibits ECM decomposition, and prevents the apoptosis and senescence of disc cells [
137].
Omentin-1 is an adipokine with anti-inflammatory and immunomodulatory effects that stimulates IL-4 synthesis and M2 macrophage polarization and has therapeutic potential in rheumatoid arthritis [
138]. Omentin-1 also mitigates IL-1β-induced senescence in human NPCs through a SIRT1-dependent mechanism [
139]. IL-37, a member of the interleukin-1 family, is be downregulated in IVDD. It suppresses the SASP phenotype and alleviates IVDD progression by downregulating the activation of the NF-κB pathway in NPCs [
140]. Osteogenic protein-1 (OP-1), also known as bone morphogenetic protein type 7 (BMP7), alleviates TNF-α-induced NPC senescence by suppressing the senescence markers p16 and p53 [
46].
Regulating RNA expression and delivering active molecules are relatively straightforward approaches to prevent IVDD. These treatments often require continuous administration and usually target a single senescence-inducing factor, which may have side effects that limit their overall therapeutic efficacy. Given the complicated mechanisms by which multiple factors can induce IVDD, biomaterial-based therapies offer more practical and comprehensive strategies. An overview of antiaging therapies is illustrated in
Fig. 3.
5.6. Extracellular vesicles for targeting antisenescence in NPCs
Previous studies have suggested that transplanted stem cells repair tissue primarily through differentiating into tissue-specific cells. However, accumulating evidence has shown that only a small proportion of transplanted stem cells migrate to the injury site and contribute to tissue regeneration. Therefore, the therapeutic effects of stem cell transplantation are now understood to largely depend on paracrine mechanisms. Among various paracrine factors, EVs, known as natural NPs, including exosomes, microvesicles, and apoptotic bodies, have shown considerable potential due to their advantages, including low immunogenicity, minimal risk of immune rejection, excellent stability, high targeting ability, strong tissue penetration capabilities, and stable pharmacokinetics [
[141],
[142],
[143]].
Recently, MSC-derived EVs, which contain functional proteins, DNA and mRNAs, as well as noncoding RNAs, have garnered significant attention for their antioxidative effects on degenerative diseases, surpassing the interest in MSC transplantation itself. Compared with polymeric biomaterials, EVs have unique advantages as biological carriers because of their negative charge and the expression of CD47 on their surface, which helps them avoid phagocytosis by circulating monocytes [
144]. Additionally, their lipid bilayer membrane enhances flexibility, facilitating their penetration through various tissues and natural barriers such as the blood–brain barrier [
145].
The internalization of EVs by recipient cells is mediated through ligand–receptor interactions [
146]. Therefore, the targeting ability of exosomes can be further enhanced by the expression of specific ligands on their surface through chemical modifications or genetic approaches [
147,
148]. EVs can effectively mitigate damage to NPCs by decreasing the release of proinflammatory factors, scavenging ROS, and slowing the process of IVDD [
149]. EVs contribute directly to tissue regeneration by transferring therapeutic cargos, such as specific RNAs and miRNAs, into IVD cells. Different external stimuli influence the contents of EVs secreted by MSCs. The mitochondrial quality control protein BCL2-interacting protein 3 (BNIP3) is activated in hypoxic environments, promoting the formation of autolysosomes to eliminate damaged mitochondria. Hypoxia-stimulated bone marrow mesenchymal stem cells (BMSCs) release BNIP3-rich EVs, which help delay NPC senescence and stimulate ECM synthesis, as shown in
Fig. 4(a) [
150].
BMSC-derived exosomes (mExos) significantly alleviate compression-induced senescence phenotypes in nucleus pulposus-derived stem cells (NPSCs), primarily through the antisenescent effects of thioredoxin (TXN). mExo-delivered TXN activates Nrf2 transcriptional activity, upregulates the expression of antioxidant genes (such as
CAT and
SOD), and increases the DNA-binding capacity of activator protein-1 (AP-1) cellular FOS proto-oncogene (
c-FOS)/cellular JUN proto-oncogene (
c-JUN) in NPSCs. These factors form a positive feedback loop that promotes endogenous TXN expression, maintains redox homeostasis, and inhibits cellular senescence [
151].
Small EVs (sEVs) secreted by adipocytes (adipo-sEVs) are enriched with nicotinamide phosphoribosyltransferase (NAMPT), a key enzyme involved in oxidized nicotinamide adenine dinucleotide (NAD
+) biosynthesis in mammals. Decreased NAD
+ levels are associated with cellular senescence. Both
in vitro and
in vivo experiments have shown that adipo-sEVs alleviate the senescence of NPCs and cartilaginous endplate cells (EPCs) by delivering NAMPT and enhancing NAD
+ biosynthesis [
152].
MSC-derived EVs also carry specific miRNAs with therapeutic potential. For example, MSC-derived EVs pretreated with selenomethionine (Se-EVs) alleviate NPC senescence and IVDD by enriching miR-125a-5p [
153]. Intradiscal injection of sEVs from induced pluripotent stem cell-derived MSCs (iMSC-sEVs) efficiently improves NPC senescence and IVDD [
154].
The abundance of miRNA-221-3p is significantly increased in exosomes (Hi-Exos) derived from MSCs exposed to hypoxic and inflammatory environments. By delivering miRNA-221-3p to senescent NPCs, Hi-Exos target and suppress the expression of DNA damage-inducible transcript 4 (DDIT4), thereby inhibiting the activation of the NF-κB signaling pathway, markedly alleviating NPC senescence and improving the IVDD microenvironment [
155].
Oxidative stress-induced lysosomal membrane permeabilization (LMP) damage can impair lysosomal autophagic function, exacerbate NPC senescence, and promote the progression of IVDD. MSC-derived exosomes (MSC-Exos) significantly alleviate LMP damage by activating the Nrf2-mediated antioxidant pathway and inhibiting NPC senescence and IVDD progression, as shown in
Fig. 4(b) [
156].
In addition to their direct therapeutic effects, EVs also serve as carriers to deliver proteins or genes, correcting the cytoplasmic mislocalization of DNA. For example, ATR deficiency impairs genomic DNA integrity and causes genomic DNA to mislocalize into the cytosol, triggering cGAS/STING-mediated inflammatory senescence in NPCs. Delivery of an ATR-overexpressing plasmid via engineered EVs reduces DNA damage-induced NPC senescence, alleviating IVDD-related pain and disability [
64]. Cavin2-modified BMSCs exosomes (M-Exos) can deliver the Oct4, KIf4, Sox2 (OKS) overexpression plasmid, achieving partial reprogramming to reverse the senescent phenotype of NPCs and restore their cellular functions, as shown in
Fig. 4(c) [
157]. The EV-targeted antisenescent effects on NPCs are listed in
Table 2 [
64,
[150],
[151],
[152],
[153],
[154],
[155],
[156],
[157]].
5.7. Hydrogel-based biomaterials for preventing NPC senescence
Hydrogels, which have excellent biocompatibility, biodegradability, and lubrication properties, are widely used in IVDD treatment. Deficiencies in mineral ions such as Mg
2+ can trigger inflammation and oxidative stress, resulting in cellular senescence. To overcome the rapid degradation of single-component hyaluronic acid (HA) hydrogels, a double-network hydrogel microsphere system incorporating Mg
2+, termed gelatin methacrylate (GelMA)/HA-His-Mg
2+ (GHHM), was developed to form a sustained charging system powered by Mg
2+. NPCs were encapsulated within this system (GHHM@NPCs), which retained their proliferative ability and resisted ROS-induced inflammation and senescence in the harsh IVD microenvironment, as shown in
Fig. 5(a) [
158].
Mechanotransduction also plays a key role in regulating NPC senescence via micromechanical and biological signaling pathways. For example, the MicroRod structures with long-axis polarity reduced DNA damage, promoted the expression of cytoskeleton proteins, and maintained the nuclear membrane integrity and antiapoptotic ability of NPCs, as shown in
Fig. 5(b) [
159].
Moreover, lactate accumulation in the IVD has been shown to expedite NPC senescence. By utilizing this pathological feature, a pH-responsive, drug-releasing, antisenescent hydrogel was designed to alleviate IVDD and associated pain. GelMA was functionalized with PBA groups to create acid PBA-modified gelatin methacryloyl (GP). An antisenescent GPQ hydrogel was constructed by conjugating GP with quercetin (Q) to reduce the number of senescent cells, suppress the levels of inflammatory factors, and inhibit oxidative stress-induced catabolism in IVDD, as shown in
Fig. 5(c) [
108].
A novel self-assembling hydrogel (MnGAH) was developed by crosslinking manganese ions (Mn
2+) with glycyrrhizic acid (GA), exhibiting both SOD- and CAT-like antioxidant activities. This hydrogel effectively scavenged ROS, delayed the senescence of nucleus pulposus MSCs and alleviated inflammation by promoting macrophage polarization toward the anti-inflammatory M2 phenotype via the ROS–p53–p21 axis, thereby significantly improving IVDD in a rat model, as shown in
Fig. 5(d) [
160]. A multifunctional injectable quaternized chitosan (QCS)-oxidized starch (OST)/SC hydrogel was prepared using OST, QCS, and SC. This self-healing hydrogel reduced the apoptosis and senescence of NPCs, altered abnormal ECM metabolism, and slowed the progression of IVDD by activating autophagy in NPCs, as shown in
Fig. 5(e) [
124]. Hydrogel-based biomaterials targeting NPC senescence are summarized in
Table 3 [
108,
124,
[158],
[159],
[160]].
5.8. Hydrogel-based composites with antisenescent effects on NPCs
A significant limitation of the in vivo administration of EVs is their short retention time. However, considering that the IVD is the largest avascular organ in the body, the systemic delivery of MSC-EVs may be ineffective. Therefore, developing a delivery platform to preserve the biological activity of EVs is indispensable for treating IVDD locally.
Functional matrix hydrogels have been utilized as injectable biomaterials to establish a stable microenvironment for EVs, facilitating the effective delivery and sustained release of antisenescent molecules. A recent study suggested that the inflammatory degeneration of cartilage EPCs (CEPCs) can indirectly contribute to NPC senescence. To achieve simultaneous alleviation of CEPC inflammation and correction of the acidic microenvironment to reverse IVDD, CEPC-targeted engineered CAP-modified exosomes (CAP-sEXOs) loaded with salvianolic acid A were incorporated into a CaCO
3/chitosan (CS) hydrogel to form a composite gel (CAP-sEXOs@Gel). This innovative system neutralized the acidic microenvironment via CaCO
3 while alleviating the inflammatory secretory phenotype of CEPCs, thereby directly and indirectly reducing NPC senescence [
161].
As a linear polysaccharide approved by the FDA, CS possesses excellent biocompatibility, biodegradability, and gel-forming abilities, making it a promising material for hydrogel scaffolds. A three-dimensional (3D) porous exosome-loaded CS hydrogel system (QCS-OST@Exos) was developed through the conjugation of QCS, OST, and MSC-derived exosomes. This hydrogel rejuvenated senescent NPCs and attenuated abnormal ECM metabolism, thereby protecting against needle puncture-induced disc degeneration in rats, as shown in
Fig. 6(a) [
162].
Static magnetic field (SMF) exposure was reported to increase the secretion of mitochondria-containing MVs (mitoMVs) from MSCs by promoting the interaction between kinesin family member 5B (Kif5b) and ras-related protein Rab-22a (Rab22a). These mitoMVs were then encapsulated in GelMA hydrogels to mitigate NPC senescence, demonstrating significant therapeutic efficacy in IVDD [
163]. Moreover, hydrogel-EV composite materials synergistically promoted the recruitment and differentiation of NPSCs, exerting potent antisenescent effects. The RGD peptide, which is expressed on the membranes of EVs, showed a strong integrin-binding affinity, enhancing EV anchorage and bioavailability
in vivo [
164]. The combination of RGD and decellularized nucleus pulposus (DNP) hydrogel (sEV-RGD-DNP) was shown to promote the recruitment and differentiation of NPSCs into NPCs. Moreover, the antisenescent effects of sEV-RGD-DNP were strongly dependent on miR-3594-5p, which was enriched in sEV and targeted the homeodomain-interacting protein kinase 2 (HIPK2)/p53 signaling pathway, as shown in
Fig. 6(b) [
165].
Furthermore, MSC-EVs enriched with antioxidant enzymes such as GLRX3 have emerged as promising candidates for improving mitochondrial function and enhancing antioxidant defense in IVDD treatment. Dopamine (DA), known for its ROS-responsiveness, was used to synthesize a multifunctional hydrogel self-cross-linked with DA-functionalized gelatin (GelDA) and borax-coupled aldehyde-modified chondroitin sulfate (Borax-ACS). This hydrogel was then loaded with EV-GLRX3 derived from hypoxia-pretreated MSCs. The self-antioxidant hydrogel and antioxidant EVs showed synergistic effects on alleviating NPC senescence and ECM degradation in a rat model of IVDD, as shown in
Fig. 6(c) [
166].
In addition to hydrogel-EV composites, hydrogels incorporating NPs have also shown antisenescent potential in IVDD treatment. Owing to the lower cell density within the IVD, inducing the proliferation of senescent cells seems to be a more effective strategy for regeneration than eliminating these cells. Therefore, a mildly alkaline hydrogel platform (MB-ALG) was constructed to convert the acidic IVD microenvironment (pH 6.2) to a mildly alkaline state (pH 8.0) through the release of hydroxide ions (OH
−). This local alkalization attracted senescent cells to the hydrogel surface, where Mg
2+ promoted their rejuvenation [
167]. Additionally, hydrogen (H
2) reduces oxidative stress-induced cell damage by clearing excessive ROS. Therefore, an ROS-responsive hydrogel was engineered to release H
2 in the degenerative IVD microenvironment and provided mechanical protection for transplanted BMSCs, increasing the viability of BMSCs and resident NPCs, as shown in
Fig. 6(d) [
168].
The microenvironment of IVDs is characterized by low oxygen levels, low nutrition, and the accumulation of various metabolic byproducts, further complicating regeneration. Anaerobic glycolysis is the primary energy pathway for IVD cells, especially NPCs, which produce lactic acid as the predominant metabolic waste [
67]. The increased lactic acid levels promote ROS production via acid-sensitive ion channels (ASIC1, ASIC3) and activate the NF-κB signaling pathway, thus promoting NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome activation and IL-1β release, thereby accelerating ECM degradation and IVDD progression [
169]. Hydrogen ion-capturing hydrogel microspheres (GMNPs) were designed to neutralize excess hydrogen ions, inhibit the thioredoxin-interacting protein (TXNIP)/NLRP3/IL-1β cascade, and promote ECM synthesis in NPCs [
170]. The ability of hydrogel-based composites to alleviate NPC senescence is presented in
Table 4 [
[161],
[162],
[163],
[165],
[166],
[167],
[168],
170,
171].
Nucleus pulposus progenitor cell (NPPC)-targeted lipid thymine NPs (NT-LNPs) delivering Klotho circRNA (NT-KLNPs) significantly suppressed the expression of senescence-associated genes in NPPCs by activating the SIRT6/Nrf2/HO-1 signaling pathway while promoting ECM synthesis. Moreover, GAG-based hydrogels remodeled the IVD microenvironment by scavenging inflammatory chemokines, including monocyte chemoattractant protein 1 (MCP1) and IL-8. The combination of lipid NPs and the injectable GAG hydrogel synergistically reversed the senescent phenotype of NPPCs and repaired degenerated IVD tissue [
171].
5.9. Nanomaterial-mediated therapies for modulating NPC senescence
5.9.1. NPs
ROS levels are tightly regulated by key antioxidant enzymes, such as SOD, CAT, and GPx, along with nonenzymatic antioxidants. Reduced activities of these enzymes lead to increased ROS accumulation, accelerating IVDD progression [
172,
173]. However, the clinical translation of natural antioxidant enzymes is limited by their high cost, harsh reaction conditions, and instability. Nanozymes, particularly carbon dots (CDs), have emerged as promising alternatives due to their intrinsic enzyme-mimicking properties, offering high catalytic efficiency, stability, and easy modification.
Glutathione-doped CDs (GSH-CDs) are novel antioxidant nanozymes that mimic the activities of SOD, CAT, and GPx, effectively scavenging intracellular ROS. Both
in vitro and
in vivo experiments have demonstrated that GSH-CDs improve mitochondrial dysfunction and prevent NPC senescence, as shown in
Fig. 7(a) [
174]. Similarly, NAC-derived CDs (NAC-CDs) have been proven to have ideal antioxidant and antisenescent effects by maintaining mitochondrial homeostasis, as shown in
Fig. 7(b) [
135].
In recent years, Prussian blue (PB) NPs have also been reported to have enzyme-like activities (SOD and CAT) and to exert therapeutic effects on ROS scavenging and ECM synthesis through the suppression of MMPs and ADAMTS [
175]. To enhance mitochondrial targeting, a composite nanozyme (CD-PB-mitochondrial-targeting molecule 4-carboxybutyl triphenyl phosphonium bromide (TPP)) was formed by linking CD-supported PB with the TPP. This hybrid nanozyme maintained mitochondrial homeostasis, inhibited inflammation and catabolism, and effectively delayed NPC senescence [
176]. Iron sulfide-based nanozymes also exhibit substantial antioxidative properties. NAC exerts antioxidant and anti-inflammatory effects and is often synthesized into an ROS-responsive prodrug. The synergistic action of NAC and iron sulfides (NAC iron sulfides, termed greigite nanozymes) alleviates NPC senescence and delays IVDD progression by regulating the ROS–p53–p21 pathway [
134].
Senescent NPCs typically secrete SASP-related factors that reinforce cellular senescence via autocrine and paracrine signaling, facilitating IVDD progression [
177,
178]. The combination of dasatinib and quercetin (D + Q) partially alleviates IVDD resulting from S1P deficiency [
110]. ABT263 demonstrates efficacy in selectively clearing senescent NPCs and reducing IVDD severity. However, the systemic administration of senolytics faces challenges due to the avascular nature of IVDs and the risk of off-target effects. Moreover, repeated intradiscal injections pose risks of infection and further degeneration. To address these limitations, NP-based delivery systems have been developed to achieve the sustained and localized release of senolytic agents. ABT263-loaded PLGA NPs (PLGA-ABT) successfully eliminated stress-induced senescent cells in rat hearts with ischemia-reperfusion injury without causing systemic toxicity [
179]. A single intradiscal injection of PLGA-ABT significantly reduced SASP secretion (IL-6 and MMP-13), restored ECM integrity, and attenuated IVDD in an injury-induced degenerative disc model [
115]. The cellular penetration efficiency of biomaterials also affects their therapeutic efficacy. Virus-like mesoporous silica NPs (VNs) were used to deliver Serpine1 small interfering RNA (siRNA) to alleviate NPC senescence and the pathology of IVDD induced by injury or instability, as shown in
Fig. 7(c) [
180]. Moreover, aptamer-functionalized polymeric NPs targeting the fas ligand (FasL) transmembrane protein on the surface of NPCs were developed to deliver an miR-150-5p inhibitor specifically to NPCs, reducing SASP-related factor secretion and mouse IVDD, as shown in
Fig. 7(d) [
181].
To enhance targeting and cellular uptake, different cell membrane-based biomimetic NPs have been constructed. For example, a dual-functional NP platform (peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC1α) activator (SP)@NNPm) was engineered. It consisted of mesoporous silica NPs loaded with SP and then coated with an NPC membrane overexpressing natural killer group 2D (NKG2D). This system enabled pH-responsive SP release, promoted mitochondrial biogenesis via PGC1α, and inhibited oxidative stress-induced senescence and SASP-related factor expression, as shown in
Fig. 7(e) [
182]. Novel biomimetic NP membrane-coated keratin nanoparticles (MKNs) were constructed with keratin NPs as the core and the cell membrane of
N-propionylneuraminic acid (Neu5Prop)-modified adipose-derived stem cells (ADSCs) as the surface coating. These MKNs alleviated oxidative stress-induced mitochondrial dysfunction and NPC senescence, further promoting ECM synthesis and mitigating IVDD [
183].
Another biomimetic NP (HIF-1A@NNP) constructed from NPC membranes (NNP) and loaded with hypoxia-inducible factor 1 alpha (HIF-1A) plasmids was developed to achieve targeted activation of HIF-1A signaling in NPCs. These NPs effectively activated autophagy, thereby suppressing cellular senescence and apoptosis; reducing the secretion of SASP-related factors such as IL-1β, IL-6, and TNF-α; and inhibiting M1 macrophage polarization as well as the formation of an inflammatory microenvironment [
184].
5.9.2. Nanofibers
Research has demonstrated that MMP-2 serves as a key indicator of catabolic activity in IVDs [
185]. Cyclooxygenase-2 (COX-2) is a proinflammatory mediator that induces inflammatory responses and tissue damage, and its expression levels in IVDs have been shown to correlate positively with the severity of degeneration [
186]. Drug-loaded nanofibers with an enzyme-responsive peptide were developed to sequentially target MMP-2 and COX-2, enabling site-specific accumulation and on-demand release of flurbiprofen.
In vitro and
in vivo experiments showed that supramolecular nanofibers suppressed the expression of inflammation-related genes and proteins, decreased M1 macrophage polarization, upregulated the expression of ECM proteins, and reduced SASP-related factor expression and DNA damage in senescent NPCs [
132].
5.9.3. Nanogels
Tetrahedral framework nucleic acids (tFNAs) are widely employed in drug delivery and gene therapy because of their excellent biocompatibility, biodegradability, and ability to penetrate cells without the need for lipofection while maintaining structural integrity within the cytoplasm. circ
ATXN1 was enriched in senescent NPCs and degenerative disc tissue and accelerated cellular senescence and impaired mitochondrial respiration by promoting progerin translocation from the cell nucleus to the cytoplasm. To address this, a DNA nanogel (SDTET) composed of four DNA strands and siRNA linkers was engineered via tFNAs. This nanoplatform showed high gene-silencing efficiency and intrinsic ROS-scavenging properties. Silencing circ
TTXN1 efficiently reversed human NPC senescence and delayed IVDD progression [
98]. Nanomaterial-mediated therapies for modulating NPC senescence are shown in
Table 5 [
98,
115,
132,
134,
135,
174,
176,
[180],
[181],
[182],
[183],
[184]].
5.10. Microsphere biomaterial platforms for mitigating NPC senescence
Owing to their advantages in sustained and controlled drug release, targeted delivery, stability, and biocompatibility, microspheres are commonly used in IVDD treatment.
Bradykinin (BK) has been reported to protect against high glucose-induced senescence in endothelial progenitor cells [
187]. BK was also found to be expressed at a low level in IVDD. Therefore, sustained release PLGA/BK microspheres were developed, which enhanced ECM synthesis and reduced NPC senescence through the activation of BK receptor 2 (B2R) and its downstream PI3K signaling pathway [
188]. Baicalein, a natural flavonoid extracted from the traditional Chinese medicinal herb
Scutellaria baicalensis Georgi, was encapsulated in porous silk fibroin microspheres and demonstrated antisenescent activity in mouse NPCs [
118]. Microsphere platforms used to prevent NPC senescence are summarized in
Table 6 [
118].
6. Discussion and perspectives
IVDD is a common musculoskeletal disorder associated with aging, and the increase in global life expectancy has led to an increased incidence of IVDD [
189]. Although the etiology of IVDD is complex, NPC senescence and ECM degradation may represent the main pathological features of IVDD. IVDD treatment can be guided by the 3W principle, as shown in
Fig. 8: What is the minimally invasive method for IVDD treatment? What is the underlying mechanism? What is used for IVDD treatment?
6.1. Localized injection of a sustained release system: A minimally invasive approach for the management of IVDD
Currently, pharmacological treatments for IVDD primarily include nonsteroidal anti-inflammatory drugs and analgesics aimed at alleviating pain and inflammation and improving patients’ quality of life [
[190],
[191],
[192]]. Oral or intravenous administration often leads to side effects and shows limited efficacy due to the avascular nature of the intervertebral disc, which hampers drug penetration into the lesion site. In this context, local drug delivery via intradiscal injection offers a more effective therapeutic approach than does systemic drug delivery [
193,
194]. Localized injections can deliver drugs directly to the lesion area while minimizing systemic drug distribution and associated side effects and rapidly increasing the local drug concentration to relieve pain [
189].
Additionally, local injection is a minimally invasive technique that causes less trauma. Moreover, incorporating sustained-release systems into local therapy can further maintain stable therapeutic drug concentrations and reduce the need for repeated injections. Although microneedles represent another form of minimally invasive delivery, their application in IVDD is limited by shallow penetration depth, risk of breakage and drug leakage, and insufficient drug-loading capacity, which collectively restrict their therapeutic efficacy [
195,
196]. Therefore, intra-articular injection remains the primary approach for IVDD therapy at present.
6.2. Inflammation, oxidative stress, and impaired autophagy are the core mechanisms for modulating NPC senescence
Owing to the avascular microenvironment, the IVD is particularly vulnerable to oxidative stress-induced damage. Consequently, antioxidant therapy is widely regarded as one of the most promising strategies for treating IVDD. The microenvironment-responsive metal–phenolic network-based delivery platform TMP@Alg-PBA/PVA improved IVDD by scavenging ROS and reducing ECM degradation [
43]. Mitochondrial dysfunction promotes ROS production, and elevated ROS levels, in turn, exacerbate mitochondrial damage, forming a vicious cycle that promotes excessive catabolic activity in NPCs and intensifies inflammation within the IVD microenvironment [
172]. Sirtuin 3, a mitochondrial deacetylase, mitigates IVDD by delaying oxidative stress-induced senescence in NPCs [
37]. Mitochondria-targeting NPs can be synthesized by linking polymerized gallic acid (PGA-Mn) with a mitochondrial targeting peptide (TP04), effectively reducing the degree of mitochondrial damage caused by ROS [
197]. As the core drivers of IVDD, inflammation and oxidative stress mutually reinforce their deleterious effects in a positive-feedback loop, which exacerbates ECM degradation and accelerates IVDD progression. Correspondingly, inactivation of proinflammatory pathways or inflammatory mediators can efficiently counteract ROS-induced damage within IVDs [
[198],
[199],
[200]].
Mangiferin has strong free radical-scavenging activity and can thus protect against IVDD by suppressing the NF-κB signaling pathway [
201]. Conversely, regulating oxidative stress can also suppress inflammation in IVDD. For example, hydrogel microspheres (GM@CS-BP) were designed to disrupt the feedback loop between oxidative imbalance and inflammation in IVDs [
202,
203]. Therefore, a combined therapeutic strategy that involves both antioxidant and anti-inflammatory effects is increasingly recognized as a rational and effective approach for treating IVDD. A chondroitin sulfate-derived hydrogel with inherent antioxidant and anti-inflammatory properties was reported to scavenge ROS, reduce IL-1β-induced ECM degradation, and ameliorate nucleus pulposus degeneration [
204]. Furthermore, ROS-responsive magnesium-containing microspheres (Mg@PLPE MSs) have been developed to provide antioxidative and anti-inflammatory therapeutic effects in a rat IVDD model by releasing H
2 in a controlled manner, efficiently reducing ECM degradation and cell apoptosis [
205].
In addition to inflammation and oxidative stress-induced senescence, reduced autophagy is also a key mechanism underlying NPC senescence. Impaired autophagy, resulting from PIEZO1 upregulation under mechanical overloading, promotes NPC senescence and contributes to ECM degradation during IVDD [
206]. Additionally, the acidic microenvironment of NPC senescence in IVDs is related to reduced autophagy [
207].
Many studies have shown that organelles such as mitochondria, the endoplasmic reticulum, and lysosomes play pivotal roles in regulating inflammation, oxidative stress, and autophagy, further fostering NPC senescence [
[208],
[209],
[210],
[211]]. Therefore, organelle-specific therapeutic strategies against cellular senescence may become novel therapies for IVDD.
6.3. Potential antisenescent strategies in IVDD therapy
Both senolytics and senomorphics have shown therapeutic potential in mitigating disc degeneration and associated pain [
113]. In addition, cellular reprogramming, EVs, and regenerative biomaterials have tremendous potential to protect against IVDD.
6.3.1. Antisenescent agents
To date, many compounds have shown antisenescent effects on NPCs. For example, metformin can delay IVDD progression by alleviating the senescence and apoptosis of NPCs in the IVD [
212]. Senolytic components such as curcumin and o-vanillin have been proven to clear senescent human NPCs [
114]. Nevertheless, the application of these compounds in the treatment of IVDD remains at an early research stage, with limited clinical data supporting their long-term efficacy and safety, primarily owing to their immunosuppressive properties and unwanted off-target effects. For example, metformin is associated with gastrointestinal disorders [
213], whereas navitoclax induces dose-dependent thrombocytopenia by directly damaging platelets [
214]. These findings further confirm that local administration is a reasonable therapeutic approach to minimize side effects. Fortunately, a growing number of natural bioactive compounds with antisenescent properties have been identified in traditional Chinese medicine, suggesting promising prospects for antiaging therapies. Nevertheless, targeted removal of senescent cells and the low proliferative capacity of residual NPCs may further exacerbate the difficulties in tissue regeneration. Consequently, new treatments targeting endogenous cells have gained widespread attention recently.
6.3.2. Endogenous NPC-based therapies
Stem cell transplantation has shown notable effectiveness in treating IVDD. However, the senescence of transplanted stem cells impairs their regenerative capacity and hinders the self-repair of degenerated discs. Improving the proliferation of quiescent endogenous stem cells provides a novel strategy for antiaging treatments. The antisenescent potential of chondroitin sulfate-derived biomaterials (CSDBs) has been evaluated in senescence-accelerated (SAMP8) mice, which suggests that reprogramming senescent NPCs to a functional state is possible [
215]. Another study confirmed the feasibility of this strategy by rejuvenating senescent NPCs to slow IVDD [
167].
A combination of three factors (Oct4, forkhead box protein A2 (FOXA2), and TBXT (OFT)) has been shown to reprogram degenerative NPCs into induced notochordal-like cells (iNCs). Adeno-associated virus (AAV)-mediated delivery of OFT efficiently restored the histological structure and mechanical function in a rat IVDD model [
216]. The partial reprogramming of senescent NPCs with Oct3/4, Sox2, Kruppel-like factor 4 (Klf4), and cellular myelocytomatosis oncogene (c-Myc) (OSKM) factors reversed the senescent phenotype by activating energy metabolism and promoting the redistribution of the cytoskeleton, thereby delaying the progression of IVDD [
217]. Additionally, a hydrogel drug delivery system (SCGP hydrogels) consisting of sodium alginate (S) hydrogels embedded with gelatin (G) microgels enabled the spatiotemporal release of chondroitin sulfate (C) and parathyroid hormone-related peptide (P2), recruiting and promoting the differentiation of endogenous NPSCs toward NPCs and further hindering the progression of IVDD [
218]. These studies show that reprogramming endogenous somatic cells into progenitor-like cells may also be an effective approach for IVDD treatment.
6.3.3. Importance and potential of EV therapies in IVDD
EVs, particularly exosomes, are emerging as promising alternatives to stem cell-based therapies, offering advantages such as avoiding the risks linked to stem cell transplantation, including tumorigenesis, immune rejection, and embolization. The applications of exosomes in the treatment of IVDD mainly include the following methods: ① direct injection of exosomes into the degenerated IVD, allowing their bioactive molecules to function on damaged tissue, promoting ECM synthesis and inhibiting the apoptosis of NPCs [
219]. ② The use of exosomes as natural carriers to deliver drugs or genes to lesions [
220]. Specific miRNAs (such as miR-378, miR-125b-5p, and miR-146) delivered via exosomes can suppress the expression of inflammatory cytokines and matrix-degrading enzymes in
in vitro and
in vivo models of IVDD, reducing disc degeneration and improving tissue repair [
195]. ③ Use of various biomaterials, such as hydrogels, NPs, and fibrous scaffolds, as carriers to deliver exosomes directly to regenerate IVDD. Integrating exosomes with biomaterials to form composite biomaterials has been shown to improve cell viability, stimulate matrix synthesis, and reduce inflammation and apoptosis within the disc tissue [
221].
However, exosome-based therapies for IVDD face several challenges, such as large-scale and stable production. Different methods have been used to increase exosome production, but the amount secreted by cells remains insufficient to support clinical translation. The process of isolating exosomes is time-consuming and costly. There is a lack of standardization regarding the cell types, intervention conditions, and EV dosages used to treat IVDD. Ultimately, quality control remains a significant hurdle [
222]. Limited loading capacity is another challenge faced by exosomes.
Despite these challenges, exosomes remain among the most promising tools in regenerative medicine because of their natural origin and nontoxicity.
6.3.4. Regenerative biomaterials for IVDD
Various biomaterials that target senescent NPCs have been developed and are synthesized mainly from natural polymers, which include gelatin, alginate, HA, silk fibroin, and CS. Among them, gelatin and CS are the most commonly used. The advantages and disadvantages of different biomaterial platforms based on antiaging therapy in IVDD are summarized in
Table 7. Among these biomaterials, NPs (such as metal oxide NPs) exhibit cytotoxicity, potentially inducing oxidative stress and apoptosis. The metabolic pathways and clearance mechanisms of NPs in the body are not well understood and lead to their accumulation in vital organs [
223,
224]. Nanofibers exhibit high hydrophobicity or hydrophilicity, making functional modification challenging [
225]. Hydrogels have a high degradation rate, short-term stability, low mechanical strength, and limited drug loading ability [
226]. However, owing to their similarity to the main component of nucleus pulposus tissue, hydrogels are still ideal platforms for IVD regeneration, offering lubrication and exhibiting anti-inflammatory properties. Unlike EV membrane proteins, which may carry xenogeneic antigens and trigger immune responses, hydrogels inherently minimize this risk [
227].
Furthermore, the disc is a load-bearing organ, and liquid drugs alone are prone to leakage and inferior efficacy. Self-assembling hydrogels can solve these problems by encapsulating therapeutic drugs or growth factors and solidifying them in situ in response to specific stimuli, such as temperature, pH, or ionic concentration, to adapt to the IVD environment [
228]. Moreover, hydrogels are ideal delivery systems for bioactive molecules, particularly exosomes. Composite biomaterials, including hydrogel-coated exosomes, significantly reduce the degradation of exosomes
in vivo [
229]. By encapsulating exosomes within hydrogels, their release can be controlled over time, and they are better protected from degradation in the harsh environment of the degenerated disc. Emerging evidence suggests that exosome-loaded hydrogels have potential in the treatment of growth plate injuries and craniofacial bone regeneration [
230,
231]. Moreover, the exosome-loaded hydrogel Exo@KeMA promoted ECM synthesis and improved nucleus pulposus biomechanics, providing structural support for tissue regeneration [
232].
Hydrogel composite materials enhance mechanical properties and improve structural stability, serving as natural carriers for antisenescent agents to achieve targeted and controlled drug release. These advantages make hydrogel composite materials highly promising for applications in tissue engineering, drug delivery, and other fields.
6.4. Perspectives
Interdisciplinary approaches for IVDD treatment have shown great potential. First, comprehensive multiomic analyses, such as metabolomics, proteomics, and transcriptomics, are more efficient at identifying potential targets for the antiaging treatment of IVDD. With advancements in artificial intelligence, technologies such as molecular docking, network pharmacology, and deep learning are increasingly used to screen more active molecules from traditional Chinese medicines. These molecules can prevent the side effects of currently known senolytic drugs by rejuvenating senescent NPCs, stimulating the proliferation of quiescent endogenous stem cells, or promoting the differentiation of NPSCs into NPCs. Second, numerous chemical groups responsive to high ROS levels, low pH, and inflammation can be incorporated into biomaterials to enable responsiveness to stimuli in the IVDD microenvironment, allowing for controlled release as needed. Ideally, these biomaterials should also target specific surface markers of senescent cells, thereby facilitating their functional recovery.
Finally, the main challenges faced by nanomedicines in clinical translation include differences between animal models and human diseases, unsatisfactory therapeutic efficacy, and the use of biomaterial designs that do not fully consider
in vivo behavior in humans. To improve clinical translation, future research should also focus on developing more representative human disease models, optimizing biomaterial design, and conducting in-depth studies of their behavior within the human body. As an emerging 3D
in vitro model, IVD organoids can more realistically mimic the structure and function of the human IVD, providing a powerful tool for studying the pathogenesis of IVDD, screening drugs, and developing regenerative therapies [
233].
Although challenges remain, the development of antisenescent biomaterials through the integration of cell experiments, animal models, and organoids to further promote clinical translation offers a promising direction for future IVDD therapy.
CRediT authorship contribution statement
Jia-Ying Ding: Writing – original draft, Methodology, Investigation, Data curation. Yang-Shuo Ge: Writing – original draft, Methodology, Investigation, Data curation. Jun Shen: Writing – original draft, Investigation, Data curation. Wen-Yao Li: Writing – original draft, Methodology, Data curation. Chun-Meng Huang: Writing – original draft, Methodology, Data curation. Min-Jun Zhao: Writing – original draft, Methodology, Data curation. Jian-Li Yin: Writing – original draft, Investigation, Data curation. Xue-Zong Wang: Writing – original draft, Methodology, Data curation. Jian-Guang Xu: Writing – review & editing, Methodology, Investigation. Wenguo Cui: Writing – review & editing, Software, Resources, Methodology, Investigation. Dao-Fang Ding: Writing – review & editing, Supervision, Project administration, Methodology, Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (81902306 and 82174406), the Shanghai Municipal Health Commission Traditional Chinese Medicine Research Project (2024QN012), the Shanghai Science and Technology Committee (22Y11923200 and 22ZR1453000), and the Shanghai University of Traditional Chinese Medicine Science and Technology Development Project (23KFL023).
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