Progenitor cells in human CEP can differentiate into osteoblasts, adipocytes, and chondrocytes (He et al., 2018), which are named as cartilage endplate stem cells (CESCs) by researchers. As reported, CESCs can maintain the integration of structure and function of CEP by paracrine activation of the SDF-1/CXCR4 and ERK1/2 signal pathways, which pathways are conducive for accelerating the proliferation and regeneration of NPCs. Moreover, CESCs can release a type of extracellular vesicle called CESCs-derived exosomes (Phinney and Pittenger, 2017). As demonstrated, those exosomes play important roles in intercellular communication by transferring certain proteins and RNAs, to control inflammation and prevent tissue degeneration by inhibiting cell apoptosis (Fan et al., 2020; Xu et al., 2019). What’s more, such exosomes can not only induce the CESCs to differentiate into NPCs, but also can promote the proliferation of NPCs, which altogether delay the development and progression of IVDD (Luo et al., 2021a). However, CEP inflammation significantly weakens the function of exosomes released from CESCs, and accountably aggravates the progression of IVDD (Luo et al., 2021b).

Accordingly, persistent inflammation can result in the CESCs to degenerate, which changes the contents of those secreted exosomes, where such exosomes are named as degenerated CESC-derived exosomes (D-exos) by researchers. Compared to normal CESC-derived exosomes (N-exos), the efficacy of D-exos on reversing IVDD is greatly weakened, and the mechanisms are as follows: firstly, N-exos can activate autophagy associated PI3K/Akt signaling pathways to inhibit the apoptosis of NPCs; secondly, N-exos can significantly reduce apoptotic proteins such as cleaved caspase3 and Bax in NPCs, while anti-apoptotic protein Bcl-2 is increased in such process (Luo et al., 2021b). Meanwhile, N-exos can also activate HIF-1α/Wnt signaling pathways to promote the transformation of CESCs into NPCs by an autocrine mechanism (Luo et al., 2021a). Compared to N-exos, D-exos carried fewer anti-apoptotic proteins, their ability to activate PI3K/AKT signaling pathways in NPCs and HIF-1α/Wnt signaling pathways in CESCs is also decreased. Those changes in D-exos downregulate NPCs autophagy and cause the NPCs to develop apoptosis, resulting in reduction of NPCs (Figure 3) (Luo et al., 2021b).

Studies also find that N-exos can affect the progression of IVDD by their carried miRNAs. For example, miR-532-5p released by N-exos can downregulate the expression of caspase-3, caspase-9, caspase-8, and MMP-1 in NPCs, while upregulate the expression of collagen type I (Col I), collagen type II (Col II) and proteoglycan. Besides, miR-532-5p can inhibit the release of inflammatory factors such as IL-6 and IL-1β by targeting RASSF5 (Zhu et al., 2020). MiR-125-5p secreted by N-exos can inhibit the apoptosis of nucleus pulposus tissue by downregulating the gene expressions of SUV39H1, Bax, MMP13, and p62, while upregulate the gene expressions of Bcl2, ACAN, LC3-II/I. However, the expression levels of miR-532-5p and miR-125-5p are downregulated in D-exos (Chen and Jiang, 2022).

Calcification of the CEP can also lead to the development of IVDD. As we mentioned previously, the degeneration of CEP can result in the release of proinflammatory cytokines, such as tumor necrosis factor and interleukins (Brisby, 2006). Moreover, those inflammatory factors can increase the risk of the CEP to calcification and chondroid tissue formation as well (Bessueille and Magne, 2015; Joshi et al., 2016). Researchers find that the TNF-α can predispose vascular smooth muscle cells (VSMCs) to form calcium deposits (Guerrero et al., 2012; Lencel et al., 2011; Masuda et al., 2013), where the release of bone morphogenetic protein 2 (BMP-2), a potent osteosynthesis factor, is revealed to accelerate the calcification of involved tissues (Nakagawa et al., 2010). Furthermore, TNF-α can also reduce the levels of extracellular inorganic pyrophosphate (PPi), which is reported to be a potent endogenous inhibitor of calcification (Lencel et al., 2011; Zhao et al., 2012). In addition, some researchers have confirmed that the inflammation of CEP is closely associated with the calcification of IVD (Joshi et al., 2016).

Besides, the calcified CEP can affect the nutrition and oxygen exchange of the IVD as well. As we described previously, the intervertebral disc is an avascular tissue structure, where capillaries originated from the vertebral body terminate at the periphery of intervertebral disc. Therefore, nutrients of the NP cells needed, such as glucose and oxygen, can only be transported by diffusion through the CEP (Sakai and Grad, 2015). Due to this anatomy of the disc, the permeability of the CEP can be significantly decreased when the CEP becomes calcified, those changes will further decrease the glucose and oxygen delivered to the IVD, and further result in intervertebral disc degeneration over time. Extracellular Ca²⁺ content is found to elevate after CEP calcification in the microenvironment of IVD as well. As reported, the elevated Ca²⁺ can activate the extracellular calcium-sensing receptor (CASR), a C-g protein-coupled receptor (GPCR), to regulate the synthesis and secretion of parathyroid hormone (Brown, 2013). While the activation of CASR is responsible for the aggravation of denatured NP cells (Grant et al., 2016), as decreased secretion and accumulation of beneficial matrix molecules, such as Col II and proteoglycans, can be observed in the NP cells during this process (Lakstins et al., 2021; Wong et al., 2019; Zhao et al., 2007). Besides, ECM synthesis-catabolism imbalance will be developed within the NP cells during CEP calcification, where more ECM is degraded than produced (DeLucca et al., 2016; Kim et al., 2009). Over time, the water content within the IVD is significantly decreased, and intervertebral disc subsidence will then occur (Figure 4) (van Uden et al., 2017).

Moreover, calcified IVDs are more likely to rupture and herniate than non-calcified IVDs and normal IVDs, because the loading stress of the spine is abnormally distributed when CEP gets calcification, which greatly increases the risk of developing vertebral fractures and fissures (Fearing et al., 2018). In addition, calcified intervertebral disc can result in caseous degeneration due to calcified deposits, which can even be spontaneously liquefied in the late stage. Calcification can also cause damage to the annulus fibrosus or surrounding soft tissue, and inflammation can further accelerate this process, which altogether lead to the development of IVD herniation (Court et al., 2018; Yu et al., 2020; Yue et al., 2016). Besides, Calcified tissue in CEP can destroy the microstructure of bone trabeculae, which is another factor that leads to endplate fracture (Crockett et al., 2017). Moreover, the calcified tissue in CEP can result in anisotropic cracks of CEP as well. Those cracks in CEP can further accelerate the loss of water content in the IVD, while simultaneously increase the flow of inflammatory cytokines and cells (Crockett et al., 2017).

Iron is considered as the most abundant trace element in the human body. Nowadays, various studies have proved that iron’s abnormal deposition is correlated to the development of IVDD and CEP inflammation (Jing et al., 2021a; Jing et al., 2021b; Nieuwenhuizen et al., 2013). Unfortunately, our human body lacks the ability to get rid of excess iron, which causes the irons to be gradually accumulated as we become aged (Tian et al., 2022). Recent studies report that chondrocyte senescence and degeneration are largely involved in iron toxicity caused by oxidative stress and iron overload (Li et al., 2023; Yao et al., 2021). Accordingly, the balance of iron metabolism within cells is regulated by iron transporters DMT1 and FPN1, where DMT1 serves as the transporter for taking up iron and FPN1 is responsible for facilitating the outflow of iron (Galy et al., 2023), while ferritin is responsible for storing iron (Zeidan et al., 2021). Some studies have shown that when CEP is inflamed, the inflammatory cytokines IL-6 and TNF-α can upregulate the expression of DMT1 while decreasing the expression of FPN1 in cells (Li et al., 2021; Urrutia et al., 2013), which changes significantly elevate iron depositions in the body. Meanwhile, IL-6 and TNF-α also increase the expression of hepcidin (Frazier et al., 2011), an another factor that destroys iron homeostasis (Stephenson et al., 2014).

Iron overload can produce large amounts of reactive oxygen species (ROS) by electron transfer in the mitochondrial oxidative respiratory chain (Mishima et al., 2022), where excess ROS is closely associated with mitochondrial dysfunction and DNA damage (Chen et al., 2020). Scholars have proved that iron overload can aggravate oxidative stress and mitochondrial function in a dose-dependent manner (He Q. et al, 2023), which further induces iron-dependent cell death (Ferroptosis) of the CEP and NP cells (Dixon et al., 2012). Ferroptosis is a recently discovered mode of cell death, a process characterized by lipid peroxidation catalyzed by iron ions (Tang et al., 2020). Nowadays, many reports have pointed out that ferroptosis is closely related to IVDD (Zhang P. et al, 2023; Zhu et al., 2023). In a TBHP-induced oxidative stress model, the increased changes of ferroptosis-associated marker and lipid peroxidation are positively correlated with IVD cell degeneration (Fan et al., 2023).

Moreover, excess iron in NP cells can activate transcription factor 3 (ATF3), a positive regulator of ferroptosis (Fu et al., 2021; Wang et al., 2019), while inhibit glutathione peroxidase 4 (GPX4) and cystine/glutamate antiporter SLC7A11 (xCT) (Wang W. et al, 2022; Zhang et al., 2020), which changes significantly induce ferroptotic cell death in the NP cells. Another major cause of ferroptosis is lipid peroxidation (Yang et al., 2020). In the human body, fatty acids are the major components used for the synthesis of phospholipid bilayers of cell membranes, and those components are also major substrates for energy metabolism (Bersuker et al., 2019). When Fe²⁺ are largely accumulated in the cytoplasm, toxic lipid ROS are produced (Doll et al., 2016), where the polyunsaturated fatty acids (PUFA) of phospholipids in the cell membrane are more likely to be bound by ROS due to its highly expressed active hydroxyl radicals (Kayagaki et al., 2021; Yan et al., 2021). Lipid peroxidation driven by free radicals produces lipid oxygen peroxides (LOOH), a enzyme that can damage the continuity of the lipid bilayer by disrupting the integration of cell membrane, thereby inducing ferroptosis of the NP cells (Fan et al., 2023).

Iron overload is also correlated with aging and degeneration of the chondrocytes (Hou et al., 2016; Jing et al., 2021a). Although abnormal iron ions have played some beneficial roles in increasing the expression of Col10 and Runx2 to promote the formation and mineralization of CEP (Wang W. et al, 2022; Yao et al., 2019). However, excessive iron deposition upregulates matrix metalloproteinases, such as Mmp3 and Mmp13 in CEP cells, which evidently reduces the expression of Sox9 and Col II (Wang W. et al, 2022), thus facilitating the breakdown of cartilage matrix and expediting the deterioration of CEP, and finally contributes to the development of IVDD (Yuan et al., 2019).

Non-physiological mechanical load is one of the important factors known to affect IVDD as well (Belavy et al., 2016; Vergari et al., 2016; Wuertz et al., 2009). While the relationship between mechanics and biology is complex, it has been established that CEP inflammation can lead to microfracture of the cartilage endplate and abnormal mechanical loading (Feng C. et al, 2018; Xiao et al., 2018; Zheng et al., 2019). As reported, The presence of inflammatory mediators such as TNF-α increase the sensitivity of the CEP to mechanical loading, thereby aggravate mechanical stresses applied to the CEP, which contribute to the development of microfractures (Din et al., 2021). Besides, the persistent production of proinflammatory substances can expand the broken endplate, or even extend to the whole NP and AF (Crockett et al., 2017; Kameda et al., 2017). In addition, inflammation increases vascular permeability of the damaged CEP, which can allow low-virulence bacteria to infiltrate into the IVD along the newly formed capillaries, and this is another factor that is associated with the development of IVDD (Hernandez et al., 2020). Moreover, the existence of inflammatory mediators like TNF-α can aggravate the outcomes of mechanical stress exerted on the IVD (Liu et al., 2023; Snuggs et al., 2021; Zhang A. et al, 2023). Under the stimulation of inflammatory factors, abnormally changed osmotic stress elicited by mechanical loading can more fiercely disrupt the F-actin structure and cell volume of the NP cells (Kletsas et al., 2014), even a complete absence of intracellular F-actin in the NP cells can be observed (Kletsas et al., 2014). By in vitro experiments, researchers reveal that those morphological and biophysical characteristics of the NP cells suffered are irreversible, even those cells are cultured in normal conditions after the discontinuation of inflammatory stimulus (Jacobsen et al., 2021; Kletsas et al., 2014).

Interestingly, recent relevant studies reveal that cells isolated from IVD tissue with inflammation show different responses to mechanical stress compared to cells isolated from normal IVD tissue (Mainardi et al., 2022; Tavakoli et al., 2020). It has been reported that normal AF cells can maintain proteoglycan production when mechanical stretch strains of low intensity (1%) and physiological frequency (1 Hz) are applied (Pratsinis et al., 2016). Besides, NP cells can observe an anabolism response under low-to-moderate intensity stretch, and only higher intensities promote a catabolism response (Fearing et al., 2018). However, Inflammatory factors such as TNF-α would alter the sensitivity of IVD to normal mechanical stress, thereby nullifying this beneficial effect under low-to-moderate mechanical stretch (Torre et al., 2018). Increased evidences have shown that stretch and inflammatory signaling are interacted in the degeneration process of AF cells (Linder et al., 2021; Ramanathan et al., 2022; Wang A. G. et al, 2022), because scholars discover that inflammatory signalings can alter the cytoskeletal mechanical transduction of the IVDs in recent studies. In AF cells, the expression of F-actin stress fiber of α-tubulin is found to be enhanced following TNF-α treatment, which may predispose the AF cells susceptible to stress of mechanical strain (Gonçalves et al., 2022). Moreover, mechanical stretches suffered by AF cells can activate inflammatory receptors such as TLR as well (Mohd Isa et al., 2022), while the changes of those receptors are demonstrated to be correlated with cytoskeletal remodeling and biomechanical alterations of the NP cells.

Senescence is also another factor that leads to the development of IVDD. Currently, multiple studies have showed close relationships between senescent NP Cells and IVDD (Zhang K. et al, 2021; Zhang et al., 2024). Short-term exposure to senescence is harmless due to the clearance of the immunological effect, but long-term exposure will lead to uncontrolled chronic inflammation owing to the disrupted homeostasis of the immune system in this condition (Calcinotto et al., 2019; Herranz and Gil, 2018). As researchers reported, chronic inflammation of CEP is recognized as a characteristic feature of senescence (Baechle et al., 2023; Salvioli et al., 2023). In such pathological status, overproduced inflammatory factors such as IL-1β and TNF-α are observed and can subsequently lead to senescence of the NP cells (Capoor et al., 2021; Zhu et al., 2022). Studies have shown that endplate chondrocytes are involved in the regulation of cell proliferation, differentiation, and senescence through the Hippo-Yes-associated (YAP) pathways (Pan et al., 2021; Sladitschek-Martens et al., 2022), and the decrease of YAP1 can further stimulates the senescence of IVD cells during inflammation in the CEP (Kong et al., 2023).

Besides, increased inflammation of the cartilage endplate can result in elevated expression of SA-β-Gal, an age-related protein (Purmessur et al., 2013). TNF-α generated during inflammation also induces downregulated expressions of proteoglycan, Col I, and Col II in the NP cells as well. For instance, Kang et al. found the proportion of aging markers (e.g., P16 and p53) in TNF-α-induced NP cells were increased in an IVDD inflammation model (Li et al., 2019). In addition, senescence can also impact the cell cycle of the NP cells, where senescence induced by TNF-α is more likely to make the NP cells to stay in G0/G1 phase. While S phase is decreased during this process, indicating an occurrence of cell growth arrest. Besides, the proliferation of NP cells is also inhibited after senescence induced by TNF-α (Li et al., 2019). In addition, other pro-inflammatory mediators, such as IL-2, IL-4, IL-8, IFN-γ, and prostaglandin 2, are also responsible for aggravating the aging of the NP cells, which combinatorially lead to the development of IVDD (Huang et al., 2018; Miyagi et al., 2022; Risbud and Shapiro, 2013).

As cells becoming aged, a variety of pro-inflammatory cytokines, chemokines, growth factors, and MMPs can be secreted, which are then named as senescence-associated secretory phenotype (SASP) by scholars (Patil et al., 2019). Accordingly, increased production of MMPs can lead to hydrolysis of Col II and proteoglycan around the NP cells (Bedore et al., 2014; Qin et al., 2022), which disrupts the metabolic balance of the NP ECM, predisposing the human body easier to develop IVDD (Chen et al., 2022; Cherif et al., 2020). SASP can induce senescence of the neighboring cells by autocrine and paracrine mechanisms as well (He X. et al, 2023; Herranz and Gil, 2018). Moreover, the cell division cycle can be prolonged by SASP through an autocrine pathway, which makes the NP cells to more likely be stayed in a static phase (G0 phase) (Ji et al., 2023), resulting in decreased S and M phases (Figure 5) (Georgilis et al., 2018; Hubackova et al., 2015). Noteworthily, the cessation of cell cycle induced by SASP is often irreversible (Acosta et al., 2013). Besides, other cytokines such as IL-6, monocyte chemotactic protein-1, and IGF binding proteins can also make some contributions to the development of NP senescence by paracrine mechanisms (Freund et al., 2010; Jeon et al., 2017).

As reported, miRNAs are also discovered to involve in the development of IVDD induced by CEP inflammation (Chandan et al., 2020). The expressions of miRNAs such as miR-640 and miR-194 are upregulated during IVDD, which changes evidently reduce the chondrogenic differentiation of CESCs and increase the osteogenic differentiation of CESCs (Cazzanelli and Wuertz-Kozak, 2020; Ma et al., 2024). For instance, Dong et al. found that upregulated expression of miR-640 is observed in the inflamed IVD cells, and stimulation of IVD cells with TNF-α and IL-1β can reciprocally upregulate miR-640 expression as well. In addition, miR-640 is found to be involved in NP cell apoptosis, where upregulation of MMP-3 and MMP-9 and downregulation of proteoglycan and Col II of the NP cells can be observed (Dong et al., 2019). Moreover, upregulation of miR-194 of the NP cells induced by IL-6 and TNF-α is revealed to upregulate the expression of CUL4A and CUL4B genes, which are then demonstrated to be positively correlated with the severity of IVDD (Chen et al., 2019). Except for the aforementioned miRNAs, miR-194 and miR-515 are reported to involve in the development of IVDD by degrading chondroitin sulfate synthase CHSY-1/2/3 of the NP cells (Hu et al., 2017).

In recent years, abnormal changes in pain-related neural pathways have aroused great interest in scholars, and those abnormal changes are discovered to be intimately correlated with the development of IVDD (Kim et al., 2020). As we know, substance P (SP) and calcitonin gene-related peptide (CGRP) are important neurotransmitters to regulate pain perception and transmission in the nervous system. Interestingly, positive sensory and sympathetic markers of the nervous systems are detected in degenerated IVD tissues (Sainoh et al., 2014; Song et al., 2020), where increased CGRP expression is detected in painful and degenerative discs (Ahmed et al., 2019). Blocking SP or CGRP production in dorsal root ganglion (DRG) neurons of the IVD can relieve pain symptoms significantly (He et al., 2020). Of note, TNF-α and IL-1β are considered to be the main inflammatory mediators to produce new nerve fibers that lead to IVD pain (Risbud and Shapiro, 2013). When CEP is inflamed, TNF-α and IL-1β infiltrate the annulus fibrosus through the damaged cartilage endplate, which promotes local nerve endings to infiltrate into the nucleus pulposus and induce pain by stimulating nociceptor responses (Sun et al., 2022). Then, those evoked pain sensations transmit to the DRG via generated electronic potential by nociceptors, which will then result in increased secretion and transport of neurotransmitters, such as SP and CGRP, into the corresponding levels of IVD via their corresponding receptors (Seidel et al., 2013; Wise et al., 2020). Those neurotransmitters can affect the neurons in the central nervous system as well, including the ventromedial hypothalamic nucleus (VMH) and paraventricular nucleus (PVN) of the hypothalamus, where excited VMH and PVN can reciprocally increase the activities of sympathetic nervous system and DRG, by secreting corresponding neurotransmitters (Sun et al., 2022) (Figure 6).