The IVD is a fibrocartilaginous structure positioned between adjacent vertebrae, comprising three primary components: i) The nucleus pulposus (NP); ii) the annulus fibrosus (AF); and iii) the cartilaginous endplate (CEP) (11). The gelatinous NP resides at the core of the disc and is primarily composed of water, a dense network of type II collagen and proteoglycans (12). This composition creates a hydrated, gel-like structure that generates substantial osmotic pressure, which is essential for maintaining disc height, distributing mechanical loads and ensuring overall disc integrity (13).

Encapsulating the NP is the organized AF, which consists of 15–25 concentric lamellae of collagen fibers (14). This structure exhibits a morphological gradient: The inner AF is rich in type II collagen, providing elasticity and integration with the NP, whereas the outer AF is composed of robust type I collagen fibers, offering tensile strength and resistance to mechanical stress (15). This anisotropic, multi-lamellar architecture effectively contains the NP and provides structural stability to the disc (16).

The CEP are layers of hyaline cartilage situated superiorly and inferiorly to the NP. In adults, these avascular structures serve as the critical interface for nutrient diffusion and metabolic waste exchange between the vascular-rich vertebral bodies and the avascular disc interior (17).

The functional integrity of the IVD relies on the precise structural and biochemical equilibrium among the NP, AF and CEP. The disruption of this delicate homeostasis is a hallmark of IVDD pathogenesis.

IVDD is influenced by multiple risk factors, including smoking, alcohol consumption, occupational standing height, socioeconomic status, sleep disturbances, hypertension, type II diabetes and obesity. Of note, metabolic dysregulation appears to have a more profound impact on IVDD progression than biomechanical stress (18). The pathological process of IVDD is characterized by extracellular matrix dysregulation, leading to progressive proteoglycan loss and dehydration of the NP (19). In addition, excessive mechanical loading and aging impair cartilage endplate function, further exacerbating disc degeneration (20). Apoptosis and autophagy of nucleus pulposus cells (NPCs) contribute markedly to IVDD pathophysiology, whereas hypoxia and an acidic microenvironment deplete essential nutrients, ultimately resulting in AF fissures and NP herniation (21). Inflammatory cytokines, including tumor necrosis factor-α (TNF-α) and interleukins-1α/β (IL-1α/β), IL-6 and IL-17, play a critical role in promoting disc degeneration by inducing metabolic imbalances within the IVD (2).

In general, the molecular mechanisms affecting IVDD are varied, but the main factors involved are apoptosis, the release of inflammatory factors and the degradation of extracellular matrix (ECM) (22). The depletion of ECM components results from metabolic disturbances, leading to a marked reduction in type II collagen, proteoglycans and water content. Concurrently, the activity of ECM-degrading enzymes, such as matrix metalloproteinases (MMPs), aggrecanases and A disintegrin and MMP with thrombospondin motifs (ADAMTS), is increased, further accelerating ECM degradation (23). Along with ECM degradation, the activation and infiltration of immune cells, including CD4⁺ and CD8⁺ T cells, M1 macrophages, mast cells and neutrophils, as well as pyroptosis and apoptosis of NP cells, contribute to the release of pro-inflammatory cytokines and chemokines, perpetuating a self-reinforcing inflammatory cascade. These mediators, such as TNF-α, IL-1β, IL-6, IL-8 and prostaglandin E2, further accelerate ECM breakdown, thus forming a vicious cycle (24). At the same time, metabolic imbalances in ECM and the production of inflammatory substances promote the aging and death of NP cells (25). As a result, apoptosis, ECM degradation and inflammatory cytokine production are interrelated and interdependent and together lead to the degradation of IVD (Fig. 1).

Due to their closed-loop structure, circRNAs lack free 3′ and 5′ ends, rendering them stable and evolutionarily conserved in eukaryotic cells (28). As a direct target of circRNAs and as a member of ncRNAs, miRNAs function as key post-transcriptional regulators by binding to the 3′-untranslated region of target mRNAs, leading to translation inhibition or transcript degradation (46). Since miRNA and its target RNA have complementary binding sites, and both circRNAs and miRNAs are mainly located in the cytoplasm, circRNAs can regulate the translation rate of target mRNA and affect its stability by pairing with the complementary sequence of miRNA (47).

With the increased understanding of and research on ncRNAs, the intricate interactions between circRNAs and miRNAs, along with their biological implications, have become progressively clearer. The ability of circRNAs to act as miRNA sponges has been implicated in various disease processes, including IVDD. CircRNAs can modulate disc degeneration by sequestering miRNA regulatory factors, thereby suppressing the translation of target mRNAs (48).

For instance, circEYA3 has been identified as a key regulator in IVDD, promoting ECM degradation, inflammatory responses and apoptosis in NP cells. By integrating circRNA/miRNA/mRNA interactions, researchers established a circRNA/miRNA/mRNA regulatory network centered on circEYA3/miR-196a-5p/early B-cell factor 1 (EBF1). Experimental validation demonstrated that miR-196a-5p expression was inversely associated with circEYA3 and EBF1 expression levels (49). Further in vivo studies confirmed that circEYA3 acts as a sponge for miR-196a-5p, indirectly regulating EBF1 expression. In addition, Wang et al (49) demonstrated that EBF1 binds to the promoter region of inhibitor of nuclear factor kappa B kinase β, thereby enhancing its transcription. The encoded protein I-κ-B kinase 1 facilitates the phosphorylation of inhibitor of κBα, disrupting NF-κB pathway homeostasis, leading to the activation of NF-κB target genes and subsequent pathological changes that drive IVDD progression.

Through systematic screening, hsa_circ_0083756 was identified as a key regulator in NP cells (50). Experimental evidence demonstrated its role in promoting NP cell apoptosis and IL-1β production, while modulating ECM synthesis and degradation. Mechanistic studies revealed that circ_0083756 functions as a molecular sponge for miR-558, which in turn regulates triggering receptor expressed on myeloid cells 1 (TREM1) expression. This circ_0083756/miR-558/TREM1 axis was shown to notably influence NP cell proliferation, apoptosis, ECM homeostasis and inflammatory responses. The findings were further validated in animal models, confirming the relevance of the pathway in disc degeneration (50).

Inflammatory cytokines such as IL-1β and TNF-α have been shown to upregulate circ_0005918 expression in a time-dependent manner, contributing to IVDD. By designing a controlled study, research findings established a link between circ_0005918 upregulation and IVDD progression. This circRNA was found to promote NP cell proliferation, ECM degradation and inflammatory cytokine secretion, including IL-1β, IL-6 and TNF-α, through the sponging of miR-622. Compared with the control group, the degree of disc degeneration was associated with miR-622 expression levels but exhibited an inverse relationship with circ_0005918 expression (51).

Further investigation into circ_0134111 identified a binding site for miR-578, which was found to counteract the effects of circ_0134111 on NP cell proliferation and inflammatory factor expression, including IL-6, IL-8, MMP-9 and ADAMTS-5. In addition, miR-578 reversed the circ_0134111-induced downregulation of type II collagen and aggrecan (ACAN). These findings indicated that circ_0134111 contributes to disc degeneration by modulating miR-578 expression; however, whether VEGF, a gene associated with miR-578, plays a role in this pathway remains to be explored (52).

The functional role of circ-FAM169A in IVDD progression was investigated, demonstrating its activity as a competing endogenous RNA (ceRNA) targeting miR-583. Considering that key regulatory genes in IVDD affect ECM composition through SRY-box transcription factor 9 (SOX9) regulation, the study further confirmed SOX9 as a direct target of miR-583, showing an inverse association with IVDD severity. Pathway analysis suggested that miR-583 potentially mediates its effects through epidermal growth factor receptor, phosphatidylinositol 3-kinase-protein kinase B and bone morphogenetic protein pathways, although the specific mechanism of the circ-FAM169A-miR-583 pathway in IVDD requires further investigation (53).

CEP, an integral component of the IVD, facilitates nutrient diffusion, and its degradation accelerates IVDD progression (54). The study on intermittent cyclic mechanical tension revealed its association with cytoskeletal morphology and ECM protein secretion in CEP cells (55). The selection of circ_0022382 as a research target was based on its 4-fold higher expression in experimental groups compared with the controls. Further functional analysis indicated that circRNA_0022382 mitigates pressure-induced degeneration in CEP chondrocytes, as demonstrated through control group comparisons and cell staining assays. Additional findings established an antagonistic relationship between circ_0022382 and miR-4726-5p, confirming the role of circ_0022382 as a miRNA sponge that regulates TGF-β3 expression. Finally, in vivo experiments using a rat model provided evidence that the circRNA_0022382/miR-4726-5p/TGF-β3 axis alleviates disc degeneration by enhancing ECM synthesis in CEP chondrocytes (56).

Xiang et al (8) identified Hsa_circ_0044722 (circCIDN) as a target gene through heat map and microarray analyses, and showed that it had a protective effect on NP cells exposed to stress load by transfecting them. Subsequently, it was verified that circRNA-CIDN directly binds to miR-34a-5p through complementary target sites, suggesting that it may act as a miRNA sponge for miR-34a-5p in NP cells, in which sirtuin (SIRT1) is the target mRNA of miR-34a-5p. Finally, reverse transcription-quantitative PCR (RT-qPCR) was used to verify that circRNA-CIDN overexpression inhibited the expression of miR-34a-5p and weakened the inhibitory effect of miR-34a-5p on target mRNA, thereby inhibiting the compression-induced apoptosis of NP cells and ECM degradation to delay the process of disc degeneration.

The expression of circ-4099 in NP cells is influenced by TNF-α in a dose- and time-dependent manner through the MAPK and NF-κB pathways. Experimental validation indicated that circ-4099 enhances collagen II and ACAN levels, thereby restoring ECM synthesis in NP cells. Concurrently, investigations into miR-616-5p revealed its opposing role in modulating inflammatory gene expression and ECM synthesis, supporting the hypothesis that circ-4099 functions as a competitive endogenous RNA. Further findings demonstrated that circ-4099 exerts its regulatory effects by counteracting miR-616-5p's suppression of SOX9, a pivotal NP cell regulator known to mitigate IVDD by promoting matrix protein expression and reducing inflammatory responses. This highlighted circ-4099 as a promising research avenue and therapeutic target (57).

Investigations into circSNHG5 identified its downregulation in IVDD, with functional studies showing that silencing this circRNA led to decreased chondrocyte proliferation and increased ECM degradation. Fluorescence in situ hybridization and luciferase reporter assays confirmed that circSNHG5 acts as an miR-495-3p sponge, inhibiting its activity. CBP/p300-Interacting Transactivator with Glu/Asp-rich carboxy-terminal domain 2 (CITED2), a downstream target of miR-495-3p, was found to play a role in IVDD attenuation by reducing MMP13 levels while enhancing collagen II and ACAN expression. Ultimately, findings suggested that the circSNHG5/miR-495-3p/CITED2 axis serves as a protective mechanism against CEP degradation (58).

CircGLCE was stably present in the cytoplasm of NP cells, but circGLCE was found by microarray analysis and RT-qPCR to be notably downregulated in IVDD. CircGLCE silencing led to the increased apoptosis and ECM degradation of NP cells, which was verified by immunofluorescence and flow cytometry, suggesting that circGLCE exerts a regulatory effect on NP cells. Subsequently, it was determined that circGLCE served as an miR-587 sponge in NP cells, and signal transducing adaptor family member 1 (STAP1) was the target mRNA of miR-587. Silencing miR-587 could reverse the circGLCE downregulation-induced IVDD, whereas STAP1 could promote the expression of collagen II and ACAN. Finally, animal IVDD models were established to verify that circGLCE alleviates IVDD by targeting miR-587/STAP1 (59).

Emerging evidence underscores the pivotal role of circRNAs as miRNA sponges, regulating target mRNAs and influencing IVDD pathogenesis. These insights offer valuable guidance for exploring the molecular mechanisms of IVDD and developing novel therapeutic strategies.

Through the study of the sponge mechanism, circRNAs have been identified as key regulators in IVDD by acting as ceRNAs. By sequestering specific miRNAs, circRNAs modulate the expression of target genes, thereby influencing apoptosis, inflammation and ECM metabolism in NP cells and chondrocytes. However, existing studies focus on different circRNAs, each associated with distinct signaling pathways and target molecules, highlighting the complexity and heterogeneity of their regulatory roles in IVDD.

Given the pivotal role of circRNAs in IVDD pathogenesis, further research is required to elucidate their molecular mechanisms in greater depth. Expanding investigations into the circRNA-miRNA-mRNA regulatory network, particularly in relation to inflammation and mechanical stress pathways, is crucial. Furthermore, exploring the upstream and downstream regulatory mechanisms of specific circRNAs will provide deeper insights into their functional significance in IVDD.

Due to their stability and tissue specificity, circRNAs hold great promise as non-invasive diagnostic biomarkers for IVDD. Future research should focus on developing targeted therapeutic strategies, such as circRNA inhibitors or miRNA mimics, to modulate their expression effectively. There are currently a large number of ongoing research projects in this field, and significant progress has been made. Furthermore, the application of nanotechnology for the precise and stable delivery of RNA molecules presents an innovative avenue to enhancing therapeutic efficacy.

Collectively, the molecular sponge mechanism remains the most extensively studied and characterized function of circRNAs. Current evidence demonstrates that circRNA-mediated miRNA sponging exerts regulatory effects on multiple pathological processes in IVDD, including ECM metabolism, inflammatory factor expression and NP cell apoptosis, with analogous regulatory functions observed in the cartilage endplate. Nevertheless, the majority of circRNAs remain uncharacterized, and their potential involvement in IVDD pathogenesis, particularly through sponge-mediated mechanisms, requires systematic investigation. These unresolved questions represent critical directions for future research in this field.

Exosomes are extracellular vesicles with diameters ranging from 40–160 nm. They encapsulate various cellular components, including DNA, RNA, metabolic byproducts and proteins, and facilitate intercellular communication through diverse mechanisms, thereby triggering complex biological responses (60). Studies have demonstrated that circRNAs exhibit exceptional stability and are abundantly expressed in exosomes. As a novel class of genetic information molecules, exosomal circRNAs have gained considerable research attention due to their intrinsic stability, high abundance and widespread distribution. Their unique characteristics position them as promising candidates for further investigation in intercellular communication and disease biomarker discovery (61,62).

Extensive research has demonstrated that exosomes play a pivotal role in disease pathogenesis and offer novel therapeutic avenues. For instance, exosomal miRNAs are implicated in oxidative stress and immune dysregulation, serving as potential biomarkers for assessing vitiligo activity (63). In addition, exosomes contribute to the progression of cancer and cardiovascular diseases (64). In the musculoskeletal system, they have also been demonstrated to be involved in IVDD.

Chen et al (65) isolated and cultured primary human NP cells from degenerated IVDs. RT-qPCR analysis revealed a marked downregulation of circ_0036763 and U2 small nuclear RNA auxiliary factor 2 (U2AF2), whereas miR-583 was upregulated. Transfection experiments and western blotting confirmed that circ_0036763 promotes the expression of ACAN. The study further identified the U2AF2/circ_0036763/miR-583/ACAN axis as a key regulatory pathway in IVDD, wherein U2AF2 facilitates circ_0036763 maturation, and circ_0036763 sponges miR-583 to enhance ACAN expression. In addition, exosomal U2AF2 derived from bone marrow mesenchymal stem cells was shown to mitigate IVDD, highlighting the therapeutic potential of exosome-mediated RNA delivery.

Investigations have demonstrated notable findings through comparative analysis of exosomes derived from normal and degenerative disc cells, employing electron microscopy and western blotting for morphological characterization and molecular marker identification. Through comprehensive circRNA microarray profiling and subsequent PCR validation, circRNA_0000253 was identified as markedly upregulated in degenerative disc cells, and computational bioinformatics analysis established miR-141-5p as a putative target of circRNA_0000253. Mechanistic studies revealed that circRNA_0000253 regulates cellular proliferation and apoptotic processes through the modulation of SIRT1 and ECM-associated components, including collagen II and ACAN. In vivo validation using a rat IVDD model, incorporating MRI, X-ray imaging and histological staining techniques, provided compelling evidence that circRNA_0000253 promotes disc degeneration through its miR-141-5p sponge function and subsequent downregulation of SIRT1 (66).

These findings suggest that the exosome-mediated delivery of specific circRNAs holds promise as a targeted therapy for IVDD. However, challenges remain in translating laboratory findings into clinical applications. Of note, studies lack in-depth analysis of the stability and delivery efficiency of exosomes in vivo. In addition, the limited sample sizes may affect the generalizability of their conclusions, underscoring the need for further research (Table I).

As discussed earlier, circRNAs regulate DD primarily by acting as ceRNAs to sponge miRNAs; however, studies on the direct interaction between circRNAs and proteins in IVDD remain limited. This section provides an overview of the mechanisms through which circRNAs interact with proteins and their potential implications in IVDD.

CircRNAs can function in multiple ways: As protein sponges, altering the physiological functions of proteins; as protein scaffolds, facilitating interactions between proteins; or as molecular recruiters, directing proteins to specific cellular locations (67). These interactions play a critical role in regulating cellular processes such as proliferation, apoptosis, angiogenesis, mRNA translation, energy metabolism and differentiation (68). Of note, RBPs, which recognize and bind RNA through specific structural motifs, form intricate regulatory networks with circRNAs (69). RBPs orchestrate various aspects of circRNA biology, including biogenesis, degradation, nucleocytoplasmic transport and translational regulation, thereby fine-tuning circRNA function (70). In the context of IVDD, emerging evidence underscores the significance of circRNA-protein interactions in disease pathogenesis, particularly through their modulation of specific cellular processes.

CircFUNDC1 ameliorates IVDD by promoting PTEN-induced kinase 1 (PINK1)-dependent mitophagy in NPCs under oxidative stress. It executes this function not as a miRNA sponge, but by directly interacting with and stabilizing the cyclin dependent kinase (CDK)9 protein. This interaction enhances the phosphorylation of RNA polymerase II, thereby facilitating the transcription of the key mitophagy gene PINK1. This mechanism establishes a novel paradigm wherein a circRNA directly regulates a kinase to control transcriptional activation in disc homeostasis (71).

The reduced m6A methylation and elevated expression of circGPATCH2L in degenerated human IVD tissues have garnered notable attention from researchers. Overexpression and knockdown experiments revealed that circGPATCH2L promotes the degeneration of NPCs by inducing apoptosis. Furthermore, circGPATCH2L interacts with tripartite motif containing 28 to inhibit its phosphorylation, resulting in P53 accumulation and DNA damage. Furthermore, it was found that circGPATCH2L is recognized and degraded by the YTH N6-methyladenosine RNA binding protein F2 (YTHDF2)-ribosomal protein L10 (RPL10)-ribonuclease P/RNases P/mitochondrial RNA processing ribonuclease complex, which helps maintain homeostasis in normal NPCs. Finally, a mouse model study confirmed that the knockdown of circGPATCH2L can alleviate IVDD. However, the small sample size of patients in the study (4 patients in the degenerative group and 4 in the control group) may affect its statistical power. The dynamic regulation of m6A modification has not been deeply discussed in this study. The potential interactions of circGPATCH2L with other DNA damage-related proteins have also not been fully analyzed (9).

Another study identified circATXN1 as notably upregulated in NPCs from aged individuals (>70 years old) of degenerated intervertebral discs. To investigate its role, researchers constructed a circATXN1 overexpression vector and confirmed its activation of aging markers CDK inhibitor 2A, specifically the P16 isoform and P21. Further analysis revealed that circATXN1 downregulated collagen type II α1 chain and SOX9, impacting ECM homeostasis and mitochondrial function. Using RNA immunoprecipitation (RIP), RNA affinity purification and mass spectrometry, heterogeneous nuclear ribonucleoprotein A2/B1 (HNRNPA2B1) was identified as a direct binding partner of circATXN1. HNRNPA2B1 facilitates the biogenesis of circATXN1 by regulating the splicing of its precursor transcript, while concurrently reducing ATXN1 mRNA levels. In a therapeutic approach, researchers developed a small interfering RNA-embedded DNA tetrahedral nanogel to efficiently silence circATXN1, leading to improvements in tissue structure and ECM content in an aged mouse IVDD model. However, while the interaction between circATXN1 and HNRNPA2B1 has been characterized, the precise mechanism through which it regulates protein mislocalization remains unclear and warrants further investigation (72).

In the musculoskeletal system, Chen et al (73) revealed that the overexpression of circTMEFF1 aggravated muscle atrophy, while the inhibition of its expression alleviated the atrophy. circTMEFF1 mainly promotes muscular atrophy through two mechanisms; the first is by binding to TAR DNA-binding protein 43 protein, inducing mitochondrial DNA release and activating cyclic GMP-AMP synthase-stimulator of interferon gene signaling pathway, thus promoting atrophy. The other mechanism is by encoding a new protein, TMEFF1-339aa, which has a pro-atrophy effect. It was revealed that this works through a dual mechanism: Binding proteins and translation proteins. In addition, circ-Foxo3 blocks the cell cycle by binding to CDK2 and cyclin dependent kinase inhibitor 1 to form a ternary complex (74). In oncology research, circZKSCAN1 has been found to suppress cancer stemness by acting as an RBP sponge, modulating the function of fragile X mental retardation protein. The characterization of circZKSCAN1 provided a new perspective on the role of circRNAs in tumor biology (75).

Although the above studies have indicated that circRNAs interact with proteins to influence IVDD, the precise molecular mechanisms remain largely unexplored. This knowledge gap may be attributed to the complexity of protein-circRNA interactions, the technical challenges of in vitro and in vivo studies, and the early stage of research in this area. In addition, the experimental validation of circRNA-protein interactions is hindered by notable technical challenges and high costs. Furthermore, the limited availability of specialized databases and analytical tools for circRNA-protein interactions poses a substantial barrier to advancing research in this field. These objective factors collectively impede the progress of related experimental studies. Most of the existing research on circRNA-protein interactions focuses on cancer, while their role in musculoskeletal disorders, including IVDD, requires further investigation.

To date, there has been no direct evidence linking the protein-coding function of circRNAs to IVDD. However, the emerging mechanisms through which circRNAs encode proteins are noteworthy and may represent a promising new research direction in understanding the pathophysiology of disc degeneration.

Initially regarded as mere splicing byproducts, circRNAs have been increasingly recognized for their diverse physiological functions (29); among them, their capacity to encode proteins has garnered marked attention (76). CircRNAs contain ≥1 open reading frames (ORFs), which, due to their circular structure, can facilitate the translation of short peptides or even proteins >100 amino acids in length (77).

This translational mechanism has been explored in the musculoskeletal system. For example, Yin et al (78) identified an ORF in circFAM188B and demonstrated its ability to encode a novel protein, circFam188B-103AA, which regulates the proliferation and differentiation of chicken skeletal muscle satellite cells. Legnini et al (79) discovered that circ-ZNF609, containing an ORF, plays a role in myoblast proliferation and protein translation. Despite these advances, the molecular activities of such proteins and their interactions with linear mRNA counterparts remain poorly understood. Similarly, Zhu et al (80) identified a circRNA, circDdb1, that promotes muscle atrophy; their pivotal finding was that circDdb1 is translated into a novel protein, circDdb1-867aa. This protein executes the function by entering the nucleus to inhibit Pax7, a master transcription factor for muscle regeneration. This work crucially establishes a paradigm where a coding circRNA directly disrupts myogenic transcription.

While current research on circRNA-encoded proteins primarily focuses on cancer and tumor biology, this field is still in its infancy (30,81,82). Numerous circRNAs with protein-coding potential remain undiscovered, and the underlying translation mechanisms require further exploration (83). To date, no studies have directly implicated circRNA-mediated protein translation in IVDD; this gap may be attributed to the rarity of circRNAs with functional ORFs, the small molecular weight of the encoded proteins and the technical challenges associated with their detection. Nevertheless, given the established role of circRNAs in the musculoskeletal system, it is plausible that they may also influence disc degeneration, although this hypothesis awaits experimental validation.