Vasoactive Intestinal Peptide Mitigates Intervertebral Disc Degeneration Through FGF18–FGFR2–AKT Pathway

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Background

Intervertebral disc degeneration (IVDD) represents a major underlying cause of chronic low back pain. This condition is characterized by extracellular matrix (ECM) breakdown, elevated activity of matrix-degrading enzymes, and heightened release of inflammatory cytokines. The regenerative ability of disc cells is insufficient to offset these processes, resulting in progressive structural deterioration.

In addition to inflammatory mechanisms, abnormal ingrowth of nerve fibers into the nucleus pulposus (NP) has been increasingly recognized as a driver of disc pathology. Neuropeptides derived from these nerve fibers may directly influence cellular survival and matrix metabolism within NP tissue. Prior studies suggest divergent roles: calcitonin gene-related peptide (CGRP) promotes apoptosis and degeneration, while neuropeptide Y appears to confer protective effects. Vasoactive intestinal peptide (VIP), a neurotransmitter originating predominantly from sympathetic postganglionic fibers, is known for its anti-inflammatory and immunomodulatory properties, though its role in disc degeneration has remained poorly defined.

This investigation sought to examine the expression of VIP receptors in human NP tissue, delineate the functional impact of VIP on disc cells under inflammatory stress, and clarify the molecular mechanisms underlying its protective actions.

Reduced Expression of VIP Receptors in Degenerative Disc Tissue

Nucleus pulposus tissue was stratified according to the Pfirrmann grading system. Advanced degeneration correlated with decreased expression of cartilage-associated genes (ACAN, COL2A1) and elevated expression of degradative enzymes (MMP3, MMP13). Histological staining confirmed loss of glycosaminoglycans with progression of disease.

Immunohistochemistry demonstrated a marked reduction of VIP receptor 1 (VIPR1) and VIP receptor 2 (VIPR2) levels in degenerated discs. In vitro models exposed to interleukin-1β (IL-1β) or oxidative stress (H₂O₂) reproduced this reduction, further confirming the association between degeneration and diminished receptor availability.

VIP Exerts Its Protective Effects Predominantly Through VIPR2

Knockdown experiments revealed that silencing VIPR2—but not VIPR1—exacerbated inflammatory and degenerative gene expression profiles. Transcriptomic analysis showed that VIP restored VIPR2 expression under inflammatory conditions. Western blotting further demonstrated that the protective influence of VIP was abrogated when VIPR2 was silenced, establishing VIPR2 as the primary mediator of its effects.

VIP Attenuates Inflammation-Induced Apoptosis

Flow cytometry and TUNEL assays revealed that IL-1β doubled apoptotic cell populations within NP cultures. VIP treatment substantially reversed this effect, restoring apoptosis rates close to baseline. Immunostaining for cleaved caspase-3 confirmed reduced apoptotic activity following VIP administration. These findings support a strong anti-apoptotic role for VIP in disc cells.

Modulation of ECM and Inflammatory Responses by VIP

VIP reduced IL-1β-induced upregulation of pro-inflammatory mediators, including NOS2, TNF-α, ASC, and IL-1β itself. Expression of iNOS was suppressed by VIP pre-treatment. Furthermore, VIP counteracted IL-1β-mediated increases in MMP3 and preserved aggrecan expression, as validated by both immunofluorescence and Western blot analysis. This demonstrates that VIP maintains ECM homeostasis while dampening inflammatory signaling.

Involvement of PI3K–AKT Signaling in VIP-Mediated Protection

Transcriptomic profiling indicated that the PI3K–AKT pathway is a central regulator of VIP activity in NP cells. VIP increased phosphorylation of AKT, while pharmacologic inhibition of AKT (LY294002) abolished these protective effects. This suggests that AKT signaling is a key downstream mediator of VIP’s actions.

FGF18–FGFR2 Axis Bridges VIP and AKT Activation

Among genes linked to AKT signaling, fibroblast growth factor receptor 2 (FGFR2) emerged as a pivotal mediator. Suppression of FGFR2 resulted in enhanced matrix degradation, apoptosis, and inflammation, while VIP’s protective effects were negated when FGFR2 was inhibited. Fibroblast growth factor 18 (FGF18), a direct ligand of FGFR2, was also upregulated by VIP treatment. Recombinant FGF18 reproduced VIP’s protective effects on ECM stability.

Regulation via miR-15a-5p

Further mechanistic studies identified miR-15a-5p as a regulator of FGF18 expression. IL-1β elevated miR-15a-5p levels, whereas VIP reduced its expression. Reporter assays confirmed direct targeting of FGF18 by miR-15a-5p. These results suggest that VIP sustains FGF18–FGFR2 signaling by suppressing miR-15a-5p.

In Vivo Confirmation of Therapeutic Potential

A lumbar instability mouse model of IVDD was employed to validate these findings. Continuous delivery of VIP over four weeks significantly reduced disc degeneration on MRI, preserved NP tissue architecture, and improved histological scores compared to untreated animals. Expression of aggrecan was partially restored, while MMP3 was reduced. Importantly, phosphorylated FRS2—a downstream mediator of FGFR2—was elevated in VIP-treated discs, confirming pathway activation in vivo.

Discussion

This study provides compelling evidence that VIP exerts significant protective effects on NP cells exposed to inflammatory stress. The mechanism involves activation of the FGF18–FGFR2–FRS2–PI3K–AKT cascade, with miR-15a-5p acting as a regulatory checkpoint.

VIP alleviates apoptosis, suppresses inflammatory mediator release, and preserves ECM integrity. These findings are consistent with prior evidence of VIP’s anti-inflammatory and chondroprotective roles in osteoarthritis and other degenerative joint diseases. Importantly, VIPR2 appears to be the dominant receptor responsible for mediating these effects.

In vivo validation supports the translational relevance of this pathway. Targeting the VIP–FGF18–FGFR2 axis may represent a promising therapeutic strategy for halting or slowing the progression of intervertebral disc degeneration.

Conclusion

VIP protects against intervertebral disc degeneration through suppression of miR-15a-5p, restoration of FGF18–FGFR2 signaling, and subsequent activation of the PI3K–AKT pathway. These mechanisms collectively reduce apoptosis, inflammation, and ECM breakdown, both in vitro and in vivo. This study highlights the potential of VIP-based interventions as novel therapeutic approaches for degenerative disc disease.

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