Multifunctional annulus fibrosus matrix prevents disc-related pain via inhibiting neuroinflammation and sensitization

Abstract

Chronic low back pain mainly attributed to intervertebral disc (IVD) degeneration. Endogenous damage-associated molecular patterns (DAMPs) in the injured IVD, particularly mitochondria-derived nucleic acid molecules (CpG DNA), play a primary role in the inflammatory responses in macrophages. M1-type macrophages form a chronic inflammatory microenvironment by releasing pro-inflammatory factors and nerve growth factor (NGF) that induce nerve growth into the inner annulus fibrosus, resulting in persistent hyperalgesia. We fabricated an amphiphilic polycarbonate that naturally forms cationic nanoparticles (cNP) in aqueous solutions, with the hydrophobic core loaded with TrkA-IN-1, an antagonist against the NGF receptor (TrkA). The drug delivery nanoparticles were denoted as TI-cNP. TrkA-IN-1 and TI-cNP were added to the decellularized annulus fibrosus matrix (DAF) hydrogel to form hybrid hydrogels, denoted as TI-DAF and TI-cNP-DAF, respectively. As a result, TrkA-IN-1 showed a delayed release profile both in TI-DAF and TI-cNP-DAF. Each mole of cNP could bind approximately 3 mol of CpG DNA to inhibit inflammation. cNP-DAF and TI-cNP-DAF significantly inhibited the M1 phenotype induced by CpG DNA. TI-DAF and TI-cNP-DAF reduced neurite branching and axon length, and inhibited the expression of neurogenic mediators (CGRP and substance P) in the presence of NGF. Besides, TI-cNP-DAF relieved mechanical hyperalgesia, reduced CGRP and substance P expression in the dorsal root ganglion, and downregulated GFAP and c-FOS signaling in the spinal cord in the rat disc herniation model. Summarily, TI-cNP-DAF, a novel composite IVD hydrogel, efficiently mediated the inflammatory environment, inhibited nerve ingrowth and sensitization, and could be clinically applied for treating discogenic pain. STATEMENT OF SIGNIFICANCE: Discogenic lower back pain, related to intervertebral disc degeneration (IDD), imposes a tremendous health and economic burden globally. M1-type macrophages release pro-inflammatory factors and nerve growth factor (NGF) that induce nerve growth into the inner annulus fibrosus, resulting in persistent hyperalgesia and discogenic pain. Reconstructing matrix integrity and modulating the inflammatory microenvironment are promising strategies for preventing the ingrowth and activation of neurites. The TI-cNP-DAF hydrogel recovers tissue integrity, alleviates inflammation, and delivers the TrkA antagonist to inhibit the activity of NGF, thus restraining hyperinnervation and nociceptive input. Due to its simple production process, injectability, and acellular strategy, the hydrogel is operable and holds great potential for treating discogenic lower back pain.

Keywords: Drug delivery system; Intervertebral disc degeneration; Low back pain; NGF; Neuroinflammation.