Nucleus pulposus cells regulate macrophages in degenerated intervertebral discs via the integrated stress response-mediated CCL2/7-CCR2 signaling pathway

Lower back pain (LBP), which is a primary cause of disability, is largely attributed to intervertebral disc degeneration (IDD). Macrophages (M phi s) in degenerated intervertebral discs (IVDs) form a chronic inflammatory microenvironment, but how M phi s are recruited to degenerative segments and transform into a proinflammatory phenotype remains unclear. We evaluated chemokine expression in degenerated nucleus pulposus cells (NPCs) to clarify the role of NPCs in the establishment of an inflammatory microenvironment in IDD and explored the mechanisms. We found that the production of C-C motif chemokine ligand 2 (CCL2) and C-C motif chemokine ligand 7 (CCL7) was significantly increased in NPCs under inflammatory conditions, and blocking CCL2/7 and their receptor, C-C chemokine receptor type 2(CCR2), inhibited the inductive effects of NPCs on M phi infiltration and proinflammatory polarization. Moreover, activation of the integrated stress response (ISR) was obvious in IDD, and ISR inhibition reduced the production of CCL2/7 in NPCs. Further investigation revealed that activating Transcription Factor 3 (ATF3) responded to ISR activation, and ChIP-qPCR verified the DNA-binding activity of ATF3 on CCL2/7 promoters. In addition, we found that Toll-like receptor 4 (TLR4) inhibition modulated ISR activation, and TLR4 regulated the accumulation of mitochondrial reactive oxygen species (mtROS) and double-stranded RNA (dsRNA). Downregulating the level of mtROS reduced the amount of dsRNA and ISR activation. Deactivating the ISR or blocking CCL2/7 release alleviated inflammation and the progression of IDD in vivo. Moreover, M phi infiltration and IDD were inhibited in CCR2-knockout mice. In conclusion, this study highlights the critical role of TLR4/mtROS/dsRNA axis-mediated ISR activation in the production of CCL2/7 and the progression of IDD, which provides promising therapeutic strategies for discogenic LBP. Low back pain, a main reason for disability, is often due to intervertebral disc degeneration (IDD - the wear and tear of the discs between the vertebrae of the spine). This degeneration, accounting for about 40% of chronic low back pain instances, results in the damage of spinal structure and function. In this research, scientists explored the role of inflammation in IDD, focusing on the role of macrophages. They found that macrophages with different polarized phenotypes (observable traits) existed in degenerative intervertebral disc tissue. The scientists also found that the polarization of these macrophages turned proinflammatory, which intensified the inflammatory cascade reaction (series of biochemical events that lead to inflammation), worsening inflammation and degeneration of the intervertebral disc. This research offers new understanding into the role of inflammation in IDD and potential intervention targets. This summary was initially drafted using artificial intelligence, then revised and fact-checked by the author.