Molecular traces of microbial cross-kingdom migration: from the gut ecosystem to the intervertebral disc...

Low back pain is a leading cause of disability worldwide, and lumbar intervertebral disc degeneration (IVDD) is strongly associated with its development. Recent studies have shown that the gut microbiota (GM) and its metabolites may be involved in the occurrence and development of IVDD through the gut-disc axis. However, the key microbes mediating this process and their specific molecular mechanisms remain unclear.

This study aimed to identify the gut microbes that play a key role in the progression of IVDD using multiomics approaches and clarify the specific mechanisms by which these microbes participate in IVDD by regulating host cell functions.

A single center, prospective cross-sectional study.

We prospectively included 113 patients who underwent surgical treatment for symptomatic lumbar degenerative diseases from May 2022 to May 2023, and their degenerated lumbar intervertebral disc (IVD) tissues as well as paired feces samples were collected.

Metagenomic next-generation sequencing (mNGS), modified Pfirrmann typing, Single-cell RNA sequencing (scRNA-seq), Bulk RNA sequencing (Bulk RNA-seq).

Clinical IVD samples and paired fecal samples were prospectively collected and subjected to multiomics bioinformatics analysis. mNGS was used to analyze the microbial composition in IVD and paired fecal samples. scRNA-seq was employed to resolve the cellular heterogeneity of IVD tissues. Bulk RNA-seq was utilized to identify the characteristics of host response genes related to microbial exposure. Subsequent AUCell scoring was performed to evaluate the abundance of microbes in cell subsets. The CellChat algorithm was applied to analyze the microbe-mediated intercellular communication network of host cells.

The raw detection rate of mNGS in IVD tissues was 100%, with a positive rate of 60.2% (68/113) after excluding background bacteria. A total of 505 genera and 1,528 microbial species were detected, with dominant species including Stutzerimonas stutzeri and Moraxella osloensis. The mNGS detection rate in fecal samples was 100% (322 genera and 789 species), among which Phocaeicola vulgatus (PV) was a dominant species. A total of 7 bacterial species shared by GM and IVD were identified; however, only the relative abundances of PV and Bacteroides thetaiotaomicron (BT) increased gradually with the severity of IVDD. Single-cell RNA-seq identified 10 cell clusters, annotated as chondrocytes, macrophages, fibroblasts, and endothelial cells, with the proportions of the latter 3 nonchondrocyte populations being significantly higher in the severe IVDD group. Chondrocytes were further divided into subsets. Subsets MDC1 and MDC5 were related to mild degeneration with high expression of ACAN and SOX9, whereas SDC2, SDC3, SDC4, SDC6, and SDC7 were related to severe degeneration. AUCell scoring revealed that PV showed a significantly higher abundance in these pathological subsets, while BT was evenly distributed. Furthermore, chondrocytes with high PV abundance significantly upregulated matrix degradation genes including MMP13 and COL1A1, as well as cell adhesion genes such as POSTN and SPARC. These upregulated genes were significantly enriched in LPS-associated inflammatory cascades, extracellular matrix degradation, and metabolic reprogramming pathways. Crucially, LPS signaling genes including TLR4, MYD88, NFKB1, and RELA were upregulated in chondrocytes with high PV abundance, while short-chain fatty acid receptor genes were minimally expressed with no significant group differences. Finally, CellChat analysis revealed that high PV abundance amplified the communication between chondrocytes and macrophages, fibroblasts, and endothelial cells, which was mediated by the CXCL pathway for immune recruitment, the VEGF and ANGPT pathways for angiogenesis, and the TGF-β pathway for profibrotic remodeling.

This study suggests that gut-derived PV may activate the inflammatory response of chondrocytes through the LPS-mediated TLR4-MYD88 signaling axis and reshape the intercellular communication network, thereby potentially contributing to the process of IVDD. These findings provide novel mechanistic insights into the gut-disc axis theory and offer new perspectives on IVDD therapeutic strategies targeting microbe-host interactions.