Modic changes are associated with activation of intense inflammatory and host defense response pathways -- molecular insights from proteomic analysis of human intervertebral discs

Abstract

<h3>BACKGROUND CONTEXT</h3> Patients with Modic changes form a distinct clinical subset with reports of higher intensity of pain, poor clinical and surgical outcomes and higher incidence of recurrence. MC also is an independent risk factor for increased postoperative surgical site infection. <h3>PURPOSE</h3> This study aimed to investigate the biological changes at molecular level, in discs with Modic changes. <h3>STUDY DESIGN</h3> Experimental analysis. <h3>MATERIALS AND METHODS</h3> Nucleus pulposus (NP) from 24 patients undergoing microdiscectomy for disc herniation [14 discs with Modic changes and 10 without Modic changes] were procured. The overall expression of proteins, biological processes, protein-protein and metabolite interactions were analyzed and compared. Host defense response proteins (HDRPs) and immunological pathways activated in patients with Modic changes were documented and analyzed. <h3>RESULTS</h3> Label-free proteomic approach with stringent filters revealed a total of 208 proteins in Modic changes and 193 in non-Modic change groups. Forty-five proteins were specific to Modic changes; 30 to non-Modic changes and 163 common to both. Downregulated proteins in Modic changes belonged to components of extracellular matrix such as collagens (COL- 6A1, 6A2, 6A3, 11A1, 12A1, and 20A1), and proteoglycans (versican [VCAN], and biglycan [BGN]). Inflammatory molecules [plasminogen (PLG), angiogenin (ANG), fibroblast growth factor-binding protein 2 (FGFBP2), tetranectin (CLEC3B), cartilage acidic protein 1(CRTAC1), kininogen (KNG1), chitinase-3-like protein 2 (CHI3L2), and ferritin (FTL) were expressed only in the Modic changes group. The significantly altered pathways in Modic changes included Fc Fragment of IgG Receptor IIIa (FCGR3A)-mediated phagocytosis, regulation of Toll-like receptors (TLR) by endogenous ligand, neutrophil and platelet degranulation. 50 HDRPs were identified in the study, 14 of which were specific to Modic changes and included acute phase reactants, antimicrobial peptides, complement cascade proteins, inflammatory molecule and stress response proteins. Metabolite-protein interaction analysis revealed a significant interaction between 19 proteins, specifically involving ubiquitin mediating proteasome degradative pathway and an association with the metabolite-glutamic acid in the Modic changes group. Accumulation of glutamic acid in Modic changes discs was confirmed by quantitative amino acid analysis using High-performance liquid chromatography. <h3>CONLCUSION</h3> Our study confirms that Modic changes represent an intense inflammatory status due to activation of host defense response and immunological pathways. Downstream effects leading to ubiquitin mediated proteasomal degradation of ECM proteins and the resulting metabolites such as glutamic acid could cause excessive pain and needs further investigation. <i>Clinical Significance:</i> We have documented the expression of inflammatory molecules, immune mechanisms and host defense response proteins which throw molecular insights into the pathological mechanisms of Modic changes. Further, ubiquitin mediated proteasomal degradation and accumulation of glutamate in discs with Modic changes might serve as targets for molecular therapy. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.