INTRODUCTION: Intervertebral disc (IVD) degeneration (IDD) is a prevalent condition characterized by degenerative and aging changes to the IVD and surrounding tissues. Alterations to the cellular, structural, and mechanical IVD integrity can lead to degenerative spinal pathologies, including disc herniation and spondylolisthesis. Chronic metabolic disorders – such as type 2 diabetes mellitus (T2DM) – have been associated with multiple musculoskeletal disorders including osteoporosis and IDD; however, the effects of T2DM on IDD at molecular level remains underexplored. This study aimed to investigate the biological differences of IDD at a molecular level between T2DM and non-T2DM patients.
METHODS: Quantitative proteomic analysis of proteomes of nucleus pulposus (NP) tissues isolated from lumbar IVD of five T2DM-IDD and four non-T2DM-IDD patients. NP tissues were collected from IVD of five T2DM-IDD and four non-T2DM-IDD patients, and then subjected to tandem mass tag (TMT)-based quantitative proteomic analysis. The identified IVD proteins with differential expression were subjected to gene ontology analysis and network analysis for identification of signaling pathways distinctively enriched in T2DM-IDD and non-T2DM-IDD patients.
RESULTS: We identified 3,887 proteins in the surgically resected NPs, surpassing previous studies. Gene ontology analysis revealed significant enrichment of proteins from the extracellular region and cytoplasm in the IVD proteomes. Among 3,720 quantified proteins, differential expression analysis identified 221 significantly upregulated (>1.5 fold change) and 233 significantly downregulated (<0.67 fold change) proteins in T2DM-IDD compared to non-T2DM-IDD. Upregulated proteins such as mitochondrial proteins (NDUFV2, NDUFB5, NDUFA13, MPC2) indicate increased oxidative stress in the NP tissues from T2DM-IDD. Conversely, downregulated proteins such as Versican (VCAN), an extracellular matrix proteoglycan, IL17B, a pro-inflammatory cytokine, and vascular endothelial growth factor A (VEGFA) indicate alterations in ECM composition and structure that impact the mechanical properties and integrity of the IVD as well as a reduced inflammatory response in NP tissues from non-T2DM-IDD. Other pathways enriched in T2DM-IDD-included oxidative phosphorylation, MYC targets, and fatty acid metabolism, while non-T2DM-IDD showed enrichment in epithelial mesenchymal transition and hypoxia pathways. Additionally, we identified 2,278 proline hydroxylation sites on 230 proteins, with significant enrichment in collagen and collagen-containing extracellular matrix proteins. Protein-protein network analysis highlighted a highly connected network among proline-hydroxylated proteins, particularly involving collagen, which was notably upregulated in NP tissues from T2DM-IDD.
DISCUSSION: Our study identified novel protein pathways that may contribute to the pathogenesis of IDD in the context of T2DM. The broad coverage of IVD proteome using TMT-based quantitation offers a comprehensive pathophysiological search tool for lumbar nucleus pulposus tissue. These findings provide insights into the underlying mechanisms of T2DM-IDD and may have implications for the development of new therapeutic strategies.