When aging becomes degeneration: Evidence from Metabolomics of Healthy Age-stratified organ-donor and degenerated lumbar discs.

Abstract

Aging and degeneration are biologically distinct processes in intervertebral discs, but are difficult to differentiate radiologically. Metabolomics reflects real-time biochemical activity, and age-stratified metabolomic profiling of normal and degenerated discs may identify preclinical degeneration and reveal molecular signatures distinguishing normal aging from degeneration. To characterize the metabolomic changes in the MRI-normal healthy lumbar intervertebral disc and compare them with degenerated discs to identify metabolomic signatures that differentiate normal aging from degeneration. Comparative metabolomics study using human nucleus pulposus tissue from organ donors and surgical specimens, conducted at a tertiary spine care center and an affiliated research laboratory. Nucleus pulposus tissue from 21 healthy organ donors (Pfirrmann grade I) was stratified by age: young (20-30 years), middle-aged (31-50 years), and old (>50 years), and compared with 40 degenerated discs (grades III-V) from surgical specimens. Primary outcome measures included differential metabolite abundance (metabolites with Variable Importance in Projection [VIP] scores>1), pathway enrichment profiles, and identification of age-specific versus degeneration-specific metabolic signatures. Untargeted ultra-high performance liquid chromatography- tandem mass spectrometry (UHPLC-MS/MS) was performed in both positive and negative ionization modes. Metabolites were identified using Compound Discoverer v3.7, with reference to HMDB and KEGG. Statistical and pathway enrichment analyses were performed using MetaboAnalyst 6.0. Untargeted UHPLC-MS/MS analysis revealed 831 significant metabolites (VIP >1), contributing to group separation in the partial least squares discriminant analysis (PLS-DA) model. Lipids and lipid-like molecules, especially sphingolipids, fatty acyls, and steroids, constituted 39%. MRI normal organ donor discs in progressive age periods demonstrated four distinct metabolic trends: (i) progressive decline of antioxidants (ubiquinone, glutathione, N-acetyl seretonin); (ii) increased oxidative/inflammatory markers (4-HNE, prostaglandin E₂ ethanolamide, N1-acetylspermidine); (iii) transient midlife antioxidant elevation; (4OH benozoic acid, 4OH phenylpyruvic acid); (iv) partial recovery in older discs (Hypoxanthine, Paraxanthine, CerP(d18:1/18:0)). In old-aged discs, accumulation of sphingolipids (sphingosine, ceramides) and redox drift indicated enhanced senescence and energy imbalance. Degenerated discs exhibited a different profile, characterized by the suppression of bioactive lipids, particularly resolvins, PGE2, and SOFAs, accompanied by disrupted sphingolipid metabolism and reduced redox capacity. Distinct metabolomic signatures differentiate physiological aging from disc degeneration. Degeneration is characterized by disrupted sphingolipid and redox homeostasis, suggesting a pathobiological process beyond normal senescence. These data provide a metabolic framework for future translational studies to identify disc degeneration even at the preclinical stage and in therapeutic stratification.