Characterization of senescent intervertebral disc cells and their role in perturbation of matrix homeostasis\ud

Aging is the largest single risk factor for intervertebral disc degeneration (IDD). Dysregulated disc cells are thought to drive age-associated disc proteoglycan (PG) loss, a hallmark of IDD, through a combination of reduced capacity to synthesize matrix PG and increased production of proteolytic enzymes to breakdown matrix. The link between aged degenerative discs and cellular senescence has been previously observed in human discs, but it remains unknown if senescent disc cells are phenotypically different from their non-senescent counterpart in terms of matrix homeostasis. Hence, the goal of this study is to explore matrix homeostasis characteristics of senescent disc cells. \ud Outcome measures for anabolism have established that stress-induced senescence of human disc cells resulted in decreased PG synthesis, increased collagen type II expression in nucleus pulposus (hNP), and decreased collagen type I expression in annulus fibrosis (hAF). \ud For catabolism, Western analysis revealed greater levels of ADAMTS- and MMP-generated proteolytic aggrecan fragments as a result of cleavage in the aggrecan interglobular domain (IGD) in the conditioned media (CM) of H2O2–induced senescent hNP cells compared to control. In contrast, the levels of aggrecan IGD proteolytic fragments were relatively unchanged in the CM of senescent hAF cell culture compared to non-senescent hAF cell culture. ELISA and antibody array experiment showed elevated levels of many pro-inflammatory cytokines (IL-6, IL-8, PDGF-BB, GCSF), chemokines (EOTAXIN-2, IP-10, RANTES) and MMPs (MMP-1, MMP-3, MMP-10) in the CM of senescent hNP cells. These are key factors previously reported for senescence-associated secretory phenotype (SASP) of stress-induced senescent cells which impart profound catabolic effects on neighboring cells and the extracellular matrix. The total GAG content moderately decreased in senescent hAF and significantly decreased in senescent hNP cell cultures compared to non-senescent cell culture control. \ud These in vitro findings suggest that senescent disc cells perturb extracellular matrix homeostasis via acquisition of SASP, reduced matrix synthesis capacity, and increased matrix degradation. Identifying and confirming cellular senescence as a driver of disc PG loss and IDD will offer novel opportunities for targeted therapy to prevent or treat IDD, which would have a tremendous public health impact in preventing or ameliorating chronic low back pain