gms | German Medical Science

Objectives: The sympathetic neurotransmitter norepinephrine (NE) was detected in the synovial fluid of trauma and OA patients. Chondroprogenitor cells are present in the synovium and migrate into cartilage defects to contribute to extracellular matrix repair. Though, in OA, progenitors exhibit reduced chondrogenic capacitiy and the reasons are still not completely understood. In earlier studies we investigated NE influence on chondroprogenitor function under normoxic conditions. However, NE effects on chondrogenic capacity were not investigated under hypoxia, reflecting the physiological oxygen concentration in cartilage. Therefore, the aim of this study was to analyze the impact of NE on migration, proliferation, and chondrogenic differentiation of synovial adipose-derived stem cells (ASCs).

Methods: Synovial tissue was obtained from OA patients undergoing total knee replacement surgery. After ASC isolation and expansion, effect of NE or of NE-treated OA chondrocyte supernatants on ACS migration towards PDGF was analyzed in trans-well migration assays. For proliferation experiments, cells in monolayer were treated with different NE concentrations (10^-9 - 10^-6 M) or specific α- and α-adrenergic receptor (AR) agonists and incubated under hypoxia (2% O₂). After 7 days, cell number, viability and adrenergic receptor (AR) profile was analyzed. Effect of NE (10^-9 - 10^-6 M) or specific α- and α-AR agonists on chondrogenic differentiation was investigated in high-density pellet cultures (under hypoxia). After 21 days, AR profile, pellet size, sulfated glycosaminoglycan (sGAG) deposition, type II collagen synthesis and double stranded DNA content was analyzed (histologically and biochemically).

Results and conclusion: NE enhanced migration of ASCs towards PDGF and induced an increased expression of the PDGF receptor CD140b most likely mediating this effect, while conditioned OA chondrocyte supernatant had no chemotactic effect. ASCs expressed α1b, α2a, α2b, α2c and α2AR in both monolayer and pellet cultures. NE or specific receptor agonists had no significant effect on ASC proliferation and viability. Chondrogenic pellets treated with high NE concentrations (10^-7 and 10^-6 M) and with specific α2AR agonist were significantly smaller compared to untreated control cultures, while dsDNA concentration was constant among all treatment groups. NE in high concentrations or the α2AR agonist decreased sGAG content as shown by histology and sGAG assay. Type II collagen synthesis was significantly decreased after treatment with high NE concentrations and with the α2AR agonist, but interestingly also with the α2AR agonist.

Our study confirms the important role of NE in progenitor recruitment, proliferation, and chondrogenic function and provides new insights in cartilage regeneration processes and OA pathophysiology. Future studies of underlying neuroendocrine-chondrogenic signaling pathways might unravel the potential of novel therapeutic options for OA treatment.