Zonal differences in meniscus matrix turnover and cytokine response

To determine the mechanisms of meniscal degeneration and whether this varied zonally and from articular cartilage.

Normal ovine menisci were dissected into inner and outer zones and along with cartilage cultured ±IL-1α and TNFα. Glycosaminoglycan (GAG) and collagen release, and gene expression were quantified. Aggrecan proteolysis was analysed by Western blotting with neoepitope-specific antibodies. Matrix metalloproteinase (MMP)2, MMP9 and MMP13 activity was evaluated by gelatin zymography or fluorogenic assay.

Inner meniscus was more cartilaginous containing more GAG and expressing more ACAN and COL2A1 than outer zones. Higher expression of VCAN and ADAMTS4 in medial outer and both zones of the lateral meniscus reflected their embryologic origin from cells outside the cartilage anlagen. All meniscal regions released a greater % GAG in response to cytokines; only outer zones had cytokine-stimulated collagenolysis. Cytokine-induced aggrecanolysis was primarily due to increased ADAMTS cleavage in cartilage and inner menisci but MMPs in the outer menisci. Outer menisci always released more active MMP2 than other tissues and more active MMP13 in basal and TNF-stimulated cultures. Expression of ACAN, COL1A1 and COL2A1 was decreased by both cytokines in all tissues, while VCAN was increased by IL-1α in cartilage and inner menisci. Metalloproteinase expression was differentially regulated by IL-1α and TNFα: ADAMTS4, MMP1, MMP3 were upregulated more by IL-1α in inner zones whereas ADAMTS5, MMP13 and MMP9 were more upregulated by TNFα in outer zones.

Meniscal degeneration mechanisms are zonally-dependent, and may contribute to the enzymatic burden in the joint.