Abstract

We have compared knockin mice with aggrecan and type II collagen resistant to aggrecanases, collagenases, or both families of enzymes, to identify the most important proteolytic driver for cartilage erosion. Specifically, the cleavage site in the aggrecan interglobular domain (IGD) of Jaffa is changed from E^373–374ALG to E^373–374NVY and the collagenase cleavage site in the α1(II) chain of Bailey is changed from PQG^775–776LAG to PPG^775–776MPG. The Jaffa mutation does not influence aggrecanase cleavage at sites in the chondroitin sulphate-rich region of aggrecan. In contrast, blocking cleavage at the primary collagenase cleavage site prevents secondary and subsequent cleavage events in Bailey type II collagen. Juniper mice are resistant to both aggrecanases and collagenases

Aggrecan loss from explants treated with IL-1α was significantly reduced in all mutant genotypes compared with Wildtype. The hierarchy of aggrecan loss was Wildtype > Jaffa > Bailey > Juniper samples. Aggrecan loss from explants treated with retinoic acid was also significantly reduced in all mutant genotypes compared with wildtype. However, with retinoic acid treatment, the hierarchy of aggrecan loss was Wildtype > Bailey > Jaffa > Juniper samples. We measured aggrecan loss and cartilage erosion 8-weeks after DMM surgery and found that both aggrecan loss and tibial cartilage erosion were significantly reduced in Juniper compared with Wildtype. Juniper was better protected against cartilage erosion than either Jaffa or Bailey

In conclusion, mutations that partially block collagenolysis (heterozygous Bailey) and aggrecanolysis (homozygous Jaffa for the IGD site only) protect against cartilage erosion in experimental OA. Furthermore, protecting collagen II against collagenase attack is equal to, or better than, protecting aggrecan against IGD aggrecanase cleavage, in terms of cartilage erosion and aggrecan loss respectively.