Altered Integration of Matrilin-3 into Cartilage Extracellular Matrix in the Absence of Collagen IX

Abstract

The matrilins are a family of four noncollagenous oligomeric extracellular matrix proteins with a modular structure. Matrilins can act as adapters which bridge different macromolecular networks. We therefore investigated the effect of collagen IX deficiency on matrilin-3 integration into cartilage tissues. Mice harboring a deleted Col9a1 gene lack synthesis of a functional protein and produce cartilage fibrils completely devoid of collagen IX. Newborn collagen IX knockout mice exhibited significantly decreased matrilin-3 and cartilage oligomeric matrix protein (COMP) signals, particularly in the cartilage primordium of vertebral bodies and ribs. In the absence of collagen IX, a substantial amount of matrilin-3 is released into the medium of cultured chondrocytes instead of being integrated into the cell layer as in wild-type and COMP-deficient cells. Gene expression of matrilin-3 is not affected in the absence of collagen IX, but protein extraction from cartilage is greatly facilitated. Matrilin-3 interacts with collagen IX-containing cartilage fibrils, while fibrils from collagen IX knockout mice lack matrilin-3, and COMP-deficient fibrils exhibit an intermediate integration. In summary, the integration of matrilin-3 into cartilage fibrils occurs both by a direct interaction with collagen IX and indirectly with COMP serving as an adapter. Matrilin-3 can be considered as an interface component, capable of interconnecting macromolecular networks and mediating interactions between cartilage fibrils and the extrafibrillar matrix.

ACKNOWLEDGMENTS

We thank Andrea Stadtbäumer and Heidi Bracht for their excellent technical assistance, Reinhard Fässler for supporting the study by providing the collagen IX knockout mice, and Ake Oldberg for his helpful suggestions and for providing the COMP knockout mice.

This study was supported by the Deutsche Forschungsgemeinschaft (grants GR 1301/4-2 to S.G. and WA 1338/3 to R.W. and M.P.), by the SFB 492 “Extracellular Matrix” project A2 to P.B., and by the Köln Fortune program of the Medical Faculty of the University of Cologne. K.B. is a student of the International Graduate School in Genetics and Functional Genomics at the University of Cologne.