Genetic mouse models for the functional analysis of the perifibrillar components collagen IX, COMP and matrilin-3: Implications for growth cartilage differentiation and endochondral ossification.

The mutual interaction of the two supramolecular compartments, the fibrillar and extrafibrillar matrix is a prerequisite for stability and integrity of the cartilage extracellular matrix. The fibrillar periphery, composed of collagen IX, matrilins and cartilage oligomeric matrix protein (COMP) among other components, constitutes the interface which mediates interactions between the two compartments. Mutations in these peripheral macromolecules cause a broad spectrum of skeletal conditions such as pseudo-achondroplasia (PSACH) and multiple epiphyseal dysplasia (MED), which severely affect the organization and integrity of the cartilage growth matrix in humans. Transgenic and knockout mouse models for collagen IX, matrilin-3 and COMP and combinations thereof display cartilage abnormalities and pathologies of varying severity. Absence of collagen IX appears to cause the most severe growth plate phenotype with a profoundly disturbed morphological organization affecting size and shape of the long bones. Notably, similar growth plate phenotypes, including irregularities in the proteoglycan content, hypocellular central regions, disorganized proliferation columns with atypically shaped and oriented chondrocytes and alterations in the hypertrophic zone are observed in transgenic mice lacking other macromolecules or carrying mutations therein. These include collagens II and XI, integrin subunits, integrin linked kinase (ILK), HIF-1alpha, VEGFalpha and the tumor suppressor PTEN. Notably, mutations in ciliar proteins such as Kif3alpha, polaris or Smo/Gli severely affect the ability of chondrocytes to move and to become arranged in columns. Absence or mutational changes of a variety of different, non-related cartilage macromolecules apparently cause similar pathologies and abnormalities of the growth cartilage, suggesting a limited number of underlying molecular mechanisms.