The α1(VIII) and α2(VIII) collagen chains form two distinct homotrimeric proteins in vivo

The collagen superfamily of proteins now contains at least 19 proteins formally defined as collagens and an additional ten proteins that have collagen-like domains. The most abundant collagens form extracellular fibrils or network-like structures, but the others fulfill a variety of biological functions. Some of the eight highly specific post-translational enzymes involved in collagen biosynthesis have recently been cloned. Over 400 mutations in 6 different collagens cause a variety of human diseases that include osteogenesis imperfecta, chondrodysplasias, some forms of osteoporosis, some forms of osteoarthritis, and the renal disease known as the Alport syndrome. Many of the disease phenotypes have been produced in transgenic mice with mutated collagen genes. There has been increasing interest in the possibility that the unique post-translational enzymes involved in collagen biosynthesis offer attractive targets for specifically inhibiting excessive fibrotic reactions in a number of diseases. A number of experiments suggest it may be possible to inhibit collagen synthesis with oligo-nucleotides or antisense genes.

The distribution of collagen XI in fibril fragments from 17-d chick embryo sternal cartilage was determined by immunoelectron microscopy using specific polyclonal antibodies. The protein was distributed throughout the fibril fragments but was antigenically masked due to the tight packing of collagen molecules and could be identified only at sites where the fibril structure was partially disrupted. Collagens II and IX were also distributed uniformly along fibrils but, in contrast to collagen XI, were accessible to the antibodies in intact fibrils. Therefore, cartilage fibrils are heterotypically assembled from collagens II, IX, and XI. This implies that collagen XI is an integral component of the cartilage fibrillar network and homogeneously distributed throughout the tissue. This was confirmed by immunofluorescence.

It has recently become apparent that collagen fibrils may be composed of more than one kind of macromolecule. To explore this possibility, we developed a procedure to purify fibril fragments from 17-d embryonic chicken sternal cartilage. The fibril population obtained shows, after negative staining, a uniformity in the banding pattern and diameter similar to the fibrils in situ. Pepsin digestion of this fibril preparation releases collagen types II, IX, and XI in the proportion of 8:1:1. Rotary shadowing of the fibrils reveals a d-periodic distribution of 35-40-nm long projections, each capped with a globular domain, which resemble in form and dimensions the aminoterminal globular and collagenous domains, NC4 and COL3, of type IX collagen. The monoclonal antibody (4D6) specific for an epitope close to the amino terminal of the COL3 domain of type IX collagen bound to these projections, thus confirming their identity. Type IX collagen is therefore distributed in a regular d-periodic arrangement along cartilage fibrils, with the chondroitin sulfate chain of type IX collagen in intimate contact with the fibril.