Altered collagen structure in mouse tail tendon lacking the alpha 2(I) chain.
Type I collagen is the most prevalent member of the fibril forming family of collagens in higher vertebrates and its heterotrimeric form is comprised of two alpha 1(I) chains and one alpha2(I) polypeptide chain. The functional importance of having two distinct chain types in type I collagen is largely undefined. The existence of a mouse model with a Cola-2 gene mutation (termed oim) that results in non-functional pro alpha 2(I) chains presents a unique opportunity to explore changes in collagen structure resulting from the complete (oim/oim mice) and partial (oim/+ mice) loss of functional alpha 2(I) chains. Tail tendon is a tissue with a well-defined, hierarchical organization of type I collagen. X-ray diffraction studies on oim/oim versus control tendons indicate that the total absence of alpha 2(I) chains results in a decrease in the order of axial packing and a loss of crystalline lateral packing. This suggests that the non-equivalence of three chains is an important determinant of lateral interactions between adjacent molecules and may be involved in the long-range axial order in type I collagen-containing tissues. Both homotrimeric and heterotrimeric type I collagen molecules are found in heterozygous oim mice and these appear to be present in the same co-polymeric fibrils, preventing crystalline lateral packing. In addition to these changes at a fibrillar level, the absence of the alpha 2(I) chain results in an increased enzymatic susceptibility at one site. The oim/oim mice are observed to have reduced body size and smaller tendon bundles, which may be a consequence of these molecular and fibrillar changes in collagen. Furthermore, it is likely that a similar alteration in the molecular packing of collagen in bone fibrils contributes to the osteopenia and decreased bone strength in mice with the oim mutation that are also characteristic of human osteogenesis imperfecta.