The main protein in bone and other connective tissues is collagen. There are more than 20 genetically different types of collagen. The most common is type I collagen and this is found in bone, skin, ligaments and tendon and many other skeletal tissues. The collagen molecule is a triple-helix, about 300 nm long and 1.5 nm diameter. In type I collagen the triple-helix comprises three so-called alpha chains, two α1 chains and one α2 chain.
Molecules assemble into fibres or fibrils with a relatively disordered side-to-side packing of molecules but a regular axial structure. Molecules are staggered length-wise such that there is a characteristic axial repeat every 67 nm. In some tissues these fibres can grow up to nearly 1 mm wide and of unknown length, although commonly they are 10-100 um across. In conjunction with other matrix components they form all the body’s supporting tissues. Like ropes, collagen fibres are strong in tension but cannot sustain compression or bending. These tissues can be considered as fibre-composite materials in which the collagen fibres provide reinforcing to a weak gel-like matrix. Cross-links between the molecules are essential for the tensile strength of collagen fibres. Initial formation of reducible cross-links, largely based on lysine, is followed by maturation of non-reducible pyridinium and pyrrolic cross-links. In bone, the addition of a mineral phase, a form of hydroxyapatite, further stiffens the tissue. Because these tissues are fibre composites it is no longer possible to attribute tensile or compressive properties to a specific component, it is the combination and interactions between them that determine the properties.
Collagen and mineralization
The correct formation of collagen fibres is essential for generating the required mechanical properties. Defects in collagen packing or in cross-links can lead to tissue weakness, e.g. lathyrism due to deficient cross-links, or bone fragility disorders such as osteogenesis imperfect. In Aberdeen, a G to T polymorphism in the Sp1 promoter region was identified which was related to a reduced bone mineral density (Grant et al., 1996). Further work showed that cells from patients who were heterozygous for this polymorphism over-expressed the α1 chain so that instead of the ratio of α1: α2 chains being synthesized being 2:1, it was found to be 2.36 (Mann et al., 2001). We proposed that this would form a small fraction of homotrimeric collagen molecules, instead of the correct heterotrimer, and that this could be a source of increased bone fragility. Individuals with an inactivating mutation in the gene for the α2 chain have severe osteogenesis imperfecta and a similar phenotype has been found in the oim/oim mouse, which has no α2 chains. Mice which are heterozygous for the oim mutation have collagen fibrils comprising a mixture of homo- and heterotrimers and they have an osteoporotic phenotype. The polymorphism was found to result in reduced mineralization of bone nodules in vitro, supporting this hypothesis (Stewart et al., 2005). We have however, failed to find any intact homotrimer in tissue extracts from heterozygous individuals even though dissociated, monomeric collagen a chains are found in the expected ratios using SDS-PAGE. Other studies have also found altered ratios of monomers in osteoporotic and osteoarthritic patients with the same Sp1 polymorphism but, it needs to be noted, most have not looked for, and none has found, intact homotrimer. The reason for this is not yet clear.
Collagen packing in bone
Mineralization affects the packing of collagen molecules in fibrils. It decreases the side-to-side spacing of the molecules in a similar way to dehydration (Figure 1). The best way of measuring this in bone is to use neutron diffraction because the presence of mineral, which strongly scatters x-rays, makes x-ray diffraction very difficult. Although neutron diffraction has been performed on samples of cortical bone, we have shown that suitable diffraction patterns can be obtained from fully hydrated, cancellous bone (Skakle and Aspden, 2002). Using a beamline at Institut Laue-Langevin in Grenoble we did a pilot study and recorded patterns from a small number of samples of femoral head bone obtained from patients with either osteoarthritis or osteoporosis. Sample numbers were too small and a more detailed study is required, the results suggested that the molecular spacing in OA bone was slightly greater than in OP, as might be expected from the mineralization (Figure 2). However, there was a lot more scatter in the data from the OP bone, which needs also needs further investigation.
Figure 1: Collagen molecules in wet tendon or ligament exhibit a liquid-like disorder and the mean lateral spacing is about 1.4 nm. This reduces to about 1.1 nm on drying. In bone the mean spacing is about 1.2 nm.
Figure 2: The mean side-to-side spacing of collagen molecules in cancellous bone from the femoral head may be slightly greater in OA (1.253 ± 0.009 nm) bone than OP (1.233 ± 0.025 nm).
- Grant, S. F. A., Reid, D. M., Blake, G., Herd, R., Fogelman, I., & Ralston, S. H. (1996). Reduced bone density and osteoporosis associated with a polymorphic Sp1 binding site in the collagen type I alpha 1 gene, Nat.Genet., 14: 203-205
- Mann, V., Hobson, E. E., Li, B., Stewart, T. L., Grant, S. F., Robins, S. P., Aspden, R. M., & Ralston, S. H. (2001). A COL1A1 Sp1 binding site polymorphism predisposes to osteoporotic fracture by affecting bone density and quality, J Clin.Invest, 107: 899-907
- Skakle JMS, Aspden RM. (2002). Neutron diffraction studies of collagen in human cancellous bone. J. Appl. Cryst. 35:506-508
- Stewart, T. L., Roschger, P., Misof, B. M., Mann, V., Fratzl, P., Klaushofer, K., Aspden, R., & Ralston, S. H. (2005). Association of COLIA1 Sp1 Alleles with Defective Bone Nodule Formation In Vitro and Abnormal Bone Mineralization In Vivo, Calcif Tissue Int. 77: 113-118, 2005.