Collagen II is essential for the removal of the notochord and the formation of intervertebral discs

Collagen II is a fibril-forming collagen that is mainly expressed in cartil age. Collagen II-deficient mice produce structurally abnormal cartilage tha t lacks growth plates in long bones, and as a result these mice develop a s keleton without endochondral bone formation. Here, we report that Col2a1-nu ll mice are unable to dismantle the notochord. This defect is associated wi th the inability to develop intervertebral discs (IVDs). During normal embr yogenesis, the nucleus pulposus of future IVDs forms from regional expansio n of the notochord, which is simultaneously dismantled in the region of the developing vertebral bodies. However, in Col2a1-null mice, the notochord i s not removed in the vertebral bodies and persists as a rod-like structure until birth. It has been suggested that this regional notochordal degenerat ion results from changes in cell death and proliferation. Our experiments w ith wildtype mice showed that differential proliferation and apoptosis play no role in notochordal reorganization. An alternative hypothesis is that t he cartilage matrix exerts mechanical forces that induce notochord removal. Several of our findings support this hypothesis. Immunohistological analys es, in situ hybridization, and biochemical analyses demonstrate that collag ens I and III are ectopically expressed in Col2al-null cartilage. Assembly of the abnormal collagens into a mature insoluble matrix is retarded and co llagen fibrils are sparse, disorganized, and irregular. We propose that thi s disorganized abnormal cartilage collagen matrix is structurally weakened and is unable to constrain proteoglycan-induced osmotic swelling pressure. The accumulation of fluid leads to tissue enlargement and a reduction in th e internal swelling pressure. These changes may be responsible for the abno rmal notochord removal in Col2al-null mice. Our studies also show that chondrocytes do not need a collagen II environme nt to express cartilage-specific matrix components and to hypertrophy. Furt hermore, biochemical analysis of collagen XI in mutant cartilage showed tha t alpha 1(XI) and alpha 2 (XI) chains form unstable collagen XI molecules, demonstrating that the alpha 3(XI) chain, which is an alternative, posttran slationally modified form of the Col2a1 gene, is essential for assembly and stability of triple helical collagen XI.