Impaired Annulus Fibrosis Development and Vertebral Fusion Cause Severe Scoliosis in Mice with Deficiency of JNK1 and JNK2.

MAP kinases, including JNK, play an important role in the development and function of a large variety of tissues. We analyzed the skeletal phenotype of JNK1 and JNK2 double knockout (dKO) mice (JNK1fl/fl Col2-Cre/JNK2-/- ) and control genotypes, including single knockouts, at different embryonic and postnatal stages. The JNK1/2 dKO mice displayed a severe scoliotic phenotype that began during development and was grossly apparent around weaning age. Alcian blue staining of embryos (E17.5) showed abnormal fusion of the posterior spinal elements. In the adult mice, fusion of vertebral bodies and of spinous and transverse processes was noted by microCT, Alcian blue/Alizarin red stain and histology. The long bones developed normally and histological sections of the growth plate and articular cartilage did not reveal significant abnormalities. Histological sections of the vertebral column at E15.5 and E17.5 revealed an abnormal organization of the annulus fibrosus in the dKOs, with chondrocyte-like cells and fusion of dorsal processes. Spinal sections in 10-week-old dKO mice showed replacement of intervertebral disc structures (annulus fibrosus and nucleus pulposus) by cartilage and bone tissues, with cells staining for markers of hypertrophic chondrocytes including collagen X and Runx2. These findings demonstrate a requirement for both JNK1 and JNK2 in the normal development of the axial skeleton with loss of JNK signaling resulting in abnormal endochondral bone formation and subsequent severe scoliosis.