Wnt9a signaling is required for joint integrity and regulation ofIhhduring chondrogenesis

Abstract

Joints, which separate skeleton elements, serve as important signaling centers that regulate the growth of adjacent cartilage elements by controlling proliferation and maturation of chondrocytes. Accurate chondrocyte maturation is crucial for endochondral ossification and for the ultimate size of skeletal elements, as premature or delayed maturation results predominantly in shortened elements. Wnt9a has previously been implicated as being a player in joint induction, based on gain-of function experiments in chicken and mouse. We show that loss of Wnt9a does not affect joint induction, but results to synovial chondroid metaplasia in some joints. This phenotype can be enhanced by removal of an additional Wnt gene, Wnt4, suggesting that Wnts are playing a crucial role in directing bi-potential chondro-synovioprogenitors to become synovial connective tissue, by actively suppressing their chondrogenic potential. Furthermore, we show that Wnt9a is a temporal and spatial regulator of Indian hedgehog (Ihh), a central player of skeletogenesis. Loss of Wnt9a activity results in transient downregulation of Ihh and reduced Ihh-signaling activity at E12.5-E13.5. The canonical Wnt/␤-catenin pathway probably mediates regulation of Ihh expression in prehypertrophic chondrocytes by Wnt9a, because embryos double-heterozygous for Wnt9a and ␤-catenin show reduced Ihh expression, and in vivo chromatin immunoprecipitation demonstrates a direct interaction between the ␤-catenin/Lef1 complex and the Ihh promoter.

Temporomandibular joint formation requires two distinct hedgehog-dependent steps

“…Conversely, a large number of genes are induced specifically in the location of the future joint. Prominent among these are Wnt9a (formerly called Wnt14), a canonical Wnt ligand (9)(10)(11), and depending on the specific joint, Gdf5, Gdf6, or Gdf7, members of the BMP/TGF␤ superfamily (12)(13)(14)(15). Strikingly, these genes are expressed during and act in the formation of joints that form between the long bones by segmentation and in other classes of joints such as those between vertebrae and those between calvarial membranous bones.…”

“…Different members of the Gdf family are expressed in diverse joints, and the loss of Gdf activity results in the failure of joint formation (16). In addition, ␤-catenin activity, a key effector of the canonical Wnt pathway, is required for joint formation, and ectopic Wnt9a is sufficient to initiate the formation of a joint interzone (9,11,17).…”

Wnt signaling and skeletal development

“…While the underlying molecular mechanism still needs to be elucidated, it is clear that Wnt5a plays important roles in chondrogenesis. In Wnt9a −/− mice, there is no obvious abnormality in chondrogenesis [45]. However, ectopic cartilaginous material was detected in the interfrontal and sagittal suture regions separating frontal and parietal bones, and in the elbow joint [45].…”

“…In Wnt9a −/− mice, there is no obvious abnormality in chondrogenesis [45]. However, ectopic cartilaginous material was detected in the interfrontal and sagittal suture regions separating frontal and parietal bones, and in the elbow joint [45]. Wnt4 −/− mice do not display ectopic cartilage formation [46].…”

“…Wnt4 −/− mice do not display ectopic cartilage formation [46]. However, mice double mutants for Wnt9a and Wnt4 developed ectopic cartilage in two additional major joints, the ankle and knee [45]. It suggests that Wnt4 and Wnt9a are dispensable for chondrogenesis but are essential to maintain joint integrity.…”