Synchondrosis Development and Growth Require Wnt/ß-catenin Signaling

Objectives: The cranial base synchondroses are essential for anterior-posterior cranial base elongation, but the mechanisms regulating their development and growth remain poorly understood. Since Wnt/ß-catenin signaling is essential for skeletal development, we studied its possible roles in synchondrosis development and growth.

Methods: Mice conditionally deficient in ß-catenin were created by mating ß-catenin floxed mice with Col2a1-Cre mice (kindly provided by Dr. Yamada, NIDCR). Samples from cartilage specific conditional ß-catenin-deficient mice and their wild type littermates were analyzed by µCT, anatomical, histological and in situ hybridization procedures. For reciprocal experiments, we studied transgenic mice over-expressing a constitutive active Lef1, a transcriptional mediator of Wnt/ß-catenin signaling (CA-LEF1), under the control of Col2a1 promoter.

Results: In E17.5 ß-catenin-deficient mice, the antero-posterior length of pre-sphenoidal and basi-sphenoidal bone was about 20-25% shorter than controls, while occipital bone length was not significantly affected. Histological and histochemical analyses revealed that the mutant synchondroses remained largely cartilaginous, lacked typical growth plate organization associated with a significant thinning of perichondrial tissues, and displayed a delay in endochondral and intramembranous ossification. These developmental alterations were accompanied by marked reductions in gene expression of Indian hedgehog (Ihh) and parathyroid hormone related protein (PTHrP) as well as secreted frizzled-related protein-1 (sFRP-1), an endogenous Wnt signaling antagonist and a potential Ihh target. In CA-LEF1 embryos, synchondrosis chondrocytes underwent precocious hypertrophy, the growth plates displayed an altered organization, intermingling of immature and mature chondrocytes, and a thick perichondrium with excessive ossification.

Conclusion: The data demonstrate that Wnt/ß-catenin signaling is pivotal for cranial base synchondrosis development and growth. This pathway promotes chondrocyte maturation and ossification events and may function via modulation of the Ihh-PTHrP signaling loop.