Perichondrial expression of Wdr5 regulates chondrocyte proliferation and differentiation

Wdr5 is developmentally expressed in osteoblasts and is required for osteoblast differentiation. Mice overexpressing Wdr5 under the control of the mouse alpha(1)I collagen promoter (Col I-Wdr5) display accelerated osteoblast differentiation as well as accelerated chondrocyte differentiation, suggesting that overexpression of Wdr5 in osteoblasts affects chondrocyte differentiation. To elucidate the molecular mechanism by which overexpression of Wdr5 in the perichondrium regulates chondrocyte differentiation, studies were undertaken using skeletal elements and cultured metatarsals isolated from wild-type and Col I-Wdr5 embryos. FGF18 mRNA levels were decreased in Col I-Wdr5 humeri. Furthermore, local delivery of FGF18 to the bone collar of ex vivo cultures of metatarsals attenuated the chondrocyte phenotype of the Col I-Wdr5 metatarsals. Impairing local FGF action in wild-type metatarsals resulted in a chondrocyte phenotype analogous to that of Col I-Wdr5 metatarsals implicating impaired FGF action as the cause of the phenotype observed. The expression of Twist-1, which regulates chondrocyte differentiation, was increased in Col I-Wdr5 humeri. Chromatin immunoprecipitation analyses demonstrated that Wdr5 is recruited to the Twist-1 promoter. These findings support a model in which overexpression of Wdr5 in the perichondrium promotes chondrocyte differentiation by modulating the expression of Twist-1 and FGF18.