Fam20b-Catalyzed Glycosylation Regulates Chondrogenesis And Osteogenesis During Condylar Development

Although the roles of proteoglycans (PGs) have been well documented in the development and homeostasis of temporomandibular joint (TMJ), how the glycosaminoglycan (GAG) chains of PGs fulfill the roles still require explication. We generated Wnt1-cre;Fam20bf/f mice by inactivating Fam20b, a hexokinase essential for attaching GAG chains to core proteins of PGs, which was robustly activated in condylar mesenchyme during TMJ development, in cranial neural crest cells (CNCCs) via Wnt1-cre transgene. In this study, we found that Wnt1-cre;Fam20bf/f mice dramatically reduced the synthesis and accumulation of GAG chains, rather than the core proteins in condylar cartilage, which resulted in a hypoplastic condylar cartilage by enhancing chondrocyte hypertrophy and subchondral ossification. In the developing condyles of Wnt1-Cre;Fam20bf/f mice, the enlarged Ihh- and Col10- expressing domains indicated the premature hypertrophy resulting from an attenuated Ihh-Pthrp negative feedback in condylar chondrocytes, while the increased osteogenic markers, canonical Wnt activity and type-H angiogenesis verified the enhanced endochondral osteogenesis. Further ex vivo investigations revealed that loss of Fam20b decreased the domain area, but up-regulating Hh signaling in condylar mesenchyme. Moreover, the abrogation of GAG chains in heparan sulfate and chondroitin sulfate proteoglycans led to a rapid up- and then, down-regulation of Hh signaling in condylar chondrocytes, suggesting a "slow-release" manner of growth factors controlled by GAG chains. Fam20b-catalyzed GAG chain synthesis enhanced the chondrogenic and osteogenic differentiation of the developing TMJ condyle by accelerating Ihh release but reducing its diffusion.