The Study Of Wnt16in Intramembranous Ossification And Osteoblast Differentiation

The craniofacial skeleton includes the neurocranium and the facial bones. The bones are formed either through intramembranous or endochondral ossification. Intramembranous ossification, in which mesenchymal cells differentiate directly into osteoblasts, occurs in the calvaria, the maxilla, and the palate. Endochondral ossification starts with the formation of a cartilaginous template, which eventually is being replaced by bone. This occurs in most parts of the skeleton including the axial and appendicular skeletal elements as well as the basicranium. Some bones such as the mandible are formed through simultaneous endochondral and intramembranous ossification [1-3]. Both processes are controlled by complex signaling cascades induced by a number of growth factors such as members of the wingless-related MMTV integration site gene family (WNT).The WNT protein family consists of signaling glycoproteins that are involved in cell differentiation, proliferation, migration, and polarity, and thereby regulate embryonic development, adult tissue homeostasis and stem cell self-renewal. Nineteen different WNT proteins have been described in humans to date. According to their downstream signal transduction, WNTs have been divided into "canonical" and "noncanonical" signaling proteins. Canonical signaling is defined by the stabilization of β-catenin, which activates target gene expression in cooperation with lymphoid-enhancer-binding factor/T cell factor DNA-binding proteins (Lef/Tcf). Non-canonical signaling is β-catenin independent, with less well-characterized intracellular pathways.WNT16, which is highly expressed in kidney, pancreas, placenta, brain, heart, and spleen, and participates in cell fate specification, senescence and proliferation. Wnt proteins are known to play important roles in vertebrate skeletal development. Recently, Wnt16was reported to be expressed in the perichondrium and periosteum of developing long bones. Furthermore, gene variation in WNT16is associated with bone mineral density and osteoporotic fractures, and Wntl6knockout mice show a substantial decrease in cortical bone thickness and strength. All of this indicates that Wntl6is crucial in the development of long bones and the regulation of bone mass. However, the exact role of Wntl6in craniofacial bone development remains unclear.The initial purpose of the present study was to investigate the expression of Wnt16during craniofacial bone development. Wntl6was expressed in osteoid tissue during intramembranous ossification in mouse embryo heads. Therefore, the effect of Wnt16on preosteoblast proliferation and differentiation was investigated using MC3T3-E1preosteoblasts, a cell line established from newborn mouse calvaria. The data suggest that Wntl6suppresses osteoblast differentiation through the canonical β-catenin pathway, while it has no effect on proliferation.