Noncanonical Wnt Signaling through G Protein-Linked PKCδ Activation Promotes Bone Formation

Abstract

Wnt signaling regulates a variety of developmental processes in animals. Although the beta-catenin-dependent (canonical) pathway is known to control cell fate, a similar role for noncanonical Wnt signaling has not been established in mammals. Moreover, the intracellular cascades for noncanonical Wnt signaling remain to be elucidated. Here, we delineate a pathway in which Wnt3a signals through the Galpha(q/11) subunits of G proteins to activate phosphatidylinositol signaling and PKCdelta in the murine ST2 cells. Galpha(q/11)-PKCdelta signaling is required for Wnt3a-induced osteoblastogenesis in these cells, and PKCdelta homozygous mutant mice exhibit a deficit in embryonic bone formation. Furthermore, Wnt7b, expressed by osteogenic cells in vivo, induces osteoblast differentiation in vitro via the PKCdelta-mediated pathway; ablation of Wnt7b in skeletal progenitors results in less bone in the mouse embryo. Together, these results reveal a Wnt-dependent osteogenic mechanism, and they provide a potential target pathway for designing therapeutics to promote bone formation.

Predominant role of PDGF receptor transactivation in Wnt3a-induced osteoblastic cell proliferation

“…In vivo studies have shown that both Lrp5/6-21 and Fzddependent pathways (32,39) influence osteoblast function and bone metabolism. However, the cellular and molecular mechanisms involved in the regulation of osteoblastic cells and bone mass by Wnt proteins remain unclear.…”

“…For instance, in mesenchymal ST2 cells, a PKCd-dependent pathway that contributes to Wnt-induced bone formation has been described. (32) This pathway is activated by Gaq heterotrimeric proteins in response to Wnt3a, probably through frizzled receptors. A Rac1-JNK noncanonical pathway has also been described in the same mesenchymal cell line and plays an important role in limb development.…”

Wnt signaling and skeletal development

“…Wnt3a can also enhance ALP activity and BSP in conditional medium, enhance osteocalcin expression in ST2 cells in nondifferentiation medium, and increase mineralization in differentiation medium [86]. Wnt3a protein dose dependently induced ALP activity in ST2 cells in a serum-free medium [86].…”

“…An initial report on the Wnt7b −/− E18.5 embryo did not show any clear skeletal phenotype [52]. However, ablation of Wnt7b from skeletal progenitors by using CreloxP technique with Dermo1-Cre results in a deficiency in embryonic bone formation, which is at least partly due to a deficit in osterix activation and subsequent osteoblast differentiation [86]. BMP2 induced Wnt1 and Wnt3a expression in C3H10T1/2 cells [87].…”

“…Wnt3a has been reported to induce ALP activity in C3H10T1/2 cells through transient transfection [53,87], conditional medium treatment [77,[95][96][97], purified protein treatment [96], adenoviral treatment [98], retroviral infection [99]; (in ST2 cells) transient transfection [53,87], conditional medium treatment [86,97]; (in C2C12 cells) transient transfection [87], or conditional medium treatment [97]. Wnt3a can also enhance ALP activity and BSP in conditional medium, enhance osteocalcin expression in ST2 cells in nondifferentiation medium, and increase mineralization in differentiation medium [86].…”