WNT16 Requires Gα Subunits as Intracellular Partners for Both Its Canonical and Non-Canonical WNT Signalling Activity in Osteoblasts

In the past years, WNT16 became an interesting target in the field of skeletal research, as it was identified as an essential regulator of the cortical bone compartment, with the ability to increase both cortical and trabecular bone mass and strength in vivo. Even though there are indications that these advantageous effects are coming from canonical and non-canonical WNT-signalling activity, a clear model of WNT signalling by WNT16 is not yet depicted. We, therefore, investigated the modulation of canonical (WNT/beta-catenin) and non-canonical [WNT/calcium, WNT/planar cell polarity (PCP)] signalling in human embryonic kidney (HEK) 293 T and SaOS2 cells. Here, we demonstrated that WNT16 activates all WNT-signalling pathways in osteoblasts, whereas only WNT/calcium signalling was activated in HEK293T cells. In osteoblasts, we therefore, additionally investigated the role of G alpha subunits as intracellular partners in WNT16 ' s mechanism of action by performing knockdown of G alpha 12, G alpha 13 and G alpha q. These studies point out that the above-mentioned G alpha subunits might be involved in the WNT/beta-catenin and WNT/calcium-signalling activity by WNT16 in osteoblasts, and for G alpha 12 in its WNT/PCP-signalling activity, illustrating a novel possible mechanism of interplay between the different WNT-signalling pathways in osteoblasts. Additional studies are needed to demonstrate whether this mechanism is specific for WNT16 signalling or relevant for all other WNT ligands as well. Altogether, we further defined WNT16 ' s mechanism of action in osteoblasts that might underlie the well-known beneficial effects of WNT16 on skeletal homeostasis. These findings on WNT16 and the activity of specific G alpha subunits in osteoblasts could definitely contribute to the development of novel therapeutic approaches for fragility fractures in the future.